Rewiring the functional complexity between Crc, Hfq and sRNAs to regulate carbon catabolite repression in Pseudomonas

Pseudomonas species are the most versatile of all known bacteria for metabolic flexibility and the extent of host range from plants to humans that remains unmatched. The evolution of diverse metabolic strategies in these species to adapt to the fluctuating environment guarantees high fitness as well as the ability to withstand stress at multiple levels. These abilities in Pseudomonas species are imprinted by an adaptable genetic repertoire through the integration of external and internal signals via complex regulatory networks. One of the main regulatory networks that lead to optimal growth, survival and cellular robustness is the phenomenon of carbon catabolite repression (CCR). Even though a large array of information is available, the molecular machinery and the mechanism of CCR in Pseudomonas are distinctly diverse from Escