THE FATE OF MITOCHONDRIAL LOCI IN RHO MINUS MUTANTS INDUCED BY ULTRAVIOLET IRRADIATION OF SACCHAROMYCES CEREVISIAE: EFFECTS OF DIFFERENT POST-IRRADIATION TREATMENTS

Three main features regarding the loss of mitochondrial genetic markers among rhomutants induced by ultraviolet irradiation are reported: (a) the frequency of loss of six loci examined increases with UV dose; (b) preferential loss of one region of the mitochondrial genome observed in spontaneous rho-mutants is enhanced by UV; and (c) the loss of each marker results from large deletions. Marker loss in rho-mutants was also investigated under conditions that modulate rho-induction. Liquid holding of irradiated exponential or stationary phase cells, as well as a split-dose regime applied to stationary phase cells, results in rho-mutants in which the loss of markers is correlated with rho-induction: the more sensitive the cells are to rho-induction, the more frequent are the marker losses among rho-clones derived from these cells. This correlation is not found in exponential-phase cells submitted to a split-dose treatment, suggesting that a different mechanism is involved in the latter case. It is known that UV-induced pyrimidine dimers are not excised in a controlled manner in mitochondrial DNA. How-ever, our studies indicate that an accurate repair mechanism (of the recombinational type ?) can lead to the restoration of mitochondrial genetic information in growing cells.