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 Inhibited by validoxylamine A, a non-reactive trehalose analog. Carbohydrate biosynthesis. By heat shock. Abolishes hyphal formation and impairs biofilm formation (PubMed:28743811). Sensitive to thermal stress (PubMed:28743811). Belongs to the glycosyltransferase 20 family.