Figure 1. Alleviation of CRP-c AMP-mediated repression on glnAp2 by extremely low level of nitrogen source.
When the cya mutant TP2006 harboring pKUlOl was grown on an extremely low level of nitrogen source, CRP-cAMP-mediated repression on glnAp2 was alleviated (Figure 1). This implies that over-production of NtrC-phosphate alleviates CRP-cAMP-mediated repression. Similarly, alleviation of CRP-cAMP-mediated repression was obtained when alternative activator NifA was over-expressed. Furthermore, CRP-cAMP-mediated repression effect on glnAp2 was abolished when the rpoN gene was over-expressed (data not shown). Taken together, the results indicate that over-expressions of either activator or rpoN could alleviate the repression effect on glnAp2 in vivo. Therefore, it is likely that CRP-cAMP exerts its repression effect by competing with activator for Eg54 RNA polymerase on glnAp2.
Here we show that CRP represses transcription of glnAp2 in a binding site non-essential, activator independent, and cAMP-dependent manner. We propose that CRP-cAMP exerts its repression effect by competing with the activator(s) for Ea54 RNA polymerase on glnApl. In addition, we have observed that the glnllp2 and glnK promoters from E. coli and the nifB, nifE, nifF, nifJ, nifLA and nifU promoters from K. pneumoniae are also repressed by the CRP-cAMP complex. We predict that CRP-cAMP-mediated repression on a54-dependent promoters is quite common. Thus, dual roles of CRP (activation at a70-dependent sugar catabolic promoters and repression on a54-dependent nitrogen-responsive promoters) provide a novel regulatory linkage between carbon metabolism and nitrogen assimilation in E. coli and related bacteria.
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