Role of Saccharomyces cerevisiae Chromatin Assembly Factor-I in Repair of Ultraviolet Radiation Damage in Vivo

Corresponding author: Paul D. Kaufman, 351 Donner Laboratory, Lawrence Berkeley National Laboratory, Berkeley, CA 94720., pdkaufman{at}lbl.gov (E-mail)

Communicating editor: F. WINSTON

	ABSTRACT

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

In vitro, the protein complex Chromatin Assembly Factor-I (CAF-I) from human or yeast cells deposits histones onto DNA templates after replication. In Saccharomyces cerevisiae, the CAC1, CAC2, and CAC3 genes encode the three CAF-I subunits. Deletion of any of the three CAC genes reduces telomeric gene silencing and confers an increase in sensitivity to killing by ultraviolet (UV) radiation. We used double and triple mutants involving cac1 and yeast repair gene mutations to show that deletion of the CAC1 gene increases the UV sensitivity of cells mutant in genes from each of the known DNA repair epistasis groups. For example, double mutants involving cac1 and excision repair gene deletions rad1 or rad14 showed increased UV sensitivity, as did double mutants involving cac1 and deletions of members of the RAD51 recombinational repair group. cac1 also increased the UV sensitivity of strains with defects in either the error-prone (rev3) or error-free (pol30-46) branches of RAD6-mediated postreplicative DNA repair but did not substantially increase the sensitivity of strains carrying null mutations in the RAD6 or RAD18 genes. Deletion of CAC1 also increased the UV sensitivity and rate of UV-induced mutagenesis in rad5 mutants, as has been observed for mutants defective in error-free postreplicative repair. Together, these data suggest that CAF-I has a role in error-free postreplicative damage repair and may also have an auxiliary role in other repair mechanisms. Like the CAC genes, RAD6 is also required for gene silencing at telomeres. We find an increased loss of telomeric gene silencing in rad6 cac1 and rad18 cac1 double mutants, suggesting that CAF-I and multiple factors in the postreplicative repair pathway influence chromosome structure.

IN Saccharomyces cerevisiae, the genes CAC1, CAC2, and CAC3 encode the three subunits of Chromatin Assembly Factor-I (CAF-1). These genes encode proteins homologous to human CAF-I subunits (KAUFMAN et al. 1997 ). In vitro, CAF-I from several organisms deposits histones onto DNA templates that have undergone replication or nucleotide excision repair synthesis (SMITH and STILLMAN 1989 ; GAILLARD et al. 1996 ; KAMAKAKA et al. 1996 ; KAUFMAN et al. 1997 ). Also, deletion of any of the three yeast CAC loci confers a modest increase in sensitivity to killing by ultraviolet (UV) radiation, implying a DNA repair defect. Combining the three cac gene deletions in the same strain does not give rise to further UV sensitivity, suggesting that the CAF-I protein complex becomes nonfunctional with respect to DNA repair upon loss of any one subunit (KAUFMAN et al. 1997 ).

We wished to explore the nature of the repair defect conferred by cac gene deletions. Most biochemical analysis of DNA repair has been performed using naked DNA in vitro (e.g., ABOUSSEKHRA et al. 1995 ). However, DNA repair in cells occurs in the context of chromatin proteins, and reformation of nucleosomes after repair is also required. Previous biochemical data have shown that CAF-I can catalyze nucleosome formation during nucleotide excision repair in vitro (GAILLARD et al. 1996 ). However, it is not clear how these in vitro data relate to the in vivo UV-sensitive phenotype. For example, CAF-I could have a role in excision repair, but other proteins might be able to substitute for CAF-I during this process in vivo, so that the observed UV sensitivity of cac mutants might be unrelated to excision repair. S. cerevisiae provides an opportunity to determine which repair pathways are defective in cac mutants, by double mutant analysis using null alleles of previously characterized repair genes.

In yeast, genes that influence cellular sensitivity to killing by ultraviolet radiation fall into three major groups that are usually considered to control three different types of DNA repair (GAME and COX 1973 ; COX and GAME 1974 ). These repair mechanisms function in the dark and are distinct from light-mediated photoreactivation, which represents a fourth type of repair (HARM 1976 ). The different groups of repair genes were initially defined by genetic analysis (GAME and COX 1973 ). Double mutant strains carrying mutations in two genes within the same group show a UV sensitivity no greater than that of one of the component single mutants, whereas double mutant combinations involving mutations in different groups display increased sensitivity. The three epistasis groups defined in this way are referred to by a prominent locus in each group, and they mediate repair by different mechanisms. Thus, the RAD1 group mediates nucleotide excision repair, an error-free mechanism by which thymine dimers and other UV photoproducts are recognized and excised from the DNA, after which DNA synthesis and ligation restores the DNA duplex (reviewed by SANCAR 1994 ). The RAD51 group mediates repair by recombinational mechanisms (reviewed by GAME 1993 ). This is the major repair mechanism for DNA double-strand breaks in yeast, and mutants in the RAD51 group are highly sensitive to ionizing radiation. In addition, they are mildly sensitive to UV radiation, because base damage also triggers recombinational processes. The RAD6 group mediates postreplicative repair: in wild-type cells, newly replicated low-molecular-weight DNA strands are rapidly converted to intact high-molecular-weight DNA in the presence of polymerase-blocking UV photoproducts. In contrast, rad6 mutants are unable to perform this method of repair, and gaps in DNA result when DNA polymerases are blocked by UV-induced photoproducts (DI CAPRIO and COX 1981 ; PRAKASH 1981 ; reviewed in PRAKASH et al. 1993 and LAWRENCE 1994 ). The RAD6 group is involved in both error-free and error-prone repair of these gaps; thus, rad6 mutants are highly UV sensitive and display greatly reduced levels of UV-induced mutagenesis (LAWRENCE and CHRISTENSEN 1976 ). rad6 mutants are also defective in various aspects of chromosome function, as discussed below.

