Killing of yeast cells and induction of the cytoplasmic petite mutation by partial cell irradiation with an ultraviolet microbeam

Abstract

Individual cells of Saccharomyces cerevisiae (NCYC 239) were irradiated with a UV-microbeam to characterize the target for induction of the cytoplasmic petite mutation. The organisms were taken from synchronous cultures at a stage prior to migration of the nucleus into the bud, thus permitting irradiation of either the nucleated parental portion of the cell or the anucleate (but mitochondria-containing) bud. Whole cell irradiation was carried out for comparison. The LD 50 was 0.061 ergs/cell for whole cell irradiation, about 0.047 ergs/cell for nuclear irradiation and about twice that value for bud irradiation. The spontaneous petite frequency was 4/7108 (0.056 %). The overall induced frequency was 9/540 among nucleus-irradiated cells (significantly enhanced) and 2/832 among bud-irradiated cells (insignificantly enhanced). Thus induction of the mutation did not require direct irradiation of all the mitochondria in the cell and the target was apparently associated with the nucleus.