A 150-megadalton plasmid in Rhizobium etli strain TAL182 contains genes for nodulation competitiveness on Phaseolus vulgaris L.

Rhizobium etli TAL182, a competitive strain for the nodulation of Phaseolus beans, occupied more than 99% of the nodules when co-inoculated in various proportions with Rhizobium TAL1145 or Rhizobium tropici CIAT899. Two overlapping cosmid clones, pUHR68 and pUHR69, containing genes for nodulation competitiveness from TAL182, were isolated by functional complementation of strain TAL1145. Using one of these cosmid clones, we constructed two Tn5-insertion mutants of TAL182 defective in nodulation competitiveness. The Tn5 insertions in both mutants were localized in identical positions within a 4.6-kb HindIII fragment. One mutant, RUH120, was complemented for nodulation competitiveness by this HindIII fragment. The cloned DNA in pUHR68 is a part of a plasmid, 150 MDa in size, in TAL182 and does not show homology with TAL1145 genomic DNA. The 4.6-kb HindIII fragment contains a gene(s) required for nodulation competitiveness on beans, which is present only in some R. etli strains and absent in other Rhizobium spp.