Characterization of AGT1 encoding a general α-glucoside transporter from Saccharomyces

Molecular genetic analysis is used to characterize the AGT1 gene encoding an α-glucoside transporter. AGT1 is found in many Saccharomyces cerevisiae laboratory strains and maps to a naturally occurring, partially functional allele of the MAL1 locus. Agt1p is a highly hydrophobic, postulated integral membrane protein. It is 57% identical to Mal61p, the maltose permease encoded at MAL6 , and is also a member of the 12 transmembrane domain superfamily of sugar transporters. Like Mal61p, Agt1p is a high-affinity, maltose/proton symporter, but Mal61p is capable of transporting only maltose and turanose, while Agt1p transports these two α-glucosides as well as several others including isomaltose, α-methylglucoside, maltotriose, palatinose, trehalose and melezitose. AGT1 expression is maltose inducible and induction is mediated by the Mal-activator. The sequence of the upstream region of AGT1 is identical to that of the maltose-inducible MAL61 gene over a 469 bp region containing the UAS_MAL but the 315 bp sequence immediately upstream of AGT1 shows no significant homology to the sequence immediately upstream of MAL61 . The evolutionary origin of the MAL1 allele to which AGT1 maps and the relationship of AGT1 to other α-glucoside fermentation genes is discussed.