Transcriptional repression ofgdhAinEscherichia coliis mediated by the Nac protein

Abstract

In this work we show that the nac gene from Escherichia coli is transcriptionally active, and that its expression is dependent on NRI (NtrC) and sigma-54. Northern blot experiments show a monocistronic nac-specific mRNA that is detected when wild-type cells are grown in nitrogen-limiting conditions. Our data also show that in nitrogen-limiting conditions Nac is involved in the transcriptional repression of the gdhA gene (encoding glutamate dehydrogenase) except when L-glutamine is used as the only nitrogen source. Moreover, the high level of GDH activity observed in a nac mutant strain is reduced when a wild-type nac gene is introduced under control of the lac promoter in N-limiting conditions, but not in L-glutamine or N-excess. These results suggest the existence of an additional mechanism responsible for overcoming repression by Nac.

Metabolic regulation of Escherichia coli and its gdhA, glnL, gltB, D mutants under different carbon and nitrogen limitations in the continuous culture

“…It has been reported that there is no NR I -P binding sites in the gdhA regulatory region [31], and it is unlikely for NR I to directly repress gdhA promoter [32]. As it has been shown that Nac is involved in the transcriptional repression of gdhA gene under N-limitation [32], Nac seems to repress gdhA gene as shown in Fig.…”

Nitrogen Assimilation in Escherichia coli: Putting Molecular Data into a Systems Perspective

“…However, growth under this condition represents a metabolic oddity, because these cells exhibit several paradoxical features: (i) even though ample external glutamine is available and is supporting rapid growth (557), the cells display a nitrogen-deficient response, as judged from a high expression level of NRI (180), an increased amount of GS (486), and a high expression level of the glnK-amtB operon (281); (ii) this apparent nitrogen deficiency shows up in spite of the fact that several glutamine transporters are known to exist, not only one or more lowaffinity transport systems but also a high-affinity ATP-dependent ABC transporter (GlnHPQ); (iii) the exponential phase of the growth curve is biphasic, where high-level glnA, glnK, and nac promoter activities show up only in the second part (401); (iv) the GDH capacity, which is expected to be low, turns out to be as high as that in cells grown with excess ammonium; and (v) with glutamine as the only nitrogen source, Nac did not repress gdhA, whereas with glutamate or low ammonium concentrations, such repression was observed (503). In light of the consensus that external nitrogen limitation is perceived as a drop in the internal glutamine pool (383), one might suggest that this pool is apparently small enough for cells to exhibit a nitrogen deficiency response when growing in a glucose-glutamine medium, but this being the case while excess glutamine is abundant in the environment remains an intriguing paradox.…”

“…Under low-ammonium conditions, GDH of E. coli was only moderately (2-fold) repressed by the nitrogen-regulated system-dependent Nac, in contrast to the strong repression (20-fold) of GDH of K. pneumoniae (K. aerogenes) (202,207). Third, an additional Nac-independent mechanism responsible for overcoming nac repression during nitrogenlimited growth is likely to be present in E. coli (202,503). Fourth, growth conditions that result in internal N limitation may or may not lead to suppression of GDH.…”

“…Fourth, growth conditions that result in internal N limitation may or may not lead to suppression of GDH. GDH is repressed during growth with excess arginine or glutamate as the N source, but its level is elevated during growth with glutamine (503). In glutamate-limited chemostat cultures, GDH was highly expressed, while it was repressed in proline-limited cultures (384).…”

Novel function assignment to a member of the essential HP1043 response regulator family of epsilon-proteobacteria

“…How the nitrogen status of the cell is sensed by HPMG440, a membrane-bound HK with two transmembrane domains flanking a periplasmic domain of 89 amino acids, is unclear. Interestingly, gdhA transcription in H. pullorum is conversely regulated compared with most other bacteria, where gdhA is repressed under conditions of nitrogen deprivation, whereas expression of glnA is induced due to the considerably higher affinity of glutamine synthetase for ammonium (Leigh & Dodsworth, 2007;Tiffert et al, 2008;Hervás et al, 2010;Harper et al, 2010;Schwacha & Bender, 1993;Ueki & Lovley, 2010;Camarena et al, 1998). However, co-regulation of gdhA and glnA in response to nitrogen limitation has been reported for Streptococcus pneumoniae, Corynebacterium glutamicum and Corynebacterium diphtheriae, and Ruminococcus flavefaciens (Kloosterman et al, 2006;Hänßler et al, 2009;Nolden et al, 2002;Antonopoulos et al, 2003).…”