Evidence for nitrogen fixation (nif) genes on indigenous Rhizobium plasmids

Abstract

The distinguishing characteristic of bacteria that belong to the genus Rhizobium is their ability to induce nodules and fix nitrogen in the roots of leguminous plants—hence their importance in agriculture. The demonstration of nitrogen-fixation by rhizobia in the absence of host plant material showed that the genes for nitrogenase synthesis and function reside in the bacterium^1–3. The presence of large plasmids (molecular weight (MW) 90–350 × 10⁶) in many Rhizobium strains is now well established^4–6 and earlier genetic evidence suggested that at least some of the genes required for symbiosis were plasmid-linked^7,8. More recently, Johnston et al.⁹ found evidence that genes required for nodulation were carried on a plasmid in Rhizobium leguminosarum. We now report hybridisation between restriction fragments of plasmid DNA from R. leguminosarum and DNA encoding part of the structural genes for Klebsiella pneumoniae nitrogenase. Our results provide evidence that at least some of the Rhizobium nif genes are carried on plasmids indigenous to the strains examined.