ALKALINE SUCROSE SEDIMENTATION STUDIES OF MMS-INDUCED DNA SINGLE-STRAND BREAKAGE AND REJOINING IN WILD-TYPE AND IN UV-SENSITIVE MUTANTS OF SACCHAROMYCES-CEREVISIAE

MMS[methyl methanesulfonate]-induced DNA single-strand breakage and rejoining was studied in the RAD strain and in rad6 and rad21 mutants, both very sensitive to this treatment as compared with the wild type. Alkaline sucrose gradient centrifugation showed that MMS treatment reduced the MW of DNA in the RAD strain and in rad6 and rad21 mutants to the same extent. Post-incubation for 4 h in synthetic growth medium after treatment with a dose of 0.4% MMS which reduced cell survival of RAD, rad21 and rad6 to 50, 20 and less than 0.01%, respectively, resulted in a significant increase in the molecular weight of DNA in the wild type, but in only slight increase in mutant strains. When the strains were exposed to a lower dose of MMS (0.04%) which led to 100% survival of RAD and 50 and 20% survival of rad21 and rad6, respectively, wild-type DNA sedimented to the position of control DNA, while in both mutants the increase in MW of DNA was less pronounced.