Coupling of Carbon and Nitrogen Regulation in Amino acid and Polyamine Catabolism through Two-component Regulatory Systems and A Novel Mechanism of Bacterial Catabolite Control

铜绿假单胞菌PAO1通过hutHUTIG操纵子编码的分解代谢途径利用组氨酸作为碳源和氮源，该操纵子的表达同时受到碳和氮的调控。琥珀酸盐（这种细菌的最佳碳源和分解代谢抑制物）部分抑制了hut操纵子的表达（约25%）。单用氨（最优氮源）对hut操纵子表达影响不大。然而，在琥珀酸盐存在的情况下，它强有力地（<5%）消除了残留表达。CbrA-CbrB和NtrB-NtrC双组分调控系统分别对组氨酸作为碳源和氮源的利用至关重要。琥珀酸盐似乎通过CbrA-CbrB系统抑制hut操纵子转录。相反，二羧酸盐似乎通过NtrB-NtrC系统增强了hut操纵子的表达，允许细胞使用组氨酸作为氮源。氨（一种氮调节剂）可拮抗NtrB-NtrC，并进一步激活hut操纵子的表达。cbrB零突变体自发产生高频率的回调物（约10^<-5>），恢复了利用组氨酸作为碳源的能力。我们在ntrC中分离出一个可逆基因，并鉴定出一个突变（在nt 251处C到T的变化）。在反向表达中，hut操纵子不再受到琥珀酸盐的分解代谢物抑制和琥珀酸盐存在下的氨抑制。CbrB和NtrC反应调节因子分别根据碳和氮状态被同源CbrA和NtrB蛋白磷酸化，并作为转录激活因子。具有Ala-84到Val的氨基酸取代的NtrC251蛋白可能具有改变的结构，模仿蛋白质的磷酸化（活性）状态，导致hut操纵子的组成表达。在ntrC251突变体中，琥珀酸盐对hut操纵子的分解代谢物抑制缺失支持了CbrB磷酸化水平的调节是分解代谢物抑制物琥珀酸盐控制hut操纵子的关键。因此，本研究揭示了细菌分解代谢控制的新机制，其中一个双组分调控系统涉及感知环境中的碳状态和调节分解代谢途径的表达。少

Pseudomonas aeruginosa PAO1 utilizes histidine via a catabolic pathway encoded by the hutHUTIG operon as both carbon and nitrogen sources, and expression of this operon is regulated by both carbon and nitrogen. Succinate (the optimal carbon source and a catabolite repressor for this bacterium) partially suppressed expression of the hut operon (by about 25%). Ammonia (the optimal nitrogen source) alone had little effect on hut operon expression. However it powerfully (<5%) abrogated residual expression in the presence of succinate. The CbrA-CbrB and NtrB-NtrC two-component regulatory systems were essential for histidine utilization as a carbon source and as a nitrogen source, respectively. Succinate appeared to suppress hut operon transcription via the CbrA-CbrB system. On the contrary, the dicarboxylate seemed to enhance hut operon expression through the NtrB-NtrC system, allowing cells to use histidine as a nitrogen source. Ammonia (a nitrogen regulator) antagonized the NtrB-NtrC depe … More ndent activation of hut operon expression.A cbrB null mutant spontaneously produced a high frequency of revertants (about 10^<-5>), that restored the ability to utilize histidine as a carbon source. We isolated a revertant and identified a mutation (a C to T change at nt 251) in ntrC. In the revertant expression of the hut operon was no longer subjected to catabolite repression by succinate and to ammonia repression in the presence of succinate. The CbrB and NtrC response regulators are phosphorylated by cognate CbrA and NtrB proteins depending on carbon and nitrogen status, respectively, and serve as transcriptional activators. The NtrC251 protein having an amino acid substitution of Ala-84 to Val might have an altered structure that mimics the phosphorylated (active)status of the protein, resulting in constitutive expression of the hut operon. The absence of catabolite repression of the hut operon by succinate in the ntrC251 mutant supports the notion that modulation of CbrB phosphorylation levels is a crucial point in control of the hut operon by the catabolite repressor, succinate. Thus, the present study disclosed a novel mechanism of bacterial catabolite control, in which a two-component regulatory system is involved in sensing carbon status in the environment and modulating expression of a catabolic pathway. Less