The Structure and Regulation of the Nodulation Genes of Rhizobium Leguminosarum

Abstract

This paper describes our current understanding of the organization of the nodulation genes in a strain of R. leguminosarum, the species that nodulates peas, Vicia, Lens and Lanthyrus. In many of the fast-growing species of Rhizobium it is well established that the genes for nodulation and nitrogen fixation are clustered on large “symbiotic” plasmids one of which is the R. leguminosarum plasmid pRL 1JI whose transfer to other species of Rhizobium allows the transconjugants to nodulate peas (Johnston et al., 1978). Further, a region, no more than 10kb in size, was sufficient to confer the ability to nodulate peas when transferred to a strain of R. phaseoli cured of its resident symbiotic plasmid (Downie et al., 1983). The fact that the nodulation and host-range genes of pRL1JI are clustered within such a small region means that they can be analysed in considerable detail using both genetical and physical techniques.