MITOTIC RECOMBINATION AND LOCALIZED DNA DOUBLE-STRAND BREAKS ARE INDUCED AFTER 8-METHOXYPSORALEN AND UVA IRRADIATION IN SACCHAROMYCES-CEREVISIAE

Mitotic recombination within the ARG4 gene of Saccharomyces cerevisiae was analysed after treatment of cells with the recombinogenic agent 8 -methoxypsoralen (8-MOP) plus UVA. The appearance of DNA double-strand breaks (DSBs) in the ARG4 region during post-treatment incubation was also tested. The results obtained after 8-MOP plus WA treatment indic ate that in mitotic cells: (1) recombination at the ARG4 locus is incr eased 30-500 fold per survivor depending on the strains and the doses employed, (2) the increase of recombination results essentially from g ene conversion events which involve the RV site located in the 5' regi on of the ARG4 gene twice as often as the Bgl site at the 3' end, (3) depending on 8-MOP/UVA dose, ectopic gene conversion is associated wit h reciprocal translocation, (4) DSBs occur preferentially in the ARG 5 ' region during post-treatment incubation, as well as in other interge nic regions containing both promoters or/and terminators of transcript ion, and (5) changes in sequence content in the 5' region of ARG4, whi ch influences positions and frequencies of DSBs formed during repair, are correlated with a modification of the local chromatin structure.