CLONING OF NOD GENE REGIONS FROM MESQUITE RHIZOBIA AND BRADYRHIZOBIA AND NUCLEOTIDE-SEQUENCE OF THE NODD GENE FROM MESQUITE RHIZOBIA

Nitrogen-fixing symbiosis between bacteria and the tree legume mesquit e (Prosopis glandulosa) is important for the maintenance of many deser t ecosystems. Genes essential for nodulation and for extending the hos t range to mesquite were isolated from cosmid libraries of Rhizobium ( mesquite) sp. strain HW17b and Bradyrhizobium (mesquite) sp. strain HW 10h and were shown to be closely linked. All of the cosmid clones of r hizobia that extended the host range of Rhizobium (Parasponia) sp. str ain NGR234CS to mesquite also supported nodulation of a Sym(-) mesquit e strain. The cosmid clones of bradyrhizobia that extended the host ra nge of Rhizobium (Parasponia) sp. strain NGR234CS to mesquite were onl y able to confer nodulation ability in the Sym- mesquite strain if the y also contained a nodD-hybridizing region. Subclones containing just the nodD genes of either genus did not extend the host range of Rhizob ium (Parasponia) sp. to mesquite, indicating that the nodD gene is ins ufficient for mesquite nodulation. The nodD gene region is conserved a mong mesquite-nodulating rhizobia regardless of the soil depth from wh ich they were collected, indicating descent from a common ancestor. In a tree of distance relationships, the NodD amino acid sequence from m esquite rhizobia clusters with homologs from symbionts that can infect both herbaceous and tree legumes, including Rhizobium tropici, Rhizob ium leguminosarum bv. phaseoli, Rhizobium loti, and Bradyrhizobium jap onicum.