The upstream region of the nodD3 gene of Sinorhizobium meliloti carries 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 re sults suggested that the response to the nitrogen status is mediated by the two-component NtrB/NtrC system which controls transcription of the nodD3 g ene, encoding a positive regulatory protein for the activation of nodABC tr anscription. Here we confirm by DNase I footprinting and gel shift assays t hat NtrC, when phosphorylated by NtrB, is able to interact with the enhance r sequences present upstream of nodD3. A model is proposed whereby NtrC fun ctions to control the transcription from the two promoters in the upstream region of nodD3 in response to nitrogen status. (C) 1999 Federation of Euro pean Microbiological Societies. Published by Elsevier Science B.V. All righ ts reserved.