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 the expression of the benzoate degradation genes. These genes are organized into several transcriptional units. We show that Crc directly inhibits the expression of the peripheral genes that transform benzoate into catechol (the ben genes) but that its effect on genes corresponding to further steps of the pathway (the cat and pca genes of the central catechol and beta-ketoadipate pathways) is indirect, since these genes are not induced because the degradation intermediates, which act as inducers, are not produced. Crc inhibits the translation of target genes by binding to mRNA. The expression of the ben, cat, and pca genes requires the BenR, CatR, and PcaR transcriptional activators, respectively. Crc significantly reduced 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 fusion of the benR and lacZ genes was sensitive to Crc, but a transcriptional fusion was not. We propose that Crc acts by reducing the translation of benR mRNA, decreasing BenR levels below those required for the full expression of the benABCD genes. This strategy provides 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.