Cloning of nod gene regions from mesquite rhizobia and bradyrhizobia and nucleotide sequence of the nodDgene from mesquite rhizobia

Nitrogen-fixing symbiosis between acteria and the tree legume mesquite (<i>Prosopis glandulosa</i>) is important for the maintenance of many desert ecosystems. Genes essential for nodulation and for extending the host range to mesquite were isolated from cosmid libraries of <i>Rhizobium</i> (mesquite) sp. strain HW17b and <i>Bradyrhizobium</i> (mesquite) sp. strain HW10h and were shown to be closely linked. All of the cosmid clones of rhizobia that extneded the host range of <i>Rhizobium</i> (<i>Parasponia</i>) sp. strain NGR234CS to mesquite also supported nodulatoin of a Sym^- mesquite strain. The cosmid clones of bradyrhizobia that extended the host range of <i>Rhizobium</i> (<i>Parasponia</i>) sp. strain NGR234CS to mesquite were only able to confer nodulation ability in the Sym^- mesquite strain if they also contained a <i>nodD</i>-hybridizing region. Subclones containing just the <i>nodD</i> genes of either genus did not extend the host range of <i>Rhizobium</i> (<i>Parasponia</i>) sp. to mesquite, indicating that the <i>nodD</i> gene is insufficient for mesquite nodulation. The <i>nodD</i> gene region is conserved among mesquite-nodulating rhizobia regardless of the soil depth from which they were collected, indicating descent from a common ancestor. In a tree of distance relationships. the NodD amino acid sequence from mesquite rhizobia clusters with homologs from symbionts that can infect both herbaceous and tree legumes, including <i>Rhizobium tropici</i>, <i>Rhizobium leguminosarum</i> bv. <i>phaseoli</i>, <i>Rhizobium loti</i>, and <i>Bradyrhizobium japonicum</i>.