The upstream region of thenodD3gene ofSinorhizobium meliloticarries enhancer sequences for the transcriptional activator NtrC

In Sinorhizobium meliloti the expression of the nodulation genes nodABC is regulated in response to the level of fixed nitrogen (ammonia). Previous results suggested that the response to the nitrogen status is mediated by the two-component NtrB/NtrC system which controls transcription of the nodD3 gene, encoding a positive regulatory protein for the activation of nodABC transcription. Here we confirm by DNase I footprinting and gel shift assays that NtrC, when phosphorylated by NtrB, is able to interact with the enhancer sequences present upstream of nodD3. A model is proposed whereby NtrC functions to control the transcription from the two promoters in the upstream region of nodD3 in response to nitrogen status.