Metabolic regulation and overproduction of primary metabolites.
Bottom Line: Inducers, effectors, inhibitors and various signal molecules play a role in different types of overproduction.The development of modern tools of molecular biology enabled more rational approaches for strain improvement.Techniques of transcriptome, proteome and metabolome analysis, as well as metabolic flux analysis. have recently been introduced in order to identify new and important target genes and to quantify metabolic activities necessary for further strain improvement.
Affiliation: Departamento de Biologia Molecular y Biotecnologia, Instituto de Investigaciones Biomedicas, Universidad Nacional Autonoma de Mexico (UNAM), Mexico City, Mexico.Show MeSH
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Mentions: A regulatory gene (glnG) in E. coli and other enteric bacteria encodes nitrogen regulator I (NRI), a dimer protein with a subunit weight of 54 000. NRI is produced at a high level (90 molecules per cell) under nitrogen limitation and at a low level (5 molecules per cell) under nitrogen excess. It activates transcription of glnA, encoding glutamine synthetase, under nitrogen limitation and represses it under conditions of excess nitrogen. NRI binds to DNA at or near the promoter of glnA and is thought to regulate production of all nitrogen‐regulated systems (Shiau et al., 1992). Furthermore in enteric organisms, glutamine synthetase and the other enzymes are regulated in positive and negative directions by three genes (ntrA, ntrB, ntrC). Two of these, ntrB and ntrC, are located next to the glutamine synthetase structural gene; ntrA is at a distance. NtrA is a sigma factor also known as RpoN or σ54 (Elderkin et al., 2005). The nitrogen regulation (ntr) system in enteric bacteria is responsible for activation of the glutamine synthetase operon (glnAntrBC), the uptake systems for glutamine (glnHPQ), arginine (argT) and histidine (hisJQMP), nitrate and nitrite assimilation (nasFEDCBA), and nitrogen fixation (nifLA). The system is complex (Merrick and Edwards, 1995) and responds to the sufficiency or limitation of the intracellular nitrogen pool. It is composed of four proteins: (i) response regulator NtrC, (ii) sensor histidine protein kinase NtrB, (iii) PII, a small protein encoded by glnB, and (iv) uridyltransferase/uridyl‐removing enzyme (Utase/UR) encoded by glnD (Fig. 1). For nitrogen‐controlled genes to be turned on, they need phosphorylated NtrC. NtrC is the response regulator of the signal transduction system; NtrB is its partner sensor kinase. In its phosphorylated state (NtrC‐P), it activates transcription of the nitrogen‐regulated genes. It binds to DNA having a helix–turn–helix motif in its C‐terminal domain. NtrB, the sensor protein kinase, catalyses its own phosphorylation and then NtrC phosphorylation under conditions of nitrogen deficiency. On the contrary, NtrC dephosphorylation occurs under nitrogen sufficiency. NtrB is cytoplasmic and dephosphorylates NtrC‐P only when it interacts with protein PII and can only phosphorylate NtrC when it interacts with PII(UMP)3. Protein PII can either be in its native state (PII) or its uridylated state [PII(UMP)3]. The uridylation reaction is carried out by Utase under conditions of nitrogen deficiency. The deuridylation reaction is catalysed by UR under conditions of nitrogen sufficiency. This uridylation/deuridylation system responds to the glutamine/α‐ketoglutarate ratio. Low ratios indicate nitrogen limitation leading to PII‐uridylation and hence NtrC‐P, whereas high ratios indicate nitrogen sufficiency leading to PII deuridylation and NtrC.
Affiliation: Departamento de Biologia Molecular y Biotecnologia, Instituto de Investigaciones Biomedicas, Universidad Nacional Autonoma de Mexico (UNAM), Mexico City, Mexico.