Molecular expression and regulation of the galactose pathway genes in Saccharomyces cerevisiae. Distinct messenger RNAs specified by the Gali and Gal7 genes in the Gal7-Gal10-Gal1 cluster.

Three of the four inducible enzymes required for galactose utilization by Saccharomyces cerevisiae are specified by three genes (GAL7, GALlO, and GALI) comprising a genetically tight cluster near the centromere of chromosome II. The inducibility of all four enzyme activities is coordinately mediated by the action of a positive regulatory gene, GAL4, which is not genetically linked to the structural genes. We have developed an assay for functional GALl-specified galactokinase mRNA based on cell-free translation and immunoprecipitation in order to study the expression and regulation of gene expression in this system. By the use of a gal4 amber mutant and an amber suppressing (SUP-a) revertant in conjunction with the galactokinase mRNA assay we provide evidence here that the GAL4 gene is required for the galactose-specific appearance of functional GAL1 mRNA. Since the GAL4 gene was previously shown to be required for the galactose-specific induction of the GAL7 encoded uridyl transferase mRNA, the present results now establish that the regulatory coordination of galactose gene activities exerted by the GAL4 gene product is mediated at the mRNA level. With the combined use of the GAL1 and GAL7 mRNA assays we have initiated a study of the translational organization of the galactose gene cluster. A fractionation analysis of the in vivo polysomal mRNAs specified by the GAL1 and GAL7 genes was carried out using continuous elution preparative polyacrylamide gel electrophoresis. The mRNA coding for galactokinase (GALl) migrates more slowly than the mRNA coding for uridyl transferase (GAL7). Based on the distinctly different migration patterns shown by these mRNAs a model of translational organization involving a polycistronic mRNA for the GAL?-GALlO-GAL1 gene cluster is ruled out.