REGULATION OF SYRM AND NODD3 IN RHIZOBIUM-MELILOTI

The early steps of symbiotic nodule formation by Rhizobium on plants r equire coordinate expression of several nod gene operons, which is acc omplished by the activating protein NodD. Three different NodD protein s are encoded by Sym plasmid genes in Rhizobium meliloti, the alfalfa symbiont. NodD1 and NodD2 activate nod operons when Rhizobium is expos ed to host plant inducers. The third, NodD3, is an inducer-independent activator of nod operons. We previously observed that nodD3 carried o n a multicopy plasmid required another closely linked gene, syrM, for constitutive nod operon expression. Here, we show that syrM activates expression of the nodD3 gene, and that nodD3 activates expression of s yrM. The two genes constitute a self-amplifying positive regulatory ci rcuit in both cultured Rhizobium and cells within the symbiotic nodule . We find little effect of plant inducers on the circuit or on express ion of nodD3 carried on pSyma. This regulatory circuit may be importan t for regulation of nod genes within the developing nodule.