COREGULATION OF STARCH DEGRADATION AND DIMORPHISM IN THE YEAST SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae, the exemplar unicellular eukaryote, can only survive and proliferate in its natural habitats through constant adap tation within the constraints of a dynamic ecosystem. In every cell cy cle of S. cerevisiae, there is a short period in the G(1) phase of the cell cycle where ''sensing'' transpires; if a sufficient amount of fe rmentable sugars is available, the cells will initiate another round o f vegetative cell division. When fermentable sugars become limiting, t he yeast can execute the diauxic shift, where it reprograms its metabo lism to utilize nonfermentable carbon sources. S. cerevisiae can also initiate the developmental program of pseudohyphal formation and invas ive growth response, when essential nutrients become limiting. S. cere visiae shares this growth form-switching ability with important pathog ens such as the human pathogen, Candida albicans, and the corn smut pa thogen Ustilago maydis. The pseudohyphal growth response of S. cerevis iae has mainly been implicated as a means for the yeast to search for nutrients. An important observation made was that starch-degrading S. cerevisiae strains have the added ability to form pseudohyphae and gro w invasively into a starch-containing medium. More significantly, it w as also shown that the STA1-3 genes encoding three glucoamylase isozym es responsible for starch hydrolysis in S. cerevisiae are coregulated with a gene, MUC1, essential for pseudohyphal and invasive growth. At least two putative transcriptional activators, Mss10p and Mss11p, are involved in this regulation. The Muc1p is a putative integral membrane -bound protein similar to mammalian mucin-like proteins that have been implicated in the ability of cancer cells to invade other tissues. Th is provided us with an excellent example of integrative control betwee n nutrient sensing, signaling, and differential development.