Synthase catalytic subunit of the trehalose synthase complex that catalyzes the production of trehalose from glucose-6-phosphate and UDP-alpha-D-glucose in a two step process. D-glucose 6-phosphate + UDP-alpha-D-glucose = alpha,alpha-trehalose 6-phosphate + H(+) + UDP Carbohydrate biosynthesis. Belongs to the glycosyltransferase 20 family.