Cyclic X-Ray Resistance to Lethal and Nonlethal Damage in Saccharomyces cerevisiae

Log-phase budding cells of two genetically marked diploid strains of S. cerevisiae are more resistant than nonbudding cells to x-ray-induced lethality and postirradiation lag, alterations in colony morphology associated with mitotic recombination, respiratory deficiency, and other nonlethal changes. The resistance of budding cells obtained from a nonbudding, small-cell population that had been stored in water at 6 to 8$sup 0$C overnight was intermediate between that of nonbudding and log-phase budding cells. Post-irradiation shifts in growth medium elicited repair of lethal and nonlethal damage in nonbudding and sensitive budding cells. Repair of lethal damage was greater in sensitive budding cells, and repair of nonlethal damage greater in nonbudding cells. Effects of budding and the medium shift in reducing lethality in sensitive budding cells were additive, while effects of budding and the shift on nonlethal damage were nonadditive. Normal budding cell resistance to x-ray lethality seems, therefore, to resemble the process elicited by medium shifts, unlike resistance to nonlethal damage. Only induced repair of lethal damage was actinomycin D sensitive. These and other observations, as well as several published but previously uncollated reports, suggest that budding cell resistance to x-ray lethality depends in part on efficient repair of chromosome breaks, and ismore » an adaptation for repairing damage in replicating DNA. (auth)« less