To determine which of the different repair pathways is affected in the cac mutants, we constructed double mutant strains incorporating a cac1 deletion and a mutation in a member of each epistasis group and determined the UV sensitivity of these strains. We also studied some triple mutant strains. We observed that deletion of the CAC1 gene increased the UV sensitivity of at least some mutants from each of the three epistasis groups. In contrast, there was almost no increase in sensitivity in rad6 cac1 and rad18 cac1 double mutants.Therefore, we propose that CAF-I functions to assist multiple RAD6-mediated repair reactions.

Like CAF-I, Rad6p participates in the formation of telomeric chromatin structures that mediate epigenetic silencing of telomere-proximal genes (ENOMOTO et al. 1997 ; HUANG et al. 1997 ; KAUFMAN et al. 1997 ). However, mutants lacking RAD18, another gene in the RAD6 epistasis group, display wild-type levels of telomeric silencing (HUANG et al. 1997 ). Here, we show that deletion of both CAC1 and either RAD6 or RAD18 causes more severe telomeric silencing defects than those observed in the single mutants. These data suggest that CAF-I and the Rad6p/Rad18p proteins contribute independently to chromatin structures responsible for telomeric gene silencing.

	MATERIALS AND METHODS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Yeast strains and plasmids:
Genotypes of relevant strains are given in Table 1. All strains used were isogenic, derived from strain W303 (THOMAS and ROTHSTEIN 1989 ) by transformation or by crossing with other strains in this background. We note that the W303 strain background carries a point mutation in the RAD5 gene, known as the rad5-535 allele (FAN et al. 1996 ). However, this mutation causes very little UV sensitivity, far less than a rad5 deletion allele (FAN et al. 1996 ), and we observed that rad5 cac1 strains display significantly greater UV sensitivity and UV-induced mutation rates than rad5-535 cac1 strains (Figure 4 and Figure 6). Together, these data suggest that the rad5-535 allele functions at a nearly wild-type level.

View larger version (28K): In this window In a new window Download PPT slide   Figure 1. UV survival data for wild-type, single mutant, and double mutant strains involving cac1 and rad mutations in the recombinational repair group and mutations in cell cycle checkpoint genes. (A) cac1 in combination with rad51. (B) cac1 in combination with rad52. (C) cac1 in combination with rad9. (D) cac1 in combination with rad53 (mec2-1 allele). Strains used were as follows: PKY028, RAD CAC1; PKY020, cac1; g1054-1B, rad51; g1054-7D, cac1 rad51; PKY065, rad52; PKY067, rad52 cac1; PKY048, rad9; PKY051, cac1 rad9; DLY67, rad53-(mec2-1); and PKY070, cac1 rad53-(mec2-1).

View larger version (21K): In this window In a new window Download PPT slide   Figure 2. UV survival data for wild-type and mutant strains involving cac1 and rad mutations in the excision repair group. (A) cac1 in combination with rad1. (B) cac1 in combination with rad14. (C) cac1 in a triple mutant combination with rad1 and rad52, compared to a rad1 rad52 double mutant. Strains used were as follows: PKY028, RAD CAC1; PKY020, cac1; g1052-2B, rad1; g1052-2D, cac1 rad1; g1062-1C, rad14; g1062-1A, rad14 cac1; g1053-4b, rad1 rad52; and g1053-10b, rad1 rad52 cac1.

View larger version (20K): In this window In a new window Download PPT slide   Figure 3. UV survival data for wild-type and mutant strains involving cac1 and deletions of RAD6 or RAD18. (A) cac1 in combination with rad6. (B) cac1 in combination with rad18. (C) cac1 in a triple mutant combination with rad6 and rad52, compared to a rad6 rad52 double mutant. Strains used were as follows: PKY028, RAD CAC1; PKY020, cac1; g1057-1A, rad6; g1060-12B, rad6 cac1; g1060-4A, rad18; g1060-4B, rad18 cac1; g1058-3b, rad6 rad52; and g1061-1b, rad6 rad52 cac1.

View larger version (26K): In this window In a new window Download PPT slide   Figure 4. UV survival data for wild-type and mutant strains involving cac1 and mutations affecting subsets of the RAD6/RAD18-mediated repair pathways. (A) cac1 in combination with rad61-9. (B) cac1 in combination with rad5. (C) cac1 in combination with pol30-46. (D) cac1 in combination with rev3. Strains used were as follows: PKY028, RAD CAC1; PKY020, cac1; PKY656, rad6 + pR661 (rad61-9); PKY659, rad6 cac1 + pR661 (rad61-9); HKY578-2C, rad5; PKY099, rad5 cac1; PKY741, pol30 CAC1 + pBL230 (POL30); PKY743, pol30 CAC1 + pBL230-46 (pol30-46); PKY766, pol30 cac1 + pBL230 (POL30); PKY768, pol30 cac1 + pBL230-46 (pol30-46); PKY718, rev3; PKY721, cac1 rev3.

