The lethality and mutagenicity of ultraviolet irradiation (UV) for both procaryotic and eucaryotic cells are attributable principally t o the induction of pyrimidine diniers in deoxyribonucleic acid (DNA) and the activities of enzymatic system for reparing that damage (KONDO 1974). I n addition, however, UV induces a variety of functional and structural disturbances in the membrane systems of eucaryotic cells whose possible contributions to cellular inactivation are as yet poorly understood ( BAHAGUNA and CHATURVEDI 1974, BEYER 1959, HILDEBRAND and STIEVE 1972, JOHNSON 1968, LIEBERMANN 1967). SARACHER and HIGGINS (1972) reported that ergosterol promotes recovery of Candida albicans from the lethal, but not mutagenic, action of UV. The phenomenon occurs only with cells that are not budding at the time of irradiation. The plausible assumption that ergosterol acts a t a membrane site was supported by the demonstration that the sterol's effect is much diminished in polyene resistant, membrane mutants of C. albicans (SARACHEK and PETTRIESS 1974). The present report compares responses to ergosterol by cells treated with UV or one of two chemical mutagens, the alkylating agent, ethyl methanesulfonate (EMS) and the oxidizing agent, nitrous acid (NA). The comparisons were made as one approach to determining whether ergosterol (i) remediates specific UV-induced lesions in the membrane, or (ii) accentuates a membrane activity involved in DNA repair, generally.
The test organisms used included a wild type strain of C. albicans, 792, a histidine auxotroph, WB-2, derived through ultraviolet irradiation from strain 792, and a methionine auxotroph, WD-2, derived similarly from the wild type strain, 526. The minimal agar medium of BUSBEE and SARA-CHER (1969) was used for all studies. When working with an auxotroph, the medium was supplemented with the required amino acid at levels of 40 yg/ml for survival determinations and 0.4 yg/ml for miltation determinations; where indicated, ergosterol was added t o a level of 20 yg/ml. Yeasts were grown for experimental use a t 25 "C 80 as to provide either stationary phase populations consisting of ca. 95% single, non-budding cells, or exponential phase populations in which m. 90% of cells possess a single bud. The procedures for obtaining such populations and for performing UV experiments were those of SARACHER and HIGGINS (1972) ; cells were UV-irradiated a t an incident dose rate of 16 ergs/mm'/sec. Cells were exposed to a 3% (V/V) solution of EMS for intervals up to 240 min, or t o NA solutions ranging from 0.01 M to 0.06 M for a 20 min period. EMS and NA studies were conducted as described by SARACHEK and BISE (1976).