REGULATION OF BASAL AND INDUCED LEVELS OF MEL1 EXPRESSION BY CARBON SOURCE AND BY THE GAL4, GAL80, AND SNF1 REGULATORY GENES IN SACCHAROMYCES CEREVISIAE (GALACTOSE, YEAST, GENETICS).
- The galactose/melibiose system in Saccharomyces cerevisiae is regulated by carbon source and by two major regulatory genes, GAL4 (positive) and GAL80 (negative). The coordinate expression of the target genes is induced by galactose or melibiose and is catabolite-repressed by glucose. Unlike the expression of the galactose-metabolizing enzymes which is dependent on galactose-induction, there is a low-level of (alpha)-galactosidase activity (MEL1) under noninducing, nonrepressing conditions. Northern analyses using cloned MEL1 sequences indicates that both the basal and induced levels of MEL1 expression are transcriptionally regulated and due to a single 1.6 kb transcript. Furthermore, both basal and induced expressions are dependent on a functional positive regulator (GAL4). In contrast, an allele of the negative regulatory gene (GAL80('s) -100) which prevents induction of the target genes does not affect the basal level of MEL1 expression.
Transcriptional expression of MEL1 is glucose-repressed in wild-type cells. However, cells carrying the moderately high-copy GAL4-containing plasmid, SJ3, or the disrupted genomic gal80-D allele have been shown to express MEL1 under glucose-repressing conditions. Cells carrying the pleiotropic snf1-31 allele are unable to derepress any genes which are catabolite repressed by glucose. Therefore, the affect on the snf1-31 block by mutant galactose/melibiose regulatory alleles, which allow constitutivity, was studied. Under nonrepressing/noninducing, repressing or inducing growth conditions, the level of (alpha)-galactosidase activity in snf1-31 cells containing either the gal80-D allele of the SJ3 plasmid was the same as the basal levels found in wild-type cells grown under nonrepressing/noninducing conditions. Only the combination of the gal80-D and SJ3 plasmid was sufficient to overcome the snf1-31 block resulting in levels of activity comparable to wild-type induced levels. In contrast, the combination of gal80-D and SJ3 in SNF1 cells resulted in levels seven to eight times higher. The results of these studies suggest that: (1) SNF1 positively regulates the galactose/melibiose system; (2) there are additional limiting factors in cells carrying either only the gal80-D allele or the SJ3 plasmid preventing the higher expression of MEL1 found in cells carrying both gal80-D and SJ3; and (3) snf1-31 cells do not respond to carbon source regulation.
- Dissertation Note:
- Ph.D. The Pennsylvania State University 1985.
- Source: Dissertation Abstracts International, Volume: 46-06, Section: B, page: 1809.
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