STRUCTURE AND REGULATION OF MALTASE GENE EXPRESSION IN THE YEAST SACCHAROMYCES
HONG, SEUNG HWAN
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In the yeast Saccharomyces, the utilization of maltose as a carbon source has been shown to be dependent upon the presence of any one of the family of five unlinked MAL loci (MAL1-4 and MAL6). Each MAL locus contains two divergently transcribed structural genes, MALS, encoding maltase ((alpha)-glucosidase, EC 126.96.36.199) and MALT, encoding maltose permease. Their expression is regulated at the transcriptional level; they are coordinately induced by maltose in the presence of a functional, positively acting regulatory gene (MALR) and carbon catabolite repressed by glucose.;To elucidate more precisely the mechanism involved in the expression of the structural genes at MAL6 locus, I have determined the nucleotide sequence of a 2,937 base pair DNA fragment containing the entire MAL6S gene, part of the MAL6T gene and the intergenic region between them. The transcription initiation sites for both genes were determined by primer extension analysis. The results show that maltase is composed of 584 amino acid with a calculated M.W. of 68,107. The coding regions for the MAL6S and MAL6T genes are 884 bp apart and divergently transcribed from respective mRNA start sites that are separated 785 bp. Both genes show common features required for the transcription and the translation of yeast structural genes.;A series of nested deletions in the intergenic region were generated upstream of the MAL6S gene to analyze its mode of regulation in detail. The results showed that inducible expression by maltose was lost when the region between 344 and 380 bp upstream of the AUG translation initiation codon was deleted. This region contained both an inverted repeat structure as well as 4 copies of 7 bp direct repeat sequence (AAANTTT) in and neighboring the inverted repeat sequence. Also, this upstream site is shown to interact with a protein(s) dependent on a functional regulatory gene. There is stretch of a T-rich sequence between the TATA box and the upstream activation site which appears to be involved in the carbon catabolite repression of the MAL6S gene.
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