CrcA, a novel player in Pseudomonas aeruginosa carbon catabolite repression
In the opportunistic human pathogen Pseudomonas aeruginosa (Pae), carbon catabolite repression (CCR) orchestrates the hierarchical utilization of N and C sources, and is known to impact virulence, antibiotic resistance and biofilm development. During CCR, the RNA chaperone Hfq and the catabolite repression control protein Crc form repressive assemblies on target mRNAs that impede translation. Affected are different uptake functions and catabolic proteins for the assimilation of less preferred C sources. After exhaustion of the preferred C-source, the levels of the regulatory RNA CrcZ increase. CrcZ binds to and acts as a decoy for Hfq, which in turn relieves translational repression of target genes. Here, we asked whether the functionality of Crc can be affected, which would likewise be anticipated to contribute to a relief of CCR after exhaustion of a preferred carbon source.  As Crc does not bind to RNA per se, we endeavoured to identify a protein interactor.  In vivo co-purification studies, co-immunoprecipitation- and biophysical assays revealed that Crc binds to Pae O1 protein PA1677. Our structural studies support bioinformatics analyses that PA1677 belongs to the isochorismatase-like superfamily. Ectopic expression of PA1677 resulted in de-repression of Hfq/Crc repressed target genes, strongly indicating that PA1677 acts as an antagonist of Crc.  This opposing function of PA1677 can be reconciled with an extended lag phase in the absence of the protein during diauxic growth on a preferred and a non-preferred carbon source, i.e. with a delayed synthesis of functions required to metabolize the non-preferred carbon source. We present a working model, wherein PA1677, which we termed CrcA (catabolite repression control protein antagonist), diminishes the formation of productive Hfq/Crc repressive complexes on target mRNAs by titrating Crc.