Suppression of the ndv mutant phenotype of Rhizobium meliloti by cloned exo genes.

The ndvA and ndvB genes of Rhizobium meliloti are involved in the export and synthesis, respectively, of the small cyclic polysaccharide beta(1,2)glucan. We have previously shown that spontaneous symbiotic pseudorevertants of ndv mutants do not produce periplasmic beta(1,2)glucan. Here we show that the pseudorevertants also do not produce extracellular beta(1,2)glucan, but do show alterations in the amount of the major acidic exopolysaccharide produced. This exopolysaccharide is not detectably different from that produced by the wild type or by the ndv mutants. A cosmid which suppresses the symbiotic defect of both ndvA and ndvB mutants was isolated from a gene bank prepared from DNA of an ndvA pseudorevertant. This cosmid contains a number of exo genes, including exoH and exoF. Subcloning and Tn5 mutagenesis were used to show that the widely separated exoH and exoF genes are both involved in suppression of the ndv mutant phenotype and that the 3.5 kb DNA fragment which contains the exoH gene does not carry the mutation responsible for second site suppression.