New insights into galactose metabolism by Schizosaccharomyces pombe: Isolation and characterization of a galactose-assimilating mutant

The fission yeast Schizosaccharomyces pombe cannot use galactose as a carbon or energy source, and little is known about galactose metabolism in this species. Here we report isolation of a galactose-assimilating mutant that grows on a medium containing galactose as a sole carbon source through use of a proofreading-deficient DNA polymerase δ variant encoded by cdc6-1. Based on comparative analysis of gene expression profiles in the wild-type and the mutant (FG2-8), we found that SPBPB2B2.10c (gal7⁺), SPBPB2B2.12c (gal10⁺) and SPBPB2B2.13 (gal1⁺), homologous to Saccharomyces cerevisiae GAL7, GAL10 and GAL1, respectively, and SPBPB2B2.08, SPBPB2B2.09c, and SPBPB2B2.11 that localize close to the gal genes, were highly expressed and dramatically induced by addition of galactose. The gal7δ strain, carrying an integrated ura4⁺ marker at the gal7⁺ locus, grew on 5-fluoroorotic acid (5-FOA)-containing medium. In contrast, the FG2-8 gal7δ strain could not grow on 5-FOA medium. In addition, expression of gal7⁺, SPBPB2B2.13, gal10⁺ and gal1⁺ genes increased in the wild-type strain when carried on a vector, and these transformants grew on galactose medium. We suggest that gal7⁺, gal10⁺, and gal1⁺ are localized close to a chromosomal terminal repressed by gene silencing in S. pombe. In contrast, gene silencing was defective in the FG2-8 strain making galactose assimilation possible.