PH SENSITIVITY OF SCHIZOSACCHAROMYCES-POMBE - EFFECT ON THE CELLULAR PHONOTYPE ASSOCIATED WITH LACZ GENE-EXPRESSION

We report on a series of experiments in Schizosaccharomyces pombe to d etect the blue-colour colony phenotype associated with expression of t he Escherichia coli lacZ gene. Increasing the pH in solid minimal medi um to optimize blue colony colour revealed a pH-sensitive phenotype in auxotrophic strains requiring uracil and leucine as external suppleme nts. This phenotype was observed among common S. pombe stock strains, 5-fluoroorotic acid (5-FOA)-selected strains, and random genetic segre gants. Growth of prototrophic S. pombe strains 972 and 975 or the aden ine auxotrophic strain NCYC 1860 were unaffected by an increase in ext ernal pH. Analysis of genetic segregants from three independent crosse s indicated that a single auxotrophic marker (ura4(-) or leu1-32) was sufficient for yeast cell-growth inhibition when the medium pH was inc reased above 6.6. In contrast, growth of a Saccharomyces cerevisiae st rain isogenic to AH22, requiring uracil, leucine and histidine, was un affected by changes in the pH of the medium. These observations sugges t that uptake of uracil and leucine into S. pombe cells is compromised by alterations in external pH. Our results have implications for dete ction of the lacZ gene-encoded bluecolour colony phenotype in S. pombe , which is optimized by growth in the presence of 5-bromo-4-chloro-3-i ndolyl-beta-D-galactoside (Xgal) at pH 7.0. We discuss the conditions under which this blue-colour phenotype can be routinely observed in S. pombe.