Involvement of the syrM and nodD3 genes of Rhizobium meliloti in nod gene activation and in optimal nodulation of the plant host.

We identified and sequenced the regulatory syrM and nodD3 genes of Rhizobium meliloti 41. Both genes were shown to contribute to optimal nodulation of alfalfa. In R. meliloti strains carrying syrM and nodD3 on plasmid, the nod genes are expressed constitutively, resulting in host-range extension to siratro. This is due to the presence of multiple syrM copies, suggesting that SyrM participates directly in nod gene activation. NodD3 activates nod genes in conjunction with flavonoids and enhances syrM expression, which is controlled also by its own product, NodD2, and two putative trans-acting factors. nodD3 is regulated by SyrM, NodD1, nodD3, the repressor NoIR, and two putative factors.