Involved in the synthesis of the major structural component of photosynthetic membranes. The 1,2-diacylglycerol substrate preference is 18:2/18:2 > 18:0/18:1 > 18:1/18:1 > 18:1/16:0 > 16:0/18:2 > 18:3/18:3 > 16:0/18:1 > 16:0/16:0 > 18:0/18:0. a 1,2-diacyl-sn-glycerol + UDP-alpha-D-galactose = a 1,2-diacyl-3-O-(beta-D-galactosyl)-sn-glycerol + H(+) + UDP Inhibited by ortho-phenanthroline and UDP (competitive inhibitor relatively to UDP-Gal only) and inactivated by citraconic anhydride, tert-butoxycarbonyl-L-methionine hydrosuccinimidyl ester (SLR) and N-ethylmaleimide (NEM). Homodimer. The C-terminal domain (308-483) is involved in UDP-Gal binding while the N-terminal domains (131-307) is involved in dyacylglycerol binding. Belongs to the glycosyltransferase 28 family.