Transcription patterns of Rhizobium meliloti symbiotic plasmid pSym: identification of nifA-independent fix genes.

We performed a systematic survey of transcription of a large region of the Rhizobium meliloti symbiotic plasmid pSym. This led to the discovery of two new sequences induced during symbiosis. The first sequence was linked to the known nitrogen fixation (nif-fix) gene cluster, and its expression depended on the nifA gene product. The second sequence was a novel fix locus (M.-H. Renalier, J. Batut, J. Ghai, B. Terzaghi, M. Gherardi, M. David, A.-M. Garnerone, J. Vasse, G. Truchet, T. Huguet, and P. Boistard, J. Bacteriol. 169:2231-2238, 1987) whose expression was independent of the nifA gene product; therefore this fix locus undergoes a novel type of symbiotic regulation.