Differential regulation of STA genes of Saccharomyces cerevisiae

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 adaptation within the constraints of a dynamic ecosystem. In every cell cycle of S. cerevisiae, there is a short period in the G1 phase of the cell cycle where "sensing" transpires; if a sufficient amount of fermentable sugars is available, the cells will initiate another round of vegetative cell division. When fermentable sugars become limiting, the yeast can execute the diauxic shift, where it reprograms its metabolism to utilize nonfermentable carbon sources. S. cerevisiae can also initiate the developmental program of pseudohyphal formation and invasive growth response, when essential nutrients become limiting. S. cerevisiae shares this growth form-switching ability with important pathogens such as the human pathogen, Candida albicans, and the corn smut pathogen Ustilago maydis. The pseudohyphal growth response of S. cerevisiae 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 grow invasively into a starch-containing medium. More significantly, it was also shown that the STA1-3 genes encoding three glucoamylase isozymes 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. This provided us with an excellent example of integrative control between nutrient sensing, signaling, and differential development.

The S1, S2 and SGA1 ancestral genes for the STA glucoamylase genes all map to chromosome IX in Saccharomyces cerevisiae

The polymorphic extracellular glucoamylase-encoding genes STA1 (chr. IV), STA2 (chr. II) and STA3 (chr. XIV), from Saccharomyces cerevisiae var. diastaticus probably evolved by genomic rearrangement of DNA regions (S1, S2 and SGA1) present in S. cerevisiae, and subsequent translocation to unlinked regions of chromosomal regions. S1, encoding a homologue to the threonine/serine-rich domain of STA glucoamylases (GAI-III), mapped to the right arm of chromosome IX. S2, encoding the hydrophobic leader peptide of GAI-III), was also mapped on the right arm of chromosome IX, next to S1, close to DAL81. The SGA1 sporulation-specific, intracellular glucoamylase-encoding gene is located on the left arm of chromosome IX, 32 kb proximal of HIS5.

Multiple positive and negative cis-acting elements of the STA2 gene regulate glucoamylase synthesis in Saccharomyces cerevisiae

Expression of the glucoamylase-encoding gene (STA2) in Saccharomyces cerevisiae was previously shown to be regulated transcriptionally by both positive and negative factors. The objective of this work was to identify the cis-acting elements responsible for STA2 transcriptional activation as well as the transcriptional repressor effects of STA10 and MATa/MAT alpha. We identified two upstream activation regions (UAS). Three repressor regions responsive to STA10-mediated repression were identified, as well as two regions for down-regulation of STA2 expression. MATa/MAT alpha repression appears to effect STA2 expression either downstream from the translational start site or, indirectly, since no functional a1/alpha 2-responsive sequence was identified in the promoter region.

A genetic analysis of an alpha-amylase super-secretor in yeast; implications for the regulatory pathway

Extracellular glucoamylase activity was increased by a gene, which is present in super-secretor, but absent in low-secretor, strains of the yeast Saccharomyces cerevisiae. Genetic data indicated that this super-secretor gene is linked to the STA3 structural gene for glucoamylase. This gene appears to act specifically since it increased the secretion of glucoamylase but not of other secreted enzymes like acid phosphatase and invertase.

Primary structure and regulation of a glucoamylase-encoding gene (STA2) in Saccharomyces diastaticus

We have determined the complete nucleotide (nt) sequence of a 5070-bp DNA fragment containing a glucoamylase-encoding gene (STA2) from Saccharomyces diastaticus. The 5' transcription start points for STA1, STA2 and STA3 were determined by primer extension of their respective mRNAs using reverse transcriptase. The sequence data show one major open reading frame (ORF) of 767 amino acids encoding GAII with a calculated Mr of 82,514. The 5' region in the ORF contains two ATG sequences within 30 nt of each other. The upstream region of STA2 was amplified by the polymerase chain reaction (PCR) and fused to the Escherichia coli lacZ gene. Some of the PCR products contained mutations in ATG1 and/or ATG2. Results indicated that both ATG1 and ATG2 encode functional translation start codons, but ATG2 was shown to encode the stronger initiator. The upstream region of STA2 contains a canonical sequence that is homologous to known sites of repression by the MATa/MAT alpha-encoded repressor. Also, consensus RAP1 (Repressor-Activator Protein 1)-binding sites are located in the 5' upstream region and within the coding region of STA2.

The glucoamylase multigene family in Saccharomyces cerevisiae var. diastaticus: an overview

Saccharomyces cerevisiae has been used widely both as a model system for unraveling the biochemical, genetic, and molecular details of gene expression and the secretion process, and as a host for the production of heterologous proteins of biotechnological interest. The potential of starch as a renewable biological resource has stimulated research into amylolytic enzymes and the broadening of the substrate range of S. cerevisiae. The enzymatic hydrolysis of starch, consisting of linear (amylose) and branched glucose polymers (amylopectin), is catalyzed by alpha- and beta-amylases, glucoamylases, and debranching enzymes, e.g., pullulanases. Starch utilization in the yeast S. cerevisiae var. diastaticus depends on the expression of the three unlinked genes, STA1 (chr. IV), STA2 (chr. II), and STA3 (chr. XIV), each encoding one of the extracellular glycosylated glucoamylases isozymes GAI, GAII, or GAIII, respectively. The restriction endonuclease maps of STA1, STA2, and STA3 are identical. These genes are absent in S. cerevisiae, but a related gene, SGA1, encoding an intracellular, sporulation-specific glucoamylase (SGA), is present. SGA1 is homologous to the middle and 3' regions of the STA genes, but lacks a 5' sequence that encodes the domain for secretion of the extracellular glucoamylases. The STA genes are positively regulated by the presence of three GAM genes. In addition to positive regulation, the STA genes are regulated negatively at three levels. Whereas strains of S. diastaticus are capable of expressing the STA genes, most strains of S. cerevisiae contain STA10, whose presence represses the expression of the STA genes in an undefined manner. The STA genes are also repressed in diploid cells, presumably by the MATa/MAT alpha-encoded repressor. STA gene expression is reduced in liquid synthetic media, it is carbon catabolite repressed by glucose, and is inhibited in petite mutants.