A 70 kbp segment of the megaplasmid from a broad host range Rhizobium strain (MPIK3030) was mapped with the aid of cosmid clones made in the vector pJB8. A 7.9 kbp EcoRI fragment from this region, 55 kbp away from the nif gene cluster, was shown to hybridize to the "common" nod genes from R. meliloti. Using several R. meliloti nod probes it was possible to delimit an 830 bp region as being the center of greatest homology. Sequence data from two sections of this region gave a nucleotide homology of 73.7% to the nodC gene of R. meliloti. Using Tn 5 mutagenesis a clone was isolated carrying Tn 5 in the highly homologous region. When tested on Macroptilium atropurpureum, this MPIK3030 derivative was shown to have a Nod- phenotype. When the wild-type allele was reintroduced into the Tn 5 mutant, nodulation was restored. Interspecies complementation also showed that both R. meliloti and Rhizobium sp. MPIK3030 nod regions were able to restore nodulation to Tn 5-induced nodC mutants from either strain.
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