View larger version (18K): In this window In a new window Download PPT slide   Figure 5. UV-induced mutagenesis in wt and cac1 strains. Strains used were PKY090 (MATa, URA3-VIIL) and isogenic derivative PKY106 (cac1::LEU2). Survival of the strains at the UV doses used is plotted in each graph on the y axis on the left. (A) Ade+ revertants per 106 viable cells is plotted vs. the UV doses given. (B) Trp+ revertants per 106 viable cells is plotted vs. the UV doses given. Each experimental point was repeated at least three times; average values with standard deviations (bars) are shown.

View larger version (15K): In this window In a new window Download PPT slide   Figure 6. UV-induced mutagenesis in rad5 and rad5 cac1 strains. Strains used were HKY578-2C (rad5) and isogenic derivative PKY099 (rad5 cac1). Survival of the strains at the indicated UV doses (left axes). (A) Ade+ or (B) Trp+ revertants per 106 viable cells is plotted vs. the UV doses (right axes). Each experimental point was repeated three times; average values with standard deviations (bars) are shown.

The cac1::LEU2 and URA3-VIIL alleles were described previously (KAUFMAN et al. 1997 ). To construct the cac1::hisG-URA3-hisG allele, plasmid pPK98 carrying a 5.6-kb genomic BamHI fragment with the CAC1 gene (KAUFMAN et al. 1997 ) was first digested with BglII and NheI. A 5.4-kb BamHI-XbaI fragment containing the URA3 gene and a kanamycin-resistance gene flanked by a direct repeat of bacterial DNA was inserted into the digested pPK98 to generate pPK102. pPK102 was digested with BamHI and used to transform wild-type diploid W303-1 cells to Ura⁺ prototrophy. Correct integration was confirmed by Southern blotting. Mutant alleles in W303-based strains provided by others were as follows: strain W1214-19b (rad1::LEU2) and strain U687 (rad51::LEU2) from R. Rothstein; strain HKY578-2C (rad5::URA3) from Hannah Klein (FAN et al. 1996 ); rad14::LEU2 from Richard Verhage; strain BL31-2c (rad52::TRP1) from A. Lustig (LI and LUSTIG 1996 ); and strains DLY67 (MATa rad53 (mec2-1)) and DLY262 (MAT rad9::HIS3 rad24::TRP1) from Ted Weinert. Plasmids used to make disruptions were pYPG101 (rev3::hisG-URA3-hisG), gift of C. Lawrence; pBL243 (pol30::hisG-URA3-hisG) (AYYAGARI et al. 1995 ), gift of P. Burgers; and pJJ239 (rad18::hisG-URA3-hisG) and pR671 (rad6::hisG-URA3-hisG), gifts of L. Prakash, as were plasmids pR67 (RAD6), pR661 (rad6_1-9), and pSCW231 (WATKINS et al. 1993 ).

Disruptions were made by lithium acetate transformation of a wild-type diploid as described by KAISER et al. 1994 . Selected colonies were colony-purified twice after transformation and then sporulated. The pol30 deletion was confirmed by Southern blotting; the lethality of this deletion was complemented by plasmids carrying the POL30 gene (see Figure 4C). For RAD6, RAD18, and REV3 gene deletions, the radiation-sensitive phenotype cosegregated with the expected nutritional marker in the progeny tetrads. We confirmed that the correct gene had been disrupted in each case by demonstrating inability of the new deletion alleles to complement known alleles of the appropriate RAD or REV gene. Loss of the URA3 gene in the cac1, rad6, and rad18 disruption strains was obtained by counterselection using 5-fluoro-orotic acid (FOA) as described by ALANI et al. 1987 .

Genetic procedures and media:
Standard procedures for genetic crosses and tetrad analysis were used. Standard yeast media for crosses and for scoring genetic marker segregations were those of KAISER et al. 1994 . YPAD is YPD medium supplemented with 50 mg/liter of adenine. FOA was added to synthetic complete medium at a concentration of 1 mg/ml.

Ultraviolet radiation source and survival curves:
Logarithmically growing cells (~10⁷ cells/ml) were assessed for UV sensitivity by preparing a dilution series in distilled water and plating immediately on solid YEPD medium or on the appropriate selective media (-Trp) to maintain the plasmids for the experiments in Figure 4A. Each plate was then irradiated for an appropriate time using a shielded apparatus containing six General Electric G8T5 tubes giving most of their radiation at 254 nM. Plates were incubated in the dark for 4 days and survival was assessed by counting visible colonies. Survival curves were performed several times with consistent results, and single curves representative of the results are shown in the figures. Error bars on the survival curves represent twice the standard error based on the number of colonies counted for the point in question for the chosen curve. In most cases these bars are smaller than the symbol used to delimit the point. This method assumes a Poisson distribution for the number of cells in random samples of equal volume for a given suspension (SCHEFLER 1979 ).

