The CRP-cAMP complex and downregulation of the glnAp2 promoter provides a novel regulatory linkage between carbon metabolism and nitrogen assimilation in Escherichia coli

In Escherichia coli, glnA (encoding glutamine synthetase) is transcribed fr om two promoters (glnAp1 and glnAp2). The glnAp1 is a sigma (70)-dependent promoter that is activated by the cAMP receptor protein (CRP). Under nitrog en-deficient growth conditions, glnAp1 is repressed by NtrC-phosphate. The downstream glnAp2 promoter is sigma (54)-dependent and is activated by NtrC -phosphate. Here, we show that glnAp2 expression is affected by different c arbon sources and that the CRP-cAMP complex inhibits the glnAp2 promoter ac tivity. Primer extension and KMnO4 footprinting analysis indicate that the inhibitory effect is at the transcriptional level in vivo. When glnAp2 is a ctivated by NifA, a similar inhibitory effect by CRP-cAMP is observed. Site -directed mutagenesis and deletion analysis indicate that the characterized and putative CRP-binding sites located in the upstream region of the glnAp 2 promoter are not essential for the inhibitory effect. CRP-cAMP inhibits s igma (54)-dependent glnAp2 strongly, by 21-fold. By activating glnAp1 and d ownregulating glnAp2, the overall effect of CRP-cAMP on glnA expression is an approximately fourfold reduction, which correlates with the reduction of gamma -glutamyl transferase activities in the cells. We propose therefore that a physiological role of CRP-cAMP activation of glnAp1 is to partially compensate for CRP-cAMP downregulation of glnAp2, allowing a low but non-ne gligible level of expression of the important genes transcribed from it. A novel regulatory linkage between carbon and nitrogen regulons is proposed.