MOLECULAR MECHANISM OF POTENTIALLY LETHAL DAMAGE REPAIR .1. ENHANCED FIDELITY OF DNA DOUBLE-STRAND BREAK REJOINING UNDER CONDITIONS ALLOWING POTENTIALLY LETHAL DAMAGE REPAIR

This study contributes to the elucidation of the molecular mechanism u nderlying potentially lethal damage (PLD) repair. Repair of DNA double -strand breaks (dsbs) is involved in PLD repair in yeast, i.e. in the enhanced survival of cells due to post-irradiation treatment under non -growth conditions before plating cells on nutrient agar (growth condi tions). However, dsbs are rejoined when cells are kept either in non-g rowth or growth medium. One possibility to explain the enhanced surviv al of cells after post-irradiation treatment in non-growth medium migh t be an enhanced fidelity of dsb rejoining under non-growth relative t o growth conditions. We have addressed this problem by using a plasmid -mediated assay. Into one of the two selectable plasmid markers a sing le dsb was introduced by a restriction enzyme. The cut plasmid was tra nsfected into an appropriate yeast mutant. Transformants that had corr ectly rejoined the dsb were selected on the basis of restoration of th e function of the cut gene. The yeast mutant was allowed to rejoin the cut plasmid under either non-growth or growth conditions. The results show that the fidelity of dsb rejoining is higher in cells kept under non-growth relative to growth conditions.