Glycogen, a branched polymer of glucose, is a storage molecule whose accumulation is under rigorous nutritional control in many cells (8). In S. cerevisiae, glycogen biosynthesis involves three processes: nucleation, elongation, and ramification, or branching (9). GLG1 and GLG2 encode self-glucosylating glycogenin glucosyltransferases (EC:2.4.1.186) involved in glycogen nucleation (8). Both Glg1p and Glg2p are able to use UDP-glucose to produce a short alpha (1,4)-glucosyl chain covalently attached to an internal tyrosine residue (10). Glycogen synthase (EC:2.4.1.11, Gsy1p and Gsy2p) is then able to extend the linear alpha (1,4)-chains of glycogen by catalyzing the formation of alpha (1,4)-glucosidic bonds from UDP-glucose at the non-reducing ends (11). Branches can be added into the glycogen molecule by Glc3p, the glycogen branching enzyme (EC:2.4.1.18) in S. cerevisiae (12). No enzyme that releases the glycogen chain from Glg1p or Glg2p has been identified (9).

GSY1 encodes approximately 10-15% of the glycogen synthase activity in S. cerevisiae (11). Although Gsy1p appears less abundant than Gsy2p, Gsy1p levels may provide important fine tuning of glycogen synthase levels under some conditions, as suggested by the complex nature of its promoter (5). The GSY1 promoter contains multiple stress response elements (STREs), also found in the GSY2 promoter, but also includes multiple negative elements which are not found in the GSY2 promoter, such as a Mig1p binding site, a Rox1p binding site, and a novel negative cis element (5). GSY1 is induced upon glucose limitation, nitrogen limitation, stationary phase, osmotic shock, and heat shock (5). GSY1 has similarity to human glycogen synthase genes GYS1 and GYS2 (13).

Last updated: 2005-08-30