Genetic locus required for antigenic maturation of Rhizobium etli CE3 lipopolysaccharide

Rhizobium edi modifies lipopolysaccharide (LPS) structure in response to en vironmental signals, such as low pH and anthocyanins. These LPS modificatio ns result in the loss of reactivity with certain monoclonal antibodies. The same antibodies fail to recognize previously isolated R. etli mutant strai n CE367, even in the absence of such environmental cues. Chemical analysis of the LPS in strain CE367 demonstrated that it lacked the terminal sugar o f the wild-type O antigen, 2,3,4-tri-O-methylfucose. A 3-kb stretch of DNA, designated as lpe3, restored wild-type antigenicity when transferred into CE367. From the sequence of this DNA, five open reading frames were postula ted. Site-directed mutagenesis and complementation analysis suggested that the genes were organized in at least two transcriptional units, both of whi ch were required for the production of LPS reactive with the diagnostic ant ibodies. Growth in anthocyanins or at low pH did not alter the specific exp ression of gusA from the transposon insertion of mutant CE367, nor did the presence of multiple copies of lpe3 situated behind a strong, constitutive promoter prevent epitope changes induced by these environmental cues. Mutat ions of the lpe genes did not prevent normal nodule development on Phaseolu s vulgaris and had very little effect on the occupation of nodules in compe tition with the wild-type strain.