Isolation and characterization of respiratory deficient mutants of Schizosaccharomyces pombe

Schizosaccharomyces pombe was originally classified as "petite-negative" or "petite-variable" because respiratory deficient strains were difficult to isolate from this species of yeast using mutagens such as acriflavine or ethidium bromide which have produced high numbers of petites in Saccharomyces Cereviaiae. This study describes the first successful mutation treatment that resulted in the isolation of high numbers of petite-type colonies. A 60% mutation rate was obtained by using (a) low concentrations of nitrosoguanidine and (b) treating growing cells in a complete medium rather than resting cells suspended in buffer. While nuclear petites have recently been isolated for this strain of Schizosaccharomyces pombe, this report is the first to describe a cytoplasmic type of a respiratory deficient petite mutant. A preliminary physiological characterization of these petites is described, also. Growth curves, as well as respiration rate measurements were carried out on two petite strains and compared to the wild type strain. The data obtained was similar to the physiological studies described for the respiratory deficient mutants of Saccharomyces cerevisiae. Almost all of the small colonies resulting from the nitrosoguanidine treatment failed to grow on glycerol (indicating respiratory deficiency) and have negative triphenyltetrazolium chloride reactions. Growth studies showed that the two petite mutant strains (1001 and 1008) had an aerobic growth rate (measured turbidimetrically) and a cellular dry weight one-fourth the value of wild type cells. Also, the petite strains were shown to grow equally well anaerobically as compared to aerobically. Genetic cross analysis showed that petite strain 1001 was a neutral petite and petite strain 1008 was a suppressive petite.