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 (PubMed:21931609). The disaccharide trehalose serves as a storage carbohydrate that is mobilized during spore germination (PubMed:21931609). D-glucose 6-phosphate + UDP-alpha-D-glucose = alpha,alpha-trehalose 6-phosphate + H(+) + UDP Carbohydrate biosynthesis. Belongs to the glycosyltransferase 20 family.