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. Can function independently of the complex. D-glucose 6-phosphate + UDP-alpha-D-glucose = alpha,alpha-trehalose 6-phosphate + H(+) + UDP Carbohydrate biosynthesis. Trehalose synthase/phosphatase complex contains three or four polypeptides of 56 kDa (TPS1), 102 kDa (TPS2), 115 kDa (TPS3) and 123 kDa (TSL1). Belongs to the glycosyltransferase 20 family.