Mutagenesis experiments:
Logarithmically growing cells were harvested at a density of ~6 x 10⁶/ml, washed with distilled water, and plated immediately in duplicate on solid YPAD medium to count viable cell number (~200 surviving cells/plate at each UV dose), and on synthetic media lacking either adenine or tryptophan (~1–2 x 10⁷ total cells/plate) to measure ade2-1 or trp1-1 reversion. After UV irradiation at the indicated doses, plates were incubated in the dark at 30° and survival was assessed by counting visible colonies on YPAD plates after 3 days. Ade⁺ and Trp⁺ revertants were counted after 6 days.

Telomeric silencing assays:
To measure telomeric silencing, the URA3-VIIL-marked telomere was used (KAUFMAN et al. 1997 ; originally described in GOTTSCHLING et al. 1990 ). This assay monitors silencing of a wild-type URA3 gene that is inserted near a telomere. Silenced cells exhibit a Ura^- phenotype, which includes resistance to FOA, whereas nonsilenced cells are Ura⁺ and FOA sensitive. We found that equivalent results were obtained by using log phase cells (A₆₀₀ = ~0.6) from liquid medium or with cells taken from plates and adjusted to A₆₀₀ = ~0.6 (e.g., RUNDLETT et al. 1996 ). To assess the frequency of silenced cells in a population, cells were plated on synthetic complete + FOA medium, and the number of FOA-resistant colonies per cell plated was determined after 8 days of incubation at 30°. Small FOA-resistant microcolonies formed by the rad18 cac1 URA3-VIIL strain were counted under a dissecting microscope, and the 8-day incubation was necessary to fully detect these microcolonies. Colonies on synthetic complete medium plates were counted after 3 days to assess the number of viable plated cells. In order to correct for variation in the potency of the FOA in different batches of plates, the fraction of FOA-resistant cells was normalized to that of the wild-type strain for each repetition of the experiment.

	RESULTS

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Epistasis analysis of the cac1 UV-sensitivity phenotype:
Previous work has shown that deletion of any of the three CAC loci confers an increase in sensitivity to UV and that double and triple cac mutant combinations confer no additional sensitivity (KAUFMAN et al. 1997 ). We sought to determine whether the UV sensitivity of a cac mutant was related to a defect in DNA damage repair mediated by one of the known epistasis groups of rad mutants. We therefore crossed strains carrying a cac1 deletion to isogenic strains with representative rad deletions and measured the UV sensitivity of the resulting double mutants to determine which type of DNA repair is affected upon loss of CAF-I.

To test the relationship between CAC1 and recombinational repair, we first obtained UV survival data for single and double mutants involving CAC1 and each of two genes in this epistasis group, RAD51 and RAD52 (Figure 1). As previously reported (KAUFMAN et al. 1997 ), the cac1 allele conferred moderate UV sensitivity, which was less severe than that conferred by the rad51 and rad52 mutations. These in turn were less sensitive than mutants in the other UV epistasis groups (e.g., COX and GAME 1974 ; Figure 1 Figure 2 Figure 3), because recombination is more important for repair of double-strand breaks than for repairing UV photoproducts. We observed an increase in sensitivity in double mutants involving cac1 and either rad51 (Figure 1A) or rad52 (Figure 1B).

Several genes involved in sensing DNA damage also confer a radiation-sensitive phenotype when mutated, including the RAD9 and RAD53 genes (WEINERT and HARTWELL 1988 ; ALLEN et al. 1994 ; WEINERT et al. 1994 ). To determine whether UV sensitivity of cac1 mutants is related to this function, we obtained UV survival data for cac1 in combination with a rad9 deletion and an allele of the RAD53 gene. We used a missense allele of RAD53 previously described as mec2-1 (WEINERT et al. 1994 ), because RAD53 is essential for viability. The rad9 cac1 double mutant (Figure 1C) and rad53 cac1 double mutant (Figure 1D) were both more UV sensitive than the rad CAC1⁺ strains.

To test the relationship between CAC1 and nucleotide excision repair, the UV sensitivity of double mutants involving the excision repair-defective mutations rad1 and rad14 in combination with cac1 were compared with the rad single mutants (Figure 2). The Rad1p protein directly binds the Rad10p protein, forming an endonuclease complex that performs an incision step during nucleotide excision repair (BARDWELL et al. 1994 ). RAD14 encodes a protein homologous to the human xeroderma pigmentosum group A (XPA) protein (BANKMANN et al. 1992 ); like XPA, Rad14p recognizes DNA containing UV photoproducts (GUZDER et al. 1993 ). The rad1 cac1 double mutant displayed increased UV sensitivity compared to the rad1 mutant strain (Figure 2A). For rad14 combined with cac1 (Figure 2B), there was an equivalent or slightly larger increase in sensitivity.

To confirm the assignment of CAC1 outside of the RAD1 and RAD52 epistasis groups for UV repair using a more sensitized assay, we constructed a triple mutant strain lacking CAC1, RAD1, and RAD52. Blocking two repair pathways will direct more UV-induced damage into the remaining pathway, thus increasing the apparent sensitivity caused by mutations in the remaining pathway (GAME and COX 1973 ). Thus, we predicted that the increased sensitivity between rad1 and cac1 would be demonstrated more readily when these mutants are also deleted for RAD52. Comparison of survival data for a triple mutant strain with data for a rad1 rad52 double mutant showed that a further increase in sensitivity was in fact conferred by the cac1 mutation (Figure 2C).

