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

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