Genetic recombination in synchronized cultures of Saccharomyces cerevisiae.

XPERIMENTAL evidence from studies in several fungi has demonstrated E that intragenic recombination is predominantly non-reciprocal whereas intergenic recombination is primarily reciprocal. These observations have led to studies to determine whether intragenic and intergenic recombination derive from the same or different mechanisms. In recent years experimental data supporting both interpretations have been obtained. The frequent association of intragenic recombination with reciprocal recombination of outside markers in yeast (FOGEL and HURST 1967), Neurospora (MITCHELL 1955), and Aspergillus (PRITCHARD 1960), suggests that both events are the results of a single mechanism. However, the observations that intragenic and intergenic recombination respond differently to heat shock (MITCHELL 1957), changes in genetic background ( STADLER 1959a; CATCHESIDE, J SSOP, and SMITH 1964), chemical mutagens (EBBS 1967; ROMAN personal communication), and display different interference patterns (STADLER 1959b), favor the view that different mechanisms are involved. The existence of at least two recombination mechanisms was proposed by ROMAN and JACOB (1958), following their observation in yeast that outside marker exchange among ultraviolet-induced intragenic recombinants declines with increasing doses of irradiation, i.e. that intragenic and intergenic recombinaton need not be obligatorily associated with one another. They suggested that intragenic recombination might occur during the period of DNA replication by a copying error whereas intergenic recombination might occur after replication by a breakage-reunion process. If it were possible to demonstrate that agents which induce recombination stimulate intragenic and intergenic events at different points in the cell cycle this would provide evidence in favor of their interpretation. However, recent studies in Ustilago maydis (HOLLIDAY 1965, 1966), indicate that both recombinational events are maximally induced during the period of DNA synthesis. The purpose of the following study was to inquire whether intragenic and intergenic recombination are maximally stimulated at the same or different points during the mitotic cell cycle of yeast. In the experiments reported below