To test the relationship between CAC1 and postreplicative DNA repair, we measured the UV sensitivity caused by combination of the cac1 mutation with deletions of RAD6 epistasis group members (Figure 3). rad6 and rad18 are the two most UV-sensitive mutants defective in postreplicative DNA repair, blocked for both error-free and error-prone mechanisms (reviewed in PRAKASH et al. 1993 ; LAWRENCE 1994 ). The cac1 mutation did not substantially increase the UV sensitivity of a rad6 (Figure 3A) or a rad18 strain (Figure 3B), even at doses at which cac1 did increase the UV sensitivity of rad1 and rad14 mutants (Figure 2). A triple mutant combination involving cac1 with both rad6 and rad52 also showed only a very small increase in UV sensitivity (Figure 3C), in contrast to the situation for the rad1 rad52 cac1 triple mutant (Figure 2C). Together these data suggest that most of the UV resistance conferred by CAF-I action requires the Rad6p protein (see DISCUSSION).

Because the rad6 and rad18 deletions block the activity of multiple proteins involved in distinct error-free and error-prone repair mechanisms, we sought to determine whether CAF-I function could be assigned to any particular subset of the RAD6 epistasis group. We therefore tested a less UV-sensitive RAD6 allele termed rad6_1-9, which encodes a protein lacking the highly conserved N-terminal nine residues. rad6_1-9 cells display increased UV-induced mutagenesis rates with respect to wild type, contrary to rad6 mutants, which display almost no UV-induced mutagenesis (LAWRENCE and CHRISTENSEN 1976 ; WATKINS et al. 1993 ). This suggests that deletion of the Rad6p N terminus results primarily in a defect in error-free postreplicative repair (WATKINS et al. 1993 ). Combination of cac1 with rad6_1-9 caused a substantial increase in UV sensitivity (Figure 4A).

We also tested cac1 in combination with mutations in other genes that affect error-free subsets of the RAD6-mediated repair processes. For example, rad5 mutants are much less UV sensitive than rad6 or rad18 mutants, and deletion of RAD5 does not increase the UV sensitivity of rad6 or rad18 mutants (JOHNSON et al. 1992 ). RAD5 contributes to error-free postreplicative repair: deletion of RAD5 increases the UV sensitivity of rev3 strains that lack the error-prone polymerase zeta (JOHNSON et al. 1992 ), and rad5 strains display increased rates of nonhomologous recombination reactions (AHNE et al. 1997 ). As observed for the rad6_1-9 mutant, a rad5 cac1 double mutant strain was more UV sensitive than a rad5 strain (Figure 4B and Figure 6). To further test the relationship between CAF-I and error-free postreplicative DNA repair, we also tested the effects of cac1 on pol30-46 strains. The pol30-46 allele contains four separate point mutations, each of which changes a charged residue to alanine in the gene encoding PCNA (Proliferating Cell Nuclear Antigen), which is required for the processivity of eukaryotic leading-strand polymerase delta (PRELICH et al. 1987 ; TSURIMOTO and STILLMAN 1989 ). pol30-46 encodes a PCNA protein able to support viability and able to stimulate polymerases in vitro, but this allele appears to block most, if not all, error-free postreplicative repair, because pol30-46 rev3 strains, which also lack RAD6-mediated error-prone repair, are nearly as UV sensitive as rad6 strains (TORRES-RAMOS et al. 1996 ). We observed that a pol30-46 cac1 strain was more UV sensitive than a pol30-46 strain (Figure 4C). In summary, the cac1 deletion increased the UV sensitivity of three mutants, rad5, rad6_1-9, and pol30-46, known to be defective in error-free postreplicative repair. This suggests that CAF-I may be required for more than one subset of RAD6-mediated repair mechanisms, or controls a subset distinct from that affected by the mutations tested (see DISCUSSION).

We also tested the effects of cac1 on a mutant defective in error-prone repair. rev3 mutants lack the error-prone DNA polymerase zeta, are mildly UV sensitive, and lack UV-induced mutagenesis (LAWRENCE and CHRISTENSEN 1976 ; NELSON et al. 1996 ). We observed that a rev3 cac1 double mutant strain was more UV sensitive than a rev3 strain (Figure 4D).

Levels of UV-induced mutagenesis in a cac1 mutant:
Thymine dimers and other photoproducts generated by UV light result in gapped DNA following replication (DI CAPRIO and COX 1981 ; PRAKASH 1981 ). Wild-type cells are able to fill these gaps by both error-free and error-prone mechanisms. Both these mechanisms are defective in rad6 and rad18 mutants (DI CAPRIO and COX 1981 ; PRAKASH 1981 ); therefore, these mutants display reduced levels of UV-induced mutagenesis (LAWRENCE and CHRISTENSEN 1976 ; CASSIER-CHAUVAT and FABRE 1991 ; ARMSTRONG et al. 1994 ). In contrast, nucleotide excision repair mutants inefficiently remove the photoproducts that stimulate postreplicative repair, including error-prone repair mechanisms, causing increased UV-induced mutagenesis levels (see, e.g., LAWRENCE and CHRISTENSEN 1976 ). If CAF-I were involved in nucleotide excision repair in vivo, cac1 mutants would be expected to display at least a slightly increased rate of UV-induced mutagenesis compared to wild type.

