Linkage of genes for nitrogenase and nodulation ability on plasmids in Rhizobium leguminosarum and R. phaseoli

Summary The ability to identify genes that specify nitrogenase (nif genes) in Rhizobium depends on the close homology between then and the corresponding nif genes of Klebsiella pneumoniae (Nuti et al. 1979; Ruvkun and Ausubel 1980). Rhizobium plasmids of high molecular weight (>100 Md) were separated on agarose gels, transferred to nitrocellulose filters and tested for their ability to hybridise with radioactively labelled pSA30, containing the nifKDH region of K. pneumoniae. Five large plasmids, each present in different strains of R. leguminosarum or R. phaseoli, were found to hybridise. Each of these plasmids had previously been shown to determine other symbiotic functions such as nodulation ability. The nif genes on three different plasmids appeared to be in conserved DNA regions since they were within an EcoRI restriction fragment of the same size.