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 Activated by fructose 6-phosphate. Inorganic phosphate inhibits the synthase activity in the complex, but activates the synthase activity in the free monomeric form. Carbohydrate biosynthesis. The trehalose synthase complex is composed of the two catalytic subunits TPS1 and TPS2 and at least one of the two regulatory subunits TPS3 or TSL1. Either partial repression by glucose or induction by galactose. Induced by heat shock. Present with 10900 molecules/cell in log phase SD medium. Belongs to the glycosyltransferase 20 family.