Homologous recombination is an important DNA repair mechanism in vegetative cells. During the repair of double-strand breaks, genetic information is transferred between the interacting DNA sequences (gene conversion). This event is often accompanied by a reciprocal exchange between the homologous molecules, resulting in crossing over. The repair of DNA damage by homologous recombination with repeated sequences dispersed throughout the genome might result in chromosomal aberrations or in the inactivation of genes. It is therefore important to understand how the suitable homologous partner for recombination is chosen. We have developed a system in the yeast Saccharomyces cerevisiae that can monitor the fate of a chromosomal double-strand break without the need to select for recombinants. The broken chromosome is efficiently repaired by recombination with one of two potential partners located elsewhere in the genome. One of the partners has homology to the broken ends of the chromosome, whereas the other is homologous to sequences distant from the break. Surprisingly, a large proportion of the repair is carried out by recombination involving the sequences distant from the broken ends. This repair is very efficient, despite the fact that it requires the processing of a large chromosomal region flanking the break. Our results imply that the homology search involves extensive regions of the broken chromosome and is not carried out exclusively by sequences adjacent to the double-strand break. We show that the mechanism that governs the choice of homologous partners is affected by the length and sequence divergence of the interacting partners, as well as by mutations in the mismatch repair genes. We present a model to explain how the suitable homologous partner is chosen during recombinational repair. The model provides a mechanism that may guard the integrity of the genome by preventing recombination between dispersed repeated sequences.Homologous recombination is a universal process; it plays a role in generating diversity during meiosis and is an important DNA repair mechanism in vegetative cells. Recombination results in the transfer of genetic information from one DNA molecule to a homologous one (gene conversion) and in the reciprocal exchange of DNA fragments between chromosomes (crossing over). Reciprocal and nonreciprocal recombination show a nonrandom association. For meiotic cells, tetrad analysis has shown that when a heterozygous marker has converted, there is often (18 to 66% of the time) an associated crossover of flanking markers (reviewed in reference 29). This coupling holds true for mitotic recombination, although the level of association seems to be lower (13,17). These results have led to the assumption that gene conversion and crossing over are mechanistically related. Double-strand breaks (DSBs) in the DNA of living organisms occur as a consequence of the natural cell metabolism or can be created by exogenous sources, such as chemical agents or radiation. In addition, DSBs are generated during cer...

Homologous recombination can result in the transfer of genetic information from one DNA molecule to another (gene conversion). These events are often accompanied by a reciprocal exchange between the interacting molecules (termed "crossing over"). This association suggests that the two types of events could be mechanistically related. We have analyzed the repair, by homologous recombination, of a broken chromosome in yeast. We show that gene conversion can be uncoupled from crossing over when the length of homology of the interacting substrates is below a certain threshold. In addition, a minimal length of homology on each broken chromosomal arm is needed for crossing over. We also show that the coupling between gene conversion and crossing over is affected by the mismatch repair system; mutations in the MSH2 or MSH6 genes cause an increase in the crossing over observed for short alleles. Our results provide a mechanism to explain how chromosomal recombinational repair can take place without altering the stability of the genome.Homologous recombination is a universal process that plays a role in generating diversity during meiosis and is an important DNA repair mechanism in vegetative cells. Recombination results in the transfer of genetic information from one DNA molecule to a homologous one (gene conversion) and in the reciprocal exchange of DNA fragments between chromosomes (crossing over). The association between gene conversion and crossing-over events has led to the assumption that they are mechanistically related (Refs. 1-5; Fig. 1). One of the characteristic features of most eukaryotic genomes is the presence of large amounts of repetitive DNA. Reciprocal recombination between dispersed repeats may result in chromosomal aberrations, such as deletions, translocations, etc., that can affect the reproductive fitness of an organism or lead to cancer. Therefore, to maintain the genome integrity, crossing over must be prevented during recombinational repair of DNA lesions involving dispersed repeats. Double-strand breaks (DSBs) 1 in the DNA of living organisms occur as a consequence of the natural cell metabolism, or they can be created by exogenous sources such as chemical agents or radiation. If left unrepaired, DSBs result in broken chromosomes and cell death (6). Mitotic recombination plays an important role in the repair of this damage. In addition, DSBs are generated during certain developmental processes such as meiosis (7) and mating-type switch in yeast (8). In different experimental systems, it was found that the level of association between gene conversion and crossing over varies, from no coupling (e.g. mating-type switch (8 -10) or recombination between direct repeats (11)) to a level of association of 70% (5). In two of the currently held models of recombination, the synthesisdependent strand annealing (SDSA) model (12) and the DSB repair model (4), recombination is initiated by the creation of a DSB in one of the two participating DNA duplexes (Fig. 1). Although the mechanism suggested by the SDS...

No abstract

Cadmium ions are bacteriocidal, resulting in exponential killing that starts immediately after exposure. We have shown that pretreatment with sublethal concentrations of cadmium induces cadmium tolerance. Protection against cadmium killing can also be obtained by preincubation at elevated temperatures, known to induce the heat-shock response. However, in contrast to pretreatment at elevated temperatures, exposure to sublethal cadmium concentrations does not induce thermotolerance.

Homologous recombination is an important DNA repair mechanism in vegetative cells. During the repair of double-strand breaks, genetic information is transferred between the interacting DNA sequences, thus creating a gene-conversion event. Gene conversion of a functional member of a gene family, which uses an inactive member (such as a pseudogene) as a template, might have deleterious consequences. It is therefore important for the cell to prevent recombination between divergent sequences. We have studied the repair of a double-strand break by recombination in a haploid yeast strain carrying 99% identical alleles located on different chromosomes. The fate of the broken chromosome was followed in the whole cell population without imposing selective constraints. Our results show that all the cells were able to repair the broken chromosome by gene conversion. During the repair, the cells arrest in the cell cycle with a "dumbbell" configuration characteristic of G2/M-arrested cells. Surprisingly, although all the cells repaired the broken chromosome, 60% of them were unable to resume growth and to form colonies after the repair was completed. The low level of cell recovery was due to the 1% divergence between the alleles, but was not dependent on the function of the mismatch-repair system. Cell death, however, could be prevented by the presence of an alternative source of perfect homology located on a different chromosome.