THE SSC PROTEIN OF ENTERIC BACTERIA HAS SIGNIFICANT HOMOLOGY TO THE ACYLTRANSFERASE LPXA OF LIPID-A BIOSYNTHESIS, AND TO 3 ACETYLTRANSFERASES

The Ssc protein, a novel essential protein affecting the function of the enterobacterial outer membrane, matched in a protein homology search best with LpxA (UDP-N-acetylglucosamine 3-hydroxymyristoyl transferase), the enzyme which catalyzes the first step of lipid A biosynthesis. The corresponding genes, located 0.56 kb apart, were 46.7% identical. The search also revealed homology to the bacterial acetyltransferases LacA and NodL, as well as to a hypothetical protein Yglm. The region of residues 109-149 Ssc displayed the highest homology and was also homologous with another bacterial acetyltransferase, CysE, and three other bacterial proteins, two of which are hypothetical. This region and the corresponding regions of all other proteins were found to have a peculiar repeated hexapeptide pattern. Each hexapeptide unit starts with isoleucine (or its equivalent leucine and valine). In most units, the second residue is glycine and the fifth residue either valine or alanine.