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

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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