Identification and structure of the Rhizobium galegae common nodulation genes: Evidence for horizontal gene transfer

Rhizobia are soil bacteria able to fix atmospheric nitrogen in symbiosis wi th leguminous plants. In response to a signal cascade coded by genes of bot h symbiotic partners, a specific plant organ, the nodule, is formed. Rhizob ial nodulation (nod) genes trigger nodule formation through the synthesis o f Nod factors, a family of chitolipooligo-saccharides that are specifically recognized by the host plant at the first stages of the nodulation process . Here, we present the organization and sequence of the common nod genes fr om Rhizobium galegae, a symbiotic member of the Rhizobiaceae. This species has an intriguing phylogenetic position, being symbiotic among pathogenic a grobacteria, which induce tumors instead of nodules in plant shoots or root s. This apparent incongruence raises special interest in the origin of the symbiotic apparatus of R. galegae. Our analysis of DNA sequence data indica ted that the organization of the common nod gene region of R. galegae was s imilar to that of Sinorhizobium meliloti and Rhizobium leguminosarum, with nodIJ downstream of nodABC and the regulatory nodD gene closely linked to t he common nod operon. Moreover, phylogenetic analyses of the nod gene seque nces showed a close relationship especially between the common nodA sequenc es of R. galegae, S. meliloti, and R. leguminosarum biovars viciae and trif olii. This relationship in structure and sequence contrasts with the phylog eny based on 16S rRNA, which groups R. galegae close to agrobacteria and se parate from most other rhizobia. The topology of the nodA tree was similar to that of the corresponding host plant tree. Taken together, these observa tions indicate that lateral nod gene transfer occurred from fast-growing rh izobia toward agrobacteria, after which the symbiotic apparatus evolved und er host plant constraint.