PREFERENTIAL INCISION OF INTERSTRAND CROSS-LINKS INDUCED BY 8-METHOXYPSORALEN PLUS UVA IN YEAST DURING THE CELL-CYCLE

Interstrand crosslink (ICL) induction by 8-methoxypsoralen plus WA and the incision step of the repair have been investigated during the mit otic cell cycle of haploid Saccharomyces cerevisiae. Cells were synchr onised by elutriation and events were examined at the level of the MAT alpha and the HML alpha loci in a SlR strain. The DNA sequence of the se two loci is identical, but the MAT alpha locus may be replicated ea rlier in S phase and is transcriptionally active while the HML alpha l ocus may be replicated later in S phase and is transcriptionally inact ive because of Sir repression that creates a heterochromatin-like stru cture at this locus. ICL were induced to similar extents in both loci during the stages of the cell cycle examined, and these levels were id entical to those reported for asynchronous cultures. Preferential inci sions occurred for ICL in the MAT alpha locus compared to those in the HML alpha locus, independently of the cell cycle phase studied. The l evels of incision were comparable for events in the early G1 phase (eG 1), late G1 phase (1G1), 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 through out the cell cycle and, surprisingly, occurs equally well in G1. Here the opportunities for recombination to further process the incised dam aged are substantially limited compared to those in the S and G2 phase s.