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O-Antigen is a part of the lipopolysaccharide present in the outer membrane of Gram-negative bacteria, which confers major antigenic variability to the cell surface. In this study, we report on a previously undefined pair of Escherichia coli and Salmonella enterica with closely related O-antigens. The O-polysaccharides were isolated from the lipopolysaccharides of E. coli O85 and S. enterica O17 by mild acid degradation and studied by sugar analysis and NMR spectroscopy. The following structure was established for the O-unit of the E. coli O85-polysaccharide: The S. enterica O17-polysaccharide has the same carbohydrate backbone and, in addition, contains an O-acetyl group at position 2 of ~80% β-Galf residues. The O-antigen gene cluster of E. coli O85 was found to be closely related to that of S. enterica O17. Screening of type strains of all E. coli and S. enterica O-serogroups revealed two genes specific to the E. coli O85 O-antigen gene cluster, which can be used for development of PCR-based assays for identification and detection of E. coli O85 strains. [ABSTRACT FROM AUTHOR]

Amt proteins constitute a class of ubiquitous integral membrane proteins that mediate movement of ammonium across cell membranes. They are homotrimers, in which each subunit contains a narrow pore through which substrate transport occurs. Two conserved histidine residues in the pore have been proposed to be necessary for ammonia conductance. By analyzing 14 engineered polar and non-polar variants of these histidines, in Escherichia coli AmtB, we show that both histidines are absolutely required for optimum substrate conductance. Crystal structures of variants confirm that substitution of the histidine residues does not affect AmtB structure. In a subgroup of Amt proteins, found only in fungi, one of the histidines is replaced by glutamate. The equivalent substitution in E. coil AmtB is partially active, and the structure of this variant suggests that the glutamate side chain can make similar interactions to those made by histidine. [ABSTRACT FROM AUTHOR]