Involved in bacillithiol (BSH) biosynthesis. Catalyzes the first step of the pathway, the formation of N-acetylglucosaminylmalate (GlcNAc-Mal) from UDP-N-acetylglucosamine (UDP-GlcNAc) and L-malate. (S)-malate + UDP-N-acetyl-alpha-D-glucosamine = (S)-malyl N-acetyl-alpha-D-glucosaminide + H(+) + UDP kcat is 28 sec(-1). Dimer of tetramers. Mutant does not produce bacillithiol. Belongs to the glycosyltransferase group 1 family. Glycosyltransferase 4 subfamily.