Short homologous sequences are dispersed throughout the
genomes of all organisms. Recombination between these sequences must be
tightly regulated to facilitate recombinational repair of DNA damage
while limiting ectopic recombination which may lead to genome
instability. Using S. cerevisiae as a model system we have
utilized deletion and insertion assays to identify and characterize two
factors which are involved in promoting recombination between short
repeats: Rad1p, a component of the RAD1/RAD10 endonuclease and
Rad50p an ATP dependent DNA binding protein. Strains bearing single null
mutations in each of these genes clearly show a significant reduction in
the recombination efficiencies in each of these assays. Furthermore, in
the insertion assay, the reduction is more pronounced as the lengths of
homology decrease. Interestingly, the rad50 null mutant exhibits
a very distinct phenotype in which the recombination efficiencies are
wild type with longer sequences but decrease as the lengths of homology
are reduced below a threshold size. This result strongly suggests that
RAD50 is not required for recombination between longer sequences
but is necessary for recombination between short regions of homology.
When we examined strains bearing both rad1 and rad50 null
alleles we observed a very strong synergistic effect in which both
deletions and insertions involving short homologous sequences are
reduced more dramatically than in either of the single mutants. This
synergistic effect indicates that at least two distinct pathways, a
RAD1 -dependent and a RAD50 -dependent pathway, are involved
in facilitating recombination between short repeated sequences in S.
cerevisiae .
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