We therefore examined UV-induced mutagenesis in a cac1 mutant. Reversion of two point mutations (ade2-1 and trp1-1) present in the W303-1 strain background was measured for several UV doses in wild-type and cac1 cells (Figure 5). At both loci, no increase in induced mutagenesis frequencies was observed for the cac1 mutant compared to wild type.

Mutation of genes in the error-free subset of the RAD6 group in some cases increases levels of UV-induced mutagenesis (WATKINS et al. 1993 ; BROOMFIELD et al. 1998 ). This presumably occurs because damage normally repaired by nonmutagenic mechanisms is instead repaired by the mutagenic DNA polymerase zeta encoded by the REV3 and REV7 genes. Such increases in mutagenesis can occur as the result of mutation of more than one gene: loss of both the RAD5 and RAD30 genes leads to a large synergistic increase in UV-induced mutagenesis (MCDONALD et al. 1997 ). We observed that cac1 significantly increased UV mutability in cac1 rad5 double mutants compared to rad5 single mutants (Figure 6). This suggests that CAF-I does play a role in error-free postreplicative DNA repair (see DISCUSSION).

Telomeric gene silencing in cac1, rad6, and rad18 mutants:
In wild-type cells, transcription of a URA3 gene adjacent to telomeres is largely suppressed by telomeric gene silencing. This results in a fraction of cells in a population being resistant to the drug FOA (GOTTSCHLING et al. 1990 ), which is rendered toxic by activity of the URA3 gene (BOEKE et al. 1987 ; see also MATERIALS AND METHODS). In these experiments, cac mutant strains with a URA3-marked telomere inserted next to the left telomere of chromosome VII generated an average of fivefold fewer FOA-resistant cells than wild type (Table 2), indicative of a reduced level of telomeric URA3 silencing. As observed previously, the colonies that did grow on FOA were smaller than those from a wild-type strain, suggesting that the ability to silence URA3 transcription is transient in cac mutants (data not shown, but see ENOMOTO et al. 1997 ; KAUFMAN et al. 1997 ; MONSON et al. 1997 ). rad6 mutants also displayed reduced levels of telomeric gene silencing (HUANG et al. 1997 ); in our strain background, the frequency of FOA-resistant colonies was ~20-fold less than wild type, and the colonies were also smaller than in wild type (Table 2 and data not shown). As previously observed, rad18 mutants displayed approximately wild-type levels of telomeric silencing (Table 2; HUANG et al. 1997 ; see below).

View this table: In this window In a new window   Table 2. Telomeric gene silencing in cac1, rad6, and rad18 mutant combinations

Because our data suggested that CAC1 may function partially outside of the RAD6 epistasis group with respect to UV sensitivity (Figure 4), we asked whether there was also an independent contribution of these genes to telomeric gene silencing. Indeed, double mutant combinations of cac1 and either rad6 or rad18 had more severe defects in telomeric silencing than the single mutants (Table 2). We observed that rad6 cac1 double mutants produced FOA-resistant colonies at a frequency ~10^-4 that of a wild-type strain; cac1 and rad6 single mutant strains generated average FOA-resistant frequencies of 0.2 and 0.05 that of wild type, respectively. The reduction of telomeric silencing was also dramatic in the case of the rad18 cac1 double mutant, which generated full-size FOA-resistant colonies at an average frequency of 2 x 10^-4 that of wild type. In addition, as observed for cac mutants, the rad18 cac1 strain generated FOA-resistant microcolonies, but in this case, the microcolonies were far smaller than those observed for cac mutants, and required the use of a dissecting microscope in order to count them. The microscopic FOA-resistant rad18 cac1 colonies arose at an average frequency of 2 x 10^-2 that of the full-size wild-type colonies. Even considering microcolonies, this resistance value is an order of magnitude below the level of FOA-resistance observed in the same experiments for the cac1 mutant alone. Control experiments using strains lacking the URA3-VIIL marker showed that none of the mutations tested caused cells to become intrinsically sensitive to FOA (Table 2). Thus, the observed effects result from changes in telomeric gene silencing.

