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 wit h a UV dose of 25 J/m2 substantially increases the repairability of da mage 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 MATa lpha and HMLalpha loci, which are, respectively, transcriptionally act ive and inactive in a 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 occurre d if cycloheximide was present only after the final UV irradiation. Th is indicated that the first result is not due to cycloheximide merely blocking the synthesis of repair enzymes associated with a hypothetica l rapid turnover of such molecules. The enhanced repairability is not the result of changes in chromatin accessibility without protein synth esis, merely caused by the repair of the damage induced by the prior i rradiation. The data clearly show that a UV-inducible removal of pyrim idine dimers has occurred which involves the synthesis of new proteins . The genes known to possess inducible promoters, and which are involv ed in excision are RAD2, RAD7, RAD16 and RAD23. Studies with the rad7 and rad16 mutants which are defective in the ability to repair HMLalph a and proficient in the repair of MATalpha showed that in rad7, preirr adiation enhanced the repair at MATalpha, whereas in rad16 this increa sed repair of MATalpha was absent. The preirradiation did not modify t he inability to repair HMLalpha in either strain. Thus RAD16 has a rol e in this inducible repair. Inducible repair is also absent in a rad2 strain which cannot repair MATa or HMLalpha after a single UV dose.