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J. Bacteriol. doi:10.1128/JB.00432-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Regulation of Carbon and Nitrogen Utilization by CbrAB and NtrBC Two-Component Systems in Pseudomonas aeruginosa

^* To whom correspondence should be addressed. Email: biocdl{at}panther.gsu.edu.

	Abstract

The global effect of the CbrAB and NtrBC two-component systems in control of carbon and nitrogen utilization in Pseudomonas aeruginosa was characterized by Phenotype Microarray^TM analyses in single and double mutants and the isogenic parent strain. The tested compounds were clustered based on the growth phenotypes of these strains, and the results clearly demonstrated the pivotal role of CbrAB and NtrBC in carbon and nitrogen utilization, respectively. Growth of the cbrAB deletion mutant on arginine, histidine, and polyamines as the sole carbon source was abolished while growth on the TCA cycle intermediates sustained. In this study, suppressors of the cbr mutant were selected from minimal medium containing L-arginine as the sole carbon and nitrogen source. These mutants fell into two groups according to the ability to utilize histidine. The genomic library of a histidine⁺ suppressor mutant was constructed and the corresponding suppressor gene was identified by complementation as an ntrB allele. Similar results were also obtained from four additional suppressor mutants, and point mutations of these ntrB alleles were identified, resulting in the following changes on residues with implications in reduced phosphatase activities: L126W, D227A, P228L and S229I. The Ntr system of these ntrB mutants became constitutively active as revealed by the activity profiles of glutamate dehydrogenase, glutamate synthase, and glutamine synthetase. As a result, these mutants not only regain the substrate-specific induction on arginine and histidine catabolic operons, but also express to a higher level than in the wild type. While the cbrAB ntrB^C mutant restored the growth on many N-containing compounds as the carbon sources, its capability to grow on TCA cycle intermediates and glucose was compromised when ammonium served as the sole nitrogen source, mostly due to an extreme imbalance of carbon and nitrogen regulatory systems. In summary, this study supports the notion that CbrAB and NtrBC form a network to control the C/N balance in P. aeruginosa. Possible molecular mechanisms of these two regulatory elements in control of arginine and histidine operons as the model systems were discussed.

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