Skip to main page content
U.S. flag
See More

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2025 Oct 21;122(42):e2518046122.
doi: 10.1073/pnas.2518046122. Epub 2025 Oct 15.

Mitotic recombination events and single-base mutations induced by ultraviolet light in G1-arrested yeast cells

Ying-Xuan Zhu  1   2 , Ke-Jing Li  1   2 , Min He  1 , Ke Zhang  3 , Dao-Qiong Zheng  1 , Thomas D Petes  2
Affiliations

Mitotic recombination events and single-base mutations induced by ultraviolet light in G1-arrested yeast cells

Ying-Xuan Zhu et al. Proc Natl Acad Sci U S A. .

Abstract

Ultraviolet light (UV) is a potent inducer of both single-base mutations and mitotic recombination. Although these genomic alterations are often attributed to the action of error-prone DNA polymerases on UV-induced DNA lesions during replicative DNA synthesis, UV damage can also result in mutagenic and recombinogenic DNA damage in nondividing cells. We examined the effects of UV on cells of the yeast Saccharomyces cerevisiae arrested in G1 of the cell cycle. By mating an irradiated haploid with an unirradiated haploid, we found that recombination was initiated only on the irradiated chromosome. This result indicates that trans effects of UV on recombination (for example, induction of recombinogenic proteins stimulating DNA breaks on the unirradiated homolog) are small or negligible. In addition, we show that most of the UV-induced mutations produced in G1-irradiated cells result in mutations at identical positions in both strands of the duplex. As observed for recombination events, mutations are almost exclusively on the irradiated chromosome, indicating the near absence of a trans effect on mutations.

Keywords: UV-induced mutagenesis; UV-induced recombination; mechanism of UV mutagenesis; mutagenesis in nondividing yeast cells.

PubMed Disclaimer

Conflict of interest statement

Competing interests statement:The authors declare no competing interest.

Figures

Fig. 1.
Fig. 1.
Patterns of LOH associated with various types of mitotic recombination as identified in daughter cells by a sectoring assay. In diploid strains that are heterozygous for an insertion of the SUP4 ochre suppressor and homozygous for the ade2-1 ochre mutation, strains that lose the ochre suppressor as a consequence of mitotic recombination form red colonies and those that duplicate the suppressor form white colonies. Chromatids or chromosomes derived from different genetic backgrounds are shown as red lines (W303 background) and blue lines (YJM789 background); centromeres are shown as ovals or circles. The black horizontal lines show the position of SUP4 on chromosome IV. (A) Reciprocal crossover between CEN4 and SUP4. A red/white sectored colony is produced with reciprocal patterns of LOH marking the position of the crossover. (B) Gene conversion event producing an interstitial LOH event without an associated crossover. Such an event does not produce a red/white sectored colony, but can be detected by whole-genome sequencing. (C) Reciprocal crossover associated with a region of gene conversion. Associated with the repair of the DSB on the red chromosome is a 3:1 gene conversion event (indicated by dotted lines) associated with the crossover. (D) DSB in G1 cell, followed by repair of the broken chromosomes resulting in conversion tracts of equal lengths. The resulting broken chromatids were repaired to produce a 4:0 gene conversion event. (E) DSB in G1 cell, followed by repair of the broken chromosomes resulting in conversion tracts of unequal lengths. The resulting repair leads to a 3:1/4:0 hybrid conversion tract. (F) DSB in G1 cell, leading to 4:0 conversion tract associated with a crossover.
Fig. 2.
Fig. 2.
Depiction of the system used by Eckardt and Haynes (20) and Kozmin and Jinks-Robertson (22) to detect two-strand mutations. Circles indicate colonies. (A) Mutations in the ADE2 or ADE1 gene of the adenine biosynthetic pathway result in red colonies. Double mutations in ADE genes earlier in the pathway result in white colonies. (B) Two- and one-strand mutations induced by UV. These strains are homozygous for the ade2 mutation. The lines with arrows represent single strands of the DNA duplex. After treatment with UV, if a single cell acquires a mutation in both strands of an ade gene that acts prior to the ade2 gene (adeX), it will form an unsectored white colony (Left side of figure). If it results in a mutation in only one strand, a red/white sectored colony would be formed (Right side of figure).
Fig. 3.
Fig. 3.
Experimental protocol used to determine whether a UV-irradiated chromosome could stimulate recombination or mutations in an unirradiated chromosome. Following UV treatment (20 or 80 J/m2), a G1-arrested MATa haploid was immediately mated to an unirradiated haploid of the opposite mating type. After 4 h, the mated cells were replica-plated to a medium on which only diploids could form colonies. Following single-colony purification, we sequenced the genomes of independent derivatives. The genetic backgrounds of the haploids were different (55,000 heterozygous SNPs), allowing us to map recombination events leading to LOH to high resolution.
Fig. 4.
Fig. 4.
Examples of LOH events diagnosed by DNA sequence coverage in YZ1 strains. Whole genomes were sequenced with >100-fold coverage. For each heterozygous SNP, the number of reads was divided by the average number of reads for SNPs assayed throughout the genome to yield the ratio on the Y-axis. The coordinates on the X-axis are based on the Saccharomyces Genome Database (SGD). The blue and red colors represent YJM789- and W303-specific SNP reads, respectively. (A) I-LOH event. This pattern is consistent with a gene conversion event with a 7 kb tract. (B) I-DEL. This 230 kb deletion of W303-derived sequences had directly oriented Ty elements at the breakpoints of the deletion. (C) T-LOH event. This pattern is consistent with either a reciprocal mitotic crossover or a BIR. (D) Monosomy. This figure shows the pattern expected for loss of the W303-derived chromosome III homolog.
Fig. 5.
Fig. 5.
Analysis of mutations in both daughter cells following irradiation: one- and two-strand mutations. This figure depicts the system used to diagnose one- vs. two-strand mutations in YZ2. This strain contains the heterozygous SUP4 mutation on chromosome IV that allows identification of the two daughter cells resulting from the first mitotic division of the diploid made by the mating of the irradiated and nonirradiated cells, since these cells form a red/white sectored colony. The paired red (W303-derived) and blue (YJM789-derived) vertical lines show the single strands of the duplex with arrows indicating the 3’ ends. (A) One-strand mutation. UV irradiation results in a C to T change in the haploid. When that strain is mated to the unirradiated strain, after the subsequent S-period and segregation of the replicated chromosomes, one cell will have one homolog with a mutation and one with the wild-type sequence. The other cell will be homozygous for the wild-type sequence. (B) Two-strand mutation. The UV irradiation mutates both strands at the same position. Following DNA replication and chromosome segregation, both daughter cells will have one homolog with the mutation and one with the wild-type sequence.
Fig. 6.
Fig. 6.
Three models to explain two-strand mutations. (A) In Model 1, a single DNA lesion is introduced on a preexisting single-stranded DNA gap. The gap is filled in by an error-prone DNA polymerase, resulting in a mutation opposite the UV-induced DNA lesion. Removal of the DNA lesion by nucleotide-excision repair and gap filling produces the two-strand mutation. (B) In Model 2, similar to that proposed previously by Kozmin and Jinks-Robertson (22), UV induces two lesions (pyrimidine dimers) on opposite strands that are close together. One lesion is removed by nucleotide-excision repair resulting in a gap that includes the second lesion. The filling in of the gap by an error-prone DNA polymerase producing a mutation (m) on one strand. Removal of the pyrimidine dimer opposite the mutant base, followed by accurate replication during gap filling generates the two-strand mutation. (C) In Model 3, a single lesion is generated and subsequently removed by NER. Error-prone repair of the gap results in introduction of a mutation and a nick on the template strand. After expansion of the nick into a gap, repair results in a two-strand mutation.
See this image and copyright information in PMC

References

    1. Esposito M. S., Wagstaff J. E., “Mechanisms of mitotic recombination” in The Molecular Biology of the Yeast Saccharomyces, Strathern J., Jones E. W., Broach J. R., Eds. (Cold Spring Harbor Press, 1981), pp. 341–370.
    1. Haynes R. H., Kunz B. A., “DNA repair and mutagenesis in yeast” in The Molecular Biology of the Yeast Saccharomyces, Strathern J., Jones E. W., Broach J. R., Eds. (Cold Spring Harbor Press, 1981), pp. 371–414.
    1. Fabre F., Roman H., Genetic evidence for inducibility of recombination competence in yeast. Proc. Natl. Acad. Sci. U.S.A. 74, 1667–1671 (1977). - PMC - PubMed
    1. Friedberg E. C., et al. , DNA Repair and Mutagenesis (ASM Press, 2006).
    1. Galli A., Schiestl R. H., Cell division transforms mutagenic lesions into deletion-recombinogenic lesions in yeast cells. Mutat. Res. 429, 13–26 (1991). - PubMed

MeSH terms

LinkOut - more resources