Catalyzes the transfer of the GlcNAc-1-phosphate moiety from UDP-GlcNAc onto the carrier lipid undecaprenyl phosphate (C55-P), yielding GlcNAc-pyrophosphoryl-undecaprenyl (GlcNAc-PP-C55), the lipid intermediate involved in the synthesis of various bacterial cell envelope components. The enzyme is highly active when tested with C35-P, instead of its natural C55-P lipid substrate, suggesting that at least a 35-carbon chain is required for the lipid to be a substrate of WecA. di-trans,octa-cis-undecaprenyl phosphate + UDP-N-acetyl-alpha-D-glucosamine = N-acetyl-alpha-D-glucosaminyl-di-trans,octa-cis-undecaprenyl diphosphate + UMP Partially inhibited by magnesium at concentration higher than 10 mM and totally inhibited at concentration higher than 250 mM. Also inhibited by tunicamycin, NaCl and KCl. Optimum pH is 8. Optimum temperature is 65 degrees Celsius. At 30 degrees Celsius activity is only 7% of the optimal value and at 80 degrees Celsius, the enzyme is totally inactive. Cell wall biogenesis; cell wall polysaccharide biosynthesis. Belongs to the glycosyltransferase 4 family. WecA subfamily.