In Rhizobium leguminosarum, NodD represses its own transcription by competing with RNA polymerase for binding sites

We isolated RNA polymerase (RNAP) from Rhizobium leguminosarum, the nitrogen-fixing symbiont of peas and Vicia. Its 91 kDa subunit, which is homologous to σ(70) of Escherichia coli RNAP, is necessary for transcription of the regulatory nodD gene, which in the presence of certain flavonoids induces transcription of other nod genes that are needed for the early steps of infection. We also show that negative autoregulation of nodD was achieved through competition between RNAP and NodD for their respective binding sites, which largely overlap. Combined with the result that high concentrations of the flavonoid inducer naringenin relieved the binding affinity of NodD for its target DNA, the way in which the nod genes are activated is discussed.