The target for the Pseudomonas putida Crc global regulator at the benzoate degradation pathway is the BenR transcriptional regulator

Departamento de Biotecnología Microbiana, Centro Nacional de Biotecnología, CSIC, Campus de la Universidad Autónoma de Madrid, Cantoblanco, 28049 - Madrid, Spain

^* To whom correspondence should be addressed. Email: frojo{at}cnb.uam.es.

	Abstract

Crc protein is a global regulator involved in catabolite repression control of several pathways for the assimilation of carbon sources in Pseudomonads when other preferred substrates are present. In Pseudomonas putida cells growing exponentially in a complete medium containing benzoate, Crc strongly inhibits expression of the benzoate degradation genes. These genes are organized in several transcriptional units. We show that Crc directly inhibits expression of the peripheral genes that transform benzoate into catechol (the ben genes), but its effect on genes corresponding to further steps of the pathway (the cat and pca genes of the central catechol and -ketoadipate pathways) is indirect, since these are not induced because the degradation intermediates, which act as inducers, are not produced. Crc inhibits translation of target genes by binding to mRNA. Expression of the ben, cat and pca genes requires the BenR, CatR and PcaR transcriptional activators, respectively. Crc reduced significantly benABCD mRNA levels, but did not affect those of benR. Crc bound to the 5'-end of benR mRNA, but not to equivalent regions of catR and pcaR mRNAs. A translational benR’-‘lacZ fusion was sensitive to Crc, but a transcriptional fusion was not. We propose that Crc acts by reducing translation of benR mRNA, decreasing BenR levels below those required for full expression of the benABCD genes. This strategy provides a great metabolic flexibility, allowing the hierarchical assimilation of different structurally related compounds that share a common central pathway by selectively regulating the entry of each substrate into the central pathway.