Inducible removal of UV-induced pyrimidine dimers from transcriptionally active and inactive genes of Saccharomyces cerevisiae.

The prior UV irradiation of alpha haploid Saccharomyces cerevisiae with a UV dose of 25 J/m2 substantially increases the repairability of damage subsequently induced by a UV dose of 70 J/m2 given 1 h after the first irradiation. This enhancement of repair is seen at both the MAT alpha and HML alpha loci, which are, respectively, transcriptionally active and inactive in alpha haploid cells. The presence in the medium of the protein synthesis inhibitor, cycloheximide in the period between the two irradiations eliminated this effect. Enhanced repair still occurred if cycloheximide was present only after the final UV irradiation. This indicated that the first result is not due to cycloheximide merely blocking the synthesis of repair enzymes associated with a hypothetical rapid turnover of such molecules. The enhanced repairability is not the result of changes in chromatin accessibility without protein synthesis, merely caused by the repair of the damage induced by the prior irradiation. The data clearly show that a UV-inducible removal of pyrimidine dimers has occurred which involves the synthesis of new proteins. The genes known to possess inducible promoters, and which are involved in excision are RAD2, RAD7, RAD16 and RAD23. Studies with the rad7 and rad16 mutants which are defective in the ability to repair HML alpha and proficient in the repair of MAT alpha showed that in rad7, preirradiation enhanced the repair at MAT alpha, whereas in rad16 this increased repair of MAT alpha was absent. The preirradiation did not modify the inability to repair HML alpha in either strain. Thus RAD16 has a role in this inducible repair.(ABSTRACT TRUNCATED AT 250 WORDS)