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 underlying 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 conditions). However, dsbs are rejoined when cells are kept either in non-growth or growth medium. One possibility to explain the enhanced survival of cells after post-irradiation treatment in non-growth medium might be an enhanced fidelity of dsb rejoining under non-growth relative to growth conditions. We have addressed this problem by using a plasmid-mediated assay. Into one of the two selectable plasmid markers a single dsb was introduced by a restriction enzyme. The cut plasmid was transfected into an appropriate yeast mutant. Transformants that had correctly rejoined the dsb were selected on the basis of restoration of the 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.