Three UDP-hexose 4-epimerases with overlapping substrate specificity coexist in E. coli O86:B7

The O-antigen gene cluster of Escherichia coli O86:B7 was sequenced previously in our lab. One UDP-hexose 4-epimerase gene (named gne2 in this paper) was found and later characterized to be able to catalyze the interconversion between UDP-GlcNAc/GalNAc and UDP-Glc/Gal with almost equal efficiency. However, sequencing of the flanking gene region upstream of the traditional O-antigen gene cluster revealed an open reading frame (gne1), sharing 100% identity with Gne from E. coli O55, previously identified as UDP-GlcNAc 4-epimerase. Furthermore, we also located the traditional galE gene in the gal operon of O86:B7, which can catalyze the interconversion of UDP-Glc to UDP-Gal. Thus, for the first time, three UDP-hexose 4-epimerases with overlapping substrate specificity were found to coexist in one bacterium. Deletion of gne1 and gne2 in O86:B7 produced two different LPS phenotypes: the gne1 mutant exhibited rough LPS, while the gne2 mutant showed semi-rough LPS phenotype. These findings provide new clues for understanding the mechanism of O-antigen biosynthesis.