RADIATION-INDUCED MITOTIC GENE CONVERSION FREQUENCY IN YEAST IS MODULATED BY THE CONDITIONS ALLOWING DNA DOUBLE-STRAND BREAK REPAIR

Repair of DNA double-strand breaks (DSB) involves recombinational proc esses which may lead to gene conversion (intragenic recombination). Us ing the diploid yeast mutant rad54-3 heteroallelic for his1 (his1-7/hi s1-1) and temperature conditional for DSB rejoining, radiation induced gene conversion was investigated as dependent on DSB repair under dif ferent postirradiation conditions. Gene conversion is negligible under conditions preventing DSB repair (36-degrees-C). In contrast, gene co nversion is observed when cells are incubated at the permissive temper ature (23-degrees-C) both under growth and nongrowth conditions. Howev er, there is a much higher yield of convertants for cells incubated un der growth as opposed to nongrowth conditions. These results can most plausibly be explained by the cell cycle regulated enhancement of the expression of genes such as PMS and POL3 known to be involved in gene conversion processes and/or the enhanced recombination in transcriptio nally active genes. 'Nutrient stress' inducible responses and/or cell cycle specific recombination pathways leading to gene conversion event s preferentially in S-phase cells seem to be less likely.