CHARACTERIZATION OF AGT1 ENCODING A GENERAL ALPHA-GLUCOSIDE TRANSPORTER FROM SACCHAROMYCES

Molecular genetic analysis is used to characterize the AGT1 gene encod ing an alpha-glucoside transporter. AGT1 is found in many Saccharomyce s cerevisiae laboratory strains and maps to a naturally occurring, par tially functional allele of the MAL1 locus. Agt1p is a highly hydropho bic, postulated integral membrane protein. It is 57% identical to Mal6 1p, 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 ca pable of transporting only maltose and turanose, while Agt1p transport s these two alpha-glucosides as well as several others including isoma ltose, alpha-methylglucoside, maltotriose, palatinose, trehalose and m elezitose. AGT1 expression is maltose inducible and induction is media ted 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 up stream of AGT1 shows no significant homology to the sequence immediate ly upstream of MAL61. The evolutionary origin of the MAL1 allele to wh ich AGT1 maps and the relationship of AGT1 to other alpha-glucoside fe rmentation genes is discussed.