Transcriptional control of glucoamylase synthesis in vegetatively growing and sporulating Saccharomyces species

Abstract

Three unlinked, homologous genes, STA1, STA2, and STA3, encode the extracellular glycosylated glucoamylase isozymes I, II, and III, respectively, in Saccharomyces species. S. cerevisiae, which is sta0 (absence of functional STA genes in haploids), does carry a glucoamylase gene, delta sta, expressed only during sporulation (W. J. Colonna and P. T. Magee, J. Bacteriol. 134:844-853, 1978; I. Yamashita and S. Fukui, Mol. Cell. Biol. 5:3069-3073, 1985). In this study we examined some of the physiological and genetic factors that affect glucoamylase expression. It was found that STA2 strains grown in synthetic medium produce glucoamylase only in the presence of either Maltrin M365 (a mixture of maltooligosaccharides) or starch. Maximal levels of glucoamylase activity were found in cells grown in rich medium supplemented with glycerol plus ethanol, starch, or Maltrin. When various sugars served as carbon sources they all supported glucoamylase synthesis, although at reduced levels. In any given growth medium glucoamylase isozyme II synthesis was modulated by functionality of the mitochondria. Synthesis of glucoamylase is continuous throughout the growth phases, with maximal secretion taking place in the early stationary phase. In the various regimens, the differences in enzyme accumulation are accounted for by differences in the levels of glucoamylase mRNA. Both glucoamylase mRNA and enzyme activity were drastically and coordinately inhibited in MATa/MAT alpha diploids and by the presence of the regulatory gene STA10. Both effects were partially overcome when the STA2 gene was present on a multicopy plasmid. The STA2 mRNA and glucoamylase were coinduced in sporulating STA2/STA2 diploids. A smaller, coinduced RNA species was also detected by Northern blotting with a STA2 probe. The same mRNA species was detected in sporulating sta0 diploids and is likely to encode the sporulation-specific glucoamylase.