Organization and Regulation of Symbiotic Nitrogen Fixation Genes from Bradyrhizobium Japonicum

Abstract

This article summarizes our recent data on the analysis of bacterial genes required for the formation of a nitrogen fixing soybean root nodule, with particular emphasis on nif and fix genes. The bacterium under study is the slow-growing Rhizobium japonicum (strain USDA 31lbllO). This species has been given a new generic name, Bradyrhizobium, to account for the many differences by which it is distinguished from the fast-growing rhizobia (Jordan 1984). The validity of this concept is supported by a phylogenetic analysis of a few Rhizobium species based on 16S rRNA oligonucleotide cataloguing (Hennecke et al. 1985). Figure 1 shows that the slow-growing Bradyrhizobium japonicum (strain 110) and R.lupini are well separated from the fast-growing R.meliloti and R.leguminosarum (by a S _AB value as low as 0.53); by this analysis all rhizobia fall into the α-subdivision of photosynthetic purple bacteria (Woese et al. 1984). Consequently, we will adopt the “Bradyrhizobium” terminology throughout this article to avoid confusion with the fast-growing R.japonicum strains (Keyser et al. 1982).