*3.4. N-Acetylglucosamine: Another Misincorporation of KfoC*

KfoC is known to incorporate GlcNAc from UDP-GlcNAc in vitro [7]. However, this incorporation has been reported to forbid further polymerization of the growing chain. The NMR experiments of our samples also show the presence of GlcNAc residues in the polysaccharide chains. The F-H3/F-C3 = 3.78/84.21 signals suggest that the third GlcNAc carbon is linked to another GlcA, which highlight the fact that in vivo, the chain elongation continues after the incorporation of a GlcNAc residue. Interestingly, the resulting motif corresponds to the hyaluronan structure. The percentage of GlcNAc incorporated (13%) in the polysaccharide chain was calculated from the <sup>1</sup>H NMR signals integration (Figure 3A) by comparing the integrals of anomeric protons and protons from the GlcNAc acetyl residue. The chemical shifts have been determined and are reported in the Supplementary Figure S2.
