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通过由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，该操纵子的表达受碳和氮的调节。琥珀酸(该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%)。氨(最佳氮源)单独对HUT操纵子的表达影响不大。然而，在琥珀酸存在的情况下，它有效地(&lt；5%)消除了残留表达。CbrA-CbrB和NtrB-NTRC两组分调控系统对于组氨酸分别作为碳源和氮源的利用是必不可少的。琥珀酸似乎通过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，允许细胞利用组氨酸作为氮源。氨(氮素调节剂)拮抗NtrB-NTRC DepE…CbrB缺失突变体自发地产生了高频率的回复突变体(约10^-5&gt；)，恢复了利用组氨酸作为碳源的能力。我们分离了一个回复突变体，并在NTRC中发现了一个突变(在第251位发生C到T的改变)。在HUT的回复表达中，操纵子不再受到琥珀酸分解代谢的抑制，也不再受到丁二酸存在下的氨抑制。CbrB和NTRC反应调节子分别被同源的CbrA和NtrB蛋白磷酸化，这取决于碳和氮的状况，并作为转录激活因子。NtrC251蛋白具有Ala-84到Val的氨基酸替换，可能具有模拟蛋白质的磷酸化(活性)状态的改变的结构，导致HUT操纵子的结构性表达。在ntrC251突变体中，琥珀酸不存在对HUT操纵子的分解代谢抑制，这支持了CbrB磷酸化水平的调节是分解代谢抑制因子琥珀酸控制HUT操纵子的关键点的观点。因此，本研究揭示了一种控制细菌分解代谢的新机制，其中一个双组分调控系统参与感知环境中的碳状态并调节分解代谢途径的表达。较少

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