THE RHIZOBIUM-MELILOTI REGION LOCATED DOWNSTREAM OF THE NOD BOX N6 ISINVOLVED IN THE SPECIFIC NODULATION OF MEDICAGO-LUPULINA

Six nod box regulatory sequences are present in the Rhizobium meliloti genome. We have analysed the DNA region located downstream of nod box n6, and identified three open reading frames, designated nolQa, nolQb and nolS. LacZ fusions in these ORFs are not induced by classical nod gene inducers, which indicates that their expression either is not un der the control of the nod box, or involves another regulatory mechani sm acting in conjunction with the NodD/nod box regulatory circuit. Mut ations in this n6 locus result in a delay in nodule formation on a par ticular host, Medicago lupulina. As this region is not strictly conser ved among different R. meliloti strains, nolQa, nolQb and nolS may con stitute auxiliary nodulation genes, for which the selection pressure i s limited to particular host plants.