REGION-II OF RHIZOBIUM SP. NGR234 INHIBITS NODULATION OF MEDICAGO-SATIVA BY RHIZOBIUM-MELILOTI NODIJ AND NODQ(1) MUTANTS

Rhizobium sp. NGR234 contains a plasmid-borne locus that hybridized st rongly to region II of R. meliloti. Surprisingly, NGR region II comple tely inhibited nodulation of Medicago sativa when conjugated into eith er R. meliloti nodIJ or nodQ(1) region II mutants. Further characteriz ation showed that region II of NGR234 contains three putative coding o pen reading frames (ORFs), which are homologous to, respectively, hypo thetical protein A of R. leguminosarum, hypothetical protein C of R. l eguminosarum, and ORF2 of Agrobacterium tumefaciens insertion sequence (IS) 66, as well as an ORF of unknown function located downstream of nodQ(1) of R. meliloti. A site-directed mutation in the IS66 homologue improved nodulation efficiency on some NGR234 hosts, but the structur e and composition of the Nod-factor family produced by the mutant were unchanged. The quantity of Nod factors secreted was reduced by two-th irds, however. Insertions or deletions of the genes encoding either th e hypothetical protein C or the IS66 homologue abolished the blocking effect observed in the ''complementation'' tests with R. meliloti regi on II mutants.