Two mutant derivatives of Rhizobium leguminosarum ANU843 defective in lipopolysaccharide (LPS) were isolated. The LPSs of both mutants lacked 0 antigen and some sugar residues of the LPS core oligosaccharides. Genetic regions previously cloned from another Rhizobium leguminosarum wild-type isolate, strain CFN42, were used to complement these mutants. One mutant was complemented to give LPS that was apparently identical to the LPS of strain ANU843 in antigenicity, electrophoretic mobility, and sugar composition. The other mutant was complemented by a second CFN42 lps genetic region. In this case the resulting LPS contained 0-antigen sugars characteristic of donor strain CFN42 and reacted weakly with antiserum against CFN42 cells, but did not react detectably with antiserum against ANU843 cells. Therefore, one of the CFN42 lps genetic regions specifies a function that is conserved between the two R. leguminosarum wild-type isolates, whereas the other region, at least in part, specifies a strain-specific LPS structure. Transfer of these two genetic regions into wild-type strains derived from R. leguminosarum ANU843 and 128C53 gave results consistent with this conclusion. The mutants derived from strain ANU843 elicited incompletely developed clover nodules that exhibited low bacterial populations and very low nitrogenase activity. Both mutants elicited normally developed, nitrogen-fixing clover nodules when they carried CFN42 Ips DNA that permitted synthesis of 0-antigen-containing LPS, regardless of whether the 0 antigen was the one originally made by strain ANU843.Leguminous plants and bacteria of the genera Rhizobium and Bradyrhizobium enter into a symbiosis characterized by nitrogen-fixing root nodules. Early steps in development of the symbiosis involve mutual recognition of plant and bacteria, induction of cortical cell division, and (in most cases) invasion of root hairs by way of infection threads. As gram-negative eubacteria, rhizobia have an outer membrane containing lipopolysaccharide (LPS). LPS is composed of two regions: lipid A, which anchors the LPS molecule in the outer leaflet of the outer membrane, and a polysaccharide portion, which projects into the environment. Being a major cell surface molecule, LPS is likely to play a role in early symbiotic events.The polysaccharide of Rhizobium leguminosarum LPS consists of a variable portion and a portion whose composition is conserved among wild-type isolates that have been studied previously (4, 6, 7). The conserved portion is released from the LPS by mild acid hydrolysis as two oligosaccharides that together are composed of galactose, mannose, galacturonic acid, and at the reducing ends, 3-deoxy-Dmanno-octulosonic acid in a 1:1:3:2 molar ratio (4, 5, 14a). The variable portion is found in a longer polysaccharide that is released by mild acid hydrolysis. Its composition varies from strain to strain (4, 26). The variable portion carries the dominant antigenic determinants of wild-type strains and is referred to as the 0 antigen, in keeping with the termin...
