Isolation and characterization of a maltose transport mutant in the yeast Saccharomyces cerevisiae.

Yeast strains carrying a functional MAL locus are inducible for the co-ordinate synthesis of both maltase and maltose permease when grown in the presence of maltose. Whether the maltose permease is encoded by a gene at the MAL loci has remained unclear due to the lack of mutants in this function. To isolate mutants defective in maltose transport, a positive selection strategy was employed in which a number of Mal(-) mutants were obtained. Among these one Mal- mutant was isolated which had normal levels of wild-type maltase in cell free extracts. This isolate, designated MGT1, has a defect in maltose transport (malT1), detected by its markedly lower uptake of [(14)C]maltose, and by its growth on media containing 10% but not 2% maltose. Since the Km of maltose uptake is altered 10-fold in this mutant and the Vmax remains unchanged, it is suggested that the mutation alters the structure of the maltose permease involved in transport of the disaccharide into the cell rather than its regulation.A genetic analysis of the malT1 mutation shows that it is in a gene allelic to one at the MAL1 locus. Transformation of this mutant to the Mal(+) phenotype using a chimeric yeast/E. coli shuttle plasmid containing a subcloned fragment of the MAL6 locus suggests that the presence of a functional analogue of the gene encoding the maltose transport function is an integral part of the MAL6 locus as well.