Preferential repair in yeast after induction of interstrand cross-links by 8-methoxypsoralen plus UVA

Abstract

The gene-specific induction and removal of 8-methoxypsoralen (8-MOP) plus UVA-induced interstrand cross-links (ICL) was studied using the genetic system MATα and HMLα in Saccharomyces cerevisiae. We first examined events in a SIRα haploid strain (K107) in which these identical sequences are respectively transcriptionally active ( MATα) and inactive ( HMLα). Induction and repair of ICL was then studied in a sir3 mutant in which HMLα is derepressed so that MATα and HMLα are both transcriptionally active. In the SIR strain at low levels of damage, no preferential repair of ICL occurred for MATα versus HMLα, whereas at high levels of ICL, those at MATα were clearly repaired more rapidly than those at HMLα. Similar experiments with the sir3 mutant revealed that the repair of ICL from both MATα and HMLα loci proceeded at the same rate at both low and high levels of damage. These data suggest that 8-MOP plus UVA-induced ICL are subject to preferential repair in yeast and that for the MATα and HMLα loci, this is dependent on their transcriptional status (i.e., the transcribed sequences are repaired more rapidly than the identical non-transcribed ones).