Characterization of Two Azospirillum brasilense Sp7 Plasmid Genes Homologous to Rhizobium meliloti nodPQ. Claire Vieille. Unité de Physiologie Cellulaire, CNRS URA 1300, Département des Biotechnologies, Institut Pasteur, 75724 Paris Cedex 15, France. Claudine Elmerich. Unité de Physiologie Cellulaire, CNRS URA 1300, Département des Biotechnologies, Institut Pasteur, 75724 Paris Cedex 15, France. MPMI 3:389-400. Accepted 18 July 1990. Copyright 1990 The American Phytopathological Society.

Bacteria belonging to the Azospirillum genus are nitrogen fixers that colonize the roots of grasses, but do not cause the formation of differentiated structures. Sequences from total DNA of several Azospirillum strains are homologous to restriction fragments containing Rhizobium meliloti nodulation genes. A 10-kilobase (kb) EcoRI fragment from A. brasilense Sp7, sharing homology with a 6.8-kb EcoRI fragment carrying nodGEFH and part of nodP of R. meliloti 41, was cloned in pUC18 to yield pAB502. The nucleotide sequence of a 3.5-kb EcoRI-SmaI fragment of the pAB502 insert revealed 60% homology with R. meliloti nodP and nodQ genes. The nodP gene product shares no homology to any known protein sequence. The Azospirillum nodQ gene product shares homology with a family of initiation and elongation factors as does the R. meliloti nodQ gene product. Since the nodQ gene overlaps the nodP gene, the two genes might be cotranscribed. Azospirillum contains large plasmids, and the nodPQ genes were found on the 90-MDa plasmid (p90). A translational nodP-lacZ fusion was constructed in the broad host range plasmid pGD926. No β-galactosidase activity was detected in Escherichia coli, but the fusion was functional in Azospirillum and constitutively expressed. Deletions and mutations of nodPQ did not modify growth, nitrogen fixation, or interaction with wheat seedlings.