Identification of a gene linked to Rhizobium meliloti ntrA whose product is homologous to a family to ATP-binding proteins.

The ntrA gene of Rhizobium meliloti has recently been identified and shown to be required for a diverse set of metabolic functions (C. W. Ronson, B. T. Nixon, L. M. Albright, and F. M. Ausubel, J. Bacteriol. 169:2424-2431, 1987). As a result of sequencing the ntrA gene and its flanking regions from R. meliloti, we identified an open reading frame directly upstream of ntrA, ORF1, whose predicted product is homologous to a superfamily of ATP-binding proteins involved in transport, cell division, nodulation, and DNA repair. The homology of ORF1 to this superfamily and its proximity to ntrA led us to investigate its role in symbiosis by mutagenesis and expression studies. We were unable to isolate an insertion mutation in ORF1, suggesting that ORF1 may code for an essential function. We identified the start of transcription for the ntrA gene in vegetative cells and bacteroids and showed that ORF1 and ntrA are transcriptionally unlinked. ORF1 appears to be in an operon with one or more upstream genes.