Preferential incision of interstrand crosslinks induced by 8-methoxypsoralen plus UVA in yeast during the cell cycle.

Interstrand crosslink (ICL) induction by 8-methoxypsoralen plus UVA and the incision step of the repair have been investigated during the mitotic cell cycle of haploid Saccharomyces cerevisiae. Cells were synchronised by elutriation and events were examined at the level of the MAT alpha and the HML alpha loci in a SIR strain. The DNA sequence of these two loci is identical, but the MAT alpha locus may be replicated earlier in S phase and is transcriptionally active while the HML alpha locus may be replicated later in S phase and is transcriptionally inactive because of Sir repression that creates a heterochromatin-like structure at this locus. ICL were induced to similar extents in both loci during the stages of the cell cycle examined, and these levels were identical to those reported for asynchronous cultures. Preferential incisions occurred for ICL in the MAT alpha locus compared to those in the HML alpha locus, independently of the cell cycle phase studied. The levels of incision were comparable for events in the early G1 phase (eG1), late G1 phase (lG1), early S phase (eS), middle S phase (mS), late S phase (lS) or G2 phase (G2). Thus the preferential incision of ICL observed previously in asynchronous cell culture is maintained throughout the cell cycle and, surprisingly, occurs equally well in G1. Here the opportunities for recombination to further process the incised damaged are substantially limited compared to those in the S and G2 phases.