Self-glucosylating initiator of glycogen synthesis. Catalyzes the formation of a short alpha (1,4)-glucosyl chain covalently attached via a glucose 1-O-tyrosyl linkage to internal tyrosine residues. These chains act as primers for the elongation reaction catalyzed by glycogen synthase. Capable of transferring glucosyl residues to unbound acceptors such as free oligoglucans or oligoglucan derivatives (By similarity). L-tyrosyl-[glycogenin] + UDP-alpha-D-glucose = alpha-D-glucosyl-L-tyrosyl-[glycogenin] + H(+) + UDP [1,4-alpha-D-glucosyl](n)-L-tyrosyl-[glycogenin] + UDP-alpha-D-glucose = [1,4-alpha-D-glucosyl](n+1)-L-tyrosyl-[glycogenin] + H(+) + UDP Induced during the diauxic transition. Belongs to the glycosyltransferase 8 family. Glycogenin subfamily.