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

Abstract

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 [¹⁴C]maltose, and by its growth on media containing 10% but not 2% maltose. Since the K_m of maltose uptake is altered 10-fold in this mutant and the V_max 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.