Recombination plays a central role in the repair of broken chromosomes in all eukaryotes. We carried out a systematic study of mitotic recombination. Using several assays, we established the chronological sequence of events necessary to repair a single double-strand break. Once a chromosome is broken, yeast cells become immediately committed to recombinational repair. Recombination is completed within an hour and exhibits two kinetic gaps. By using this kinetic framework we also characterized the role played by several proteins in the recombinational process. In the absence of Rad52, the broken chromosome ends, both 5′ and 3′, are rapidly degraded. This is not due to the inability to recombine, since the 3′ single-stranded DNA ends are stable in a strain lacking donor sequences. Rad57 is required for two consecutive strand exchange reactions. Surprisingly, we found that the Srs2 helicase also plays an early positive role in the recombination process.

Telomeres, the eukaryotic chromosomal ends, preserve genome stability and help duplicate the genome. They play important roles in aging and cancer. Here, we sum

Many overlapping surveillance and repair mechanisms operate in eukaryotic cells to ensure the stability of the genome. We have screened to isolate yeast mutants exhibiting increased levels of recombination between repeated sequences. Here we characterize one of these mutants, elg1 . Strains lacking Elg1p exhibit elevated levels of recombination between homologous and nonhomologous chromosomes, as well as between sister chromatids and direct repeats. These strains also exhibit increased levels of chromosome loss. The Elg1 protein shares sequence homology with the large subunit of the clamp loader replication factor C (RFC) and with the product of two additional genes involved in checkpoint functions and genome maintenance: RAD24 and CTF18 . Elg1p forms a complex with the Rfc2–5 subunits of RFC that is distinct from the previously described RFC-like complexes containing Rad24 and Ctf18. Genetic data indicate that the Elg1, Ctf18, and Rad24 RFC-like complexes work in three separate pathways important for maintaining the integrity of the genome and for coping with various genomic stresses.

Telomeres protect the chromosome ends from degradation and play crucial roles in cellular aging and disease. Recent studies have additionally found a correlation between psychological stress, telomere length, and health outcome in humans. However, studies have not yet explored the causal relationship between stress and telomere length, or the molecular mechanisms underlying that relationship. Using yeast as a model organism, we show that stresses may have very different outcomes: alcohol and acetic acid elongate telomeres, whereas caffeine and high temperatures shorten telomeres. Additional treatments, such as oxidative stress, show no effect. By combining genome-wide expression measurements with a systematic genetic screen, we identify the Rap1/Rif1 pathway as the central mediator of the telomeric response to environmental signals. These results demonstrate that telomere length can be manipulated, and that a carefully regulated homeostasis may become markedly deregulated in opposing directions in response to different environmental cues. Over 70 years ago, Barbara McClintock described telomeres and hypothesized about their role in protecting the integrity of chromosomes. Since then, scientists have shown that telomere length is highly regulated and associated with cell senescence and longevity, as well as with age-related disorders and cancer. Here, we show that despite their importance, the tight, highly complex regulation of telomeres may be disrupted by environmental cues, leading to changes in telomere length. We have introduced yeast cells to 13 different environmental stresses to show that some stresses directly alter telomere length. Our results indicate that alcohol and acetic acid elongate telomeres, while caffeine and high temperatures shorten telomeres. Using expression data, bioinformatics tools, and a large genetic screen, we explored the mechanisms responsible for the alterations of telomere length under several stress conditions. We identify Rap1 and Rif1, central players in telomere length maintenance, as the central proteins directly affected by external cues that respond by altering telomere length. Because many human diseases are related to alterations in telomere length that fuel the disease's pathology, controlling telomere length by manipulating simple stressing agents may point the way to effective treatment, and will supply scientists with an additional tool to study the machinery responsible for telomere length homeostasis.

Cells adapt their gene expression and their metabolism in response to a changing environment. Glucose represses expression of genes involved in the catabolism of other carbon sources in a process known as (carbon) catabolite repression. However, the relationships between “poor” carbon sources is less characterized. Here we show that in addition to the well-characterized glucose (and galactose) repression of ADH2 (alcohol dehydrogenase 2, required for efficient utilization of ethanol as a carbon source), ADH2 expression is also inhibited by acetate which is produced during ethanol catabolism. Thus, repressive regulation of gene expression occurs also between “poor” carbon sources. Acetate repression of ADH2 expression is via Haa1, independently from the well-characterized mechanism of AMPK (Snf1) activation of Adr1. The response to extracellular acetate is attenuated when all three acetate transporters (Ady2, Fps1 and Jen1) are deleted, but these deletions do not affect the acetate response resulting from growth with glucose or ethanol as the carbon source. Furthermore, genetic manipulation of the ethanol catabolic pathway affects this response. Together, our results show that acetate is sensed intracellularly and that a hierarchical control of carbon sources exists even for “poor” carbon sources.