Summary: Two transposon Tn5-induced mutants of wild-type broad-host-range sp. GRH2 were isolated and found to harbour different alterations in surface polysaccharides. These mutants, designated GRH2-14 and GRH2-50, induced a few, empty nodules on and lost the ability to nodulate most host herbaceous legumes. Whereas mutant GRH2-14 produces an acidic exopolysaccharide (EPS) similar to the wild-type, the acidic EPS of mutant GRH2-50 lacks galactose and the pyruvyl and 3-hydroxybutyryl substituents attached to this sugar moiety. In addition, both mutants GRH2-50 and GRH2-14 were altered in smooth lipopolysaccharides (LPS). DNA sequence analyses of the correspresponding Tn5 insertions revealed that strain GRH2-50 was mutated in a DNA locus homologous to , and enzyme assays indicated that the UDPglucose 4-epimerase (GalE) activity was missing in this mutant strain. DNA hybridization studies showed that the GRH2-50 mutant DNA has homologous sequences within the different biovars of However, no DNA homology to GRH2-14 altered DNA was found in those rhizobial strains, indicating that it represents a new chromosomal locus in sp. () involved in symbiotic development.


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