	DISCUSSION

TOP ABSTRACT MATERIALS AND METHODS RESULTS DISCUSSION LITERATURE CITED

Contribution of CAC1 to UV repair:
S. cerevisiae cells lacking any of the CAC1, 2, or 3 genes encoding the three subunits of CAF-I display increased UV sensitivity (KAUFMAN et al. 1997 ). We show here that loss of the CAC1 gene increased the UV sensitivity of rad51 and rad52 mutants (Figure 1). These data indicate that CAC1 does not belong to the RAD51 epistasis group and is therefore unlikely to be involved in recombinational repair. This conclusion is consistent with previous data showing that cells lacking any or all of the three CAC genes display no increase in sensitivity to gamma-irradiation (KAUFMAN et al. 1997 ), a treatment that causes double-strand breaks that are normally repaired by the recombination pathway (reviewed by GAME 1993 ). cac1 also increased the sensitivity of rad9 and rad53 (mec2-1) mutants, suggesting that CAC1 does not protect cells from UV damage through a role in S-phase checkpoint control (Figure 1). Similarly, cac1 increased the sensitivity of rad1, rad14, and rad1 rad52 mutants (Figure 2). These data place CAC1 outside of the RAD1 epistasis group responsible for nucleotide excision repair.

cac1 mutant cells display no increase in UV-induced mutagenesis compared to wild-type cells (Figure 5). Increased mutagenesis is a common phenotype of mutants defective in nucleotide excision repair mediated by the RAD1 epistasis group (see, e.g., LAWRENCE and CHRISTENSEN 1976 ). Therefore, this result is consistent with the UV-sensitivity data placing CAC1 outside the RAD1 excision repair group. However, CAF-I from vertebrate cells does deposit histones onto DNA templates undergoing nucleotide-excision repair in vitro (GAILLARD et al. 1996 ), reflecting the ability of CAF-I to recognize (directly or indirectly) DNA polymerase movement in a cell-free system. We hypothesize that other factors may be more important than or functionally redundant with CAF-I in vivo for reformation of nucleosomes after nucleotide-excision repair. Alternatively, poor nucleosome reformation after nucleotide-excision repair may not impact viability. Because both nucleotide excision repair and error-free postreplicative repair involve polymerases stimulated by PCNA (AYYAGARI et al. 1995 ; TORRES-RAMOS et al. 1996 ), one possibility suggested by these data is that these proteins are recognized by CAF-I as the cue to specifically target DNA replicated during repair for nucleosome assembly and that this targeting has lower specificity in vitro.

No significant increase in UV sensitivity was detected when cac1 was combined with rad6 or rad18 alleles (Figure 3). We note that at similarly low UV doses (5–10 J/m²), deletion of CAC1 was observed to significantly increase the UV sensitivity of rad1 and rad14 mutants (Figure 2). Furthermore, deletion of CAC1 had little effect on the UV sensitivity of a rad6 rad52 double mutant (Figure 3C). Because rad6 rad52 cells are able to survive UV damage almost exclusively by action of the remaining RAD1-dependent nucleotide excision repair pathway, this supports our conclusion that nucleotide excision repair is largely functional in cac1 mutants.

What aspect of RAD6-mediated postreplicative repair is affected by loss of CAF-I? Error-prone repair appears to be intact in cac1 mutants: cac1 increased the UV sensitivity of nonmutagenic rev3 strains (Figure 4), and UV-induced mutagenesis is at near-wild-type levels in cac1 cells (Figure 5). The strong increase in UV sensitivity observed when cac1 was combined with rad5, pol30-46, or rad6_1-9 mutations implies that CAF-I also operates outside of many known RAD6-dependent error-free repair functions (Figure 4). However, the increase in UV-induced mutagenesis in rad5 cac1 strains compared to rad5 single mutants (Figure 6) suggests a role for CAF-I in error-free postreplicative repair analogous to that observed for the RAD30 gene (MCDONALD et al. 1997 ). In that case, deletion of RAD5 in a rad30 strain also increased UV-induced mutagenesis, presumably because mutation of multiple error-free repair factors leads to increased damage repair by the error-prone polymerase zeta. Overall, we interpret our data to suggest that the majority of the UV-protective effect of CAF-I action occurs via Rad6p- and Rad18p-mediated error-free mechanisms.

Our data are consistent with two models for how CAF-I contributes to DNA repair. In one scenario, CAF-I acts as an auxiliary factor to assist in DNA repair by multiple pathways. For example, if CAF-I were important for multiple subsets of RAD6-mediated DNA repair, then it would be expected that cac1 deletions would increase the UV sensitivity of all mutants tested except ra6 and rad18 deletions, as observed. A second possibility is that nucleosome assembly by CAF-I results in some prevention of ultraviolet radiation damage to the DNA, and that in the absence of CAF-I the amount of damage caused by a given UV dose increases. This would result in the observed increase in UV sensitivity caused by cac gene deletions in combination with almost any rad mutation (Figure 1 Figure 2 Figure 3 Figure 4). We note that cac1 mutants have enlarged nuclei (ENOMOTO et al. 1997 ); perhaps this phenotype directly or indirectly causes the DNA to become more easily damaged upon UV irradiation. We note that these two possibilities are not mutually exclusive.

RAD6 and CAC1 both affect chromosome function:
RAD6 and CAC1 each contribute to position-dependent gene silencing in yeast (ENOMOTO et al. 1997 ; HUANG et al. 1997 ; KAUFMAN et al. 1997 ; MONSON et al. 1997 ; ENOMOTO and BERMAN 1998 ; KAUFMAN et al. 1998 ). In addition, the RAD6 gene is required for several other biological processes including postreplicative DNA repair, UV-induced mutagenesis and N-end-dependent protein degradation (DOHMEN et al. 1991 ; reviewed in PRAKASH et al. 1993 ; LAWRENCE 1994 ). Rad6p is a member of the E2 family of ubiquitin-conjugating enzymes (JENTSCH et al. 1987 ); mutation of the catalytic cysteine required for formation of ubiquitin conjugates destroys the biological activity of the protein with respect to all known phenotypes (SUNG et al. 1990 , SUNG et al. 1991 ; HUANG et al. 1997 ). Other genes in the RAD6 group with regard to UV sensitivity are generally involved in only a subset of these processes and often have quantitatively less severe phenotypes than rad6 mutants (reviewed in PRAKASH et al. 1993 ; LAWRENCE 1994 ).

rad18 mutants are highly UV sensitive (Figure 3; LAWRENCE and CHRISTENSEN 1976 ) and are also defective in postreplicative repair (PRAKASH 1981 ). Rad18p is a single-strand DNA-binding protein that directly interacts with Rad6p to form a heterodimer (BAILLY et al. 1994 , BAILLY et al. 1997A , BAILLY et al. 1997B ). Mutation of the Rad6p-binding domain of Rad18p leads to a UV-sensitivity phenotype very similar to that of deletion alleles (BAILLY et al. 1997A ). Such data suggest that Rad18p recruits Rad6p to single-stranded DNA segments that remain after DNA synthesis through damaged regions. The strong UV-sensitive phenotype of rad18 mutants suggests that this interaction is critical for the ability of Rad6p to function in the recovery from DNA damage.

Several phenotypes related to chromosome structure have been observed in rad6 mutants. Ty transposition is stimulated and displays a more randomized insertion pattern in rad6 mutants (PICOLOGLOU et al. 1990 ; LIEBMAN and NEWNAM 1993 ), and deletion of RAD6 reduces the transcriptional silencing and increases mitotic recombination of Ty elements located in ribosomal RNA genes (BRYK et al. 1997 ). In contrast, recombination within the ARS1 locus is reduced in a rad6 mutant (MARKVART et al. 1996 ). Also, there are effects on heterochromatic gene silencing in rad6 mutants, including diminished gene silencing at telomeres and the HML silent mating-type locus (HUANG et al. 1997 ; Figure 7). The catalytic cysteine of Rad6p is required for this function, suggesting that ubiquitination of an unknown substrate is required for silencing. Consistent with this hypothesis, a Sir-protein-binding deubiquitinating enzyme acts to antagonize telomeric silencing (MOAZED and JOHNSON 1996 ). Although Rad6p is able to ubiquitinate histones in vitro (SUNG et al. 1988 ), it is not certain that these are the biologically relevant substrates in vivo (SWERDLOW et al. 1990 ). RAD6 is also required for successful meiosis in yeast (GAME et al. 1980 ). Likewise, disruption of a RAD6-homologous gene in mice causes male sterility arising from defects in spermatogenesis during postmeiotic chromatin remodeling, suggesting that aspects of RAD6 function have been conserved in evolution (ROEST et al. 1996 ). Together, these data suggest that global aspects of chromosome structure are perturbed in the absence of RAD6 function.

We note that rad18 mutants display no defects in telomeric silencing (HUANG et al. 1997 ), but that loss of RAD6 or RAD18 further reduces telomeric silencing in the absence of CAC1 (Table 2). This suggests that that Rad6p and CAF-I play nonredundant roles in the formation of the proper chromatin structure for telomeric gene silencing. One possibility for the role of Rad18p is that it is normally functionally redundant with other factors that serve to recruit Rad6p to telomeric DNA, but that in the absence of CAF-I, these other factor(s) are unable to function properly, presumably due to changes in chromatin structure. This may result in a much more substantial reduction in Rad6p recruitment to telomeres in the absence of both CAF-I and Rad18p, leading to the observed synergistic silencing defect.

Although Rad18p appears to be functionally redundant with other factors with respect to telomeric silencing, this is clearly not the case for DNA repair. The strong UV-sensitive phenotype of rad18 mutants instead suggests that Rad18p is the most important factor for recruitment of Rad6p to sites of DNA damage, and cannot be substituted in this role regardless of the presence of CAF-I.

Recent results (SINGH et al. 1998 ) demonstrate that in the fission yeast Schizosaccharomyces pombe, rhp6 mutants defective in the gene structurally and functionally homologous to S. cerevisiae RAD6 also have position-dependent gene silencing defects at their silent mating loci. These silencing defects display a novel specificity: the silent mating cassettes are only derepressed in rhp6 mutant cells when the cis-acting sequences allowing for mating type switching are intact. This implies that the recombinational switching event itself causes a requirement for rhp6⁺ function with regard to silencing; the authors propose a model in which proper reassembly of chromatin after the switch is the critically regulated event.

	ACKNOWLEDGMENTS

We would like to thank all our colleagues who provided strains and plasmids as mentioned in the text, and Jay Chuang for excellent technical assistance. We thank K. Collins, A. Ehrenhofer-Murray, and S. Okamura for critical reading of the manuscript and S. Liebman and D. Gottschling for communicating results prior to publication. This work was supported by Department of Energy funds awarded to J.C.G. and to P.D.K. and administered through the Lawrence Berkeley National Laboratory and by National Institutes of Health grant 1 R01 GM-55712 to P.D.K.

Manuscript received August 7, 1998; Accepted for publication October 21, 1998.

	LITERATURE CITED

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