Catalyzes the conversion of GlcNAc-PP-undecaprenol into ManNAc-GlcNAc-PP-undecaprenol, the first committed lipid intermediate in the de novo synthesis of teichoic acid. N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UDP-N-acetyl-alpha-D-mannosamine = H(+) + N-acetyl-beta-D-mannosaminyl-(1->4)-N-acetyl-alpha-D-glucosaminyl di-trans,octa-cis-undecaprenyl diphosphate + UDP Optimum pH is 7-7.5. Cell wall biogenesis; poly(glycerol phosphate) teichoic acid biosynthesis. Positively regulated by WalR. Mainly expressed during exponential growth and rapidly shut off as cells enter the stationary phase. Not essential. Impaired growth. Looses rod shape of cells. No teichoic acid in cell walls. Belongs to the glycosyltransferase 26 family. TagA/TarA subfamily.