BIOSYNTHESIS OF O‐ANTIGEN in SALMONELLA TYPHIMURIUM *

Studies of the mechanism of biosynthesis of the lipopolysaccharides of the Enterobacteriaceae have, until recently, been limited to the “core” region of the molecule (FIGURE 1). (For summary see references 1 and 2.) Within the past year, however, several laboratories have reported the biosynthesis of the antigenic side chain in cell-free preparations from species of Salmonella. Zeleznick et ~ 1 . ~ used the cell envelope fraction prepared from a mutant strain of S. typhimurium deficient in the synthesis of GDP-mannose. These workers described the synthesis of a macromolecular product containing galactose, rhamnose, and mannose in the same ratio and sequence as reported for the repeating unit of the authentic 0-antigen of this Partial acid hydrolysates of the enzymic product contained the trisaccharide a-galactosylmannosyl-rhamnose. Abequose, the. remaining component of authentic 0-antigen side chains, occurs as nofireducing monosaccharide branches on the trisaccharide repeating units, and it was concluded that the absence of its precursor (CDP-abequose) from the reaction mixtures did not preclude the formation of the macromolecule. Comparable results were reported by Nikaido and Nikaido,‘ who employed a mutant strain of s. typhimurium deficient in the synthesis of TDPrhamnose. With cell-free preparations of S . anatum, Robbins et ~ 1 . ~ observed the synthesis of a similar macromolecule from the appropriate sugar nucleotides. The 0-antigenic side chain of lipopolysaccharide from the latter organism contains the sugars galactose, mannose, and rhamnose in the same sequence as is found in S. typhimurium but is lacking in abequose. The structure of the core region of the molecule has been established by chemical analysis of the lipopolysaccharide isolated from a variety of rough mutants and by biosynthetic studies utilizing mutants deficient in the biosynthesis of UDP-galactose and/or UDP-glucose. This portion of the lipopolysaccharide molecule does not contain repeating sequences of sugars (FIGURE 1) and the biosynthetic work indicates that it is formed by the stepwise addition of sugars according to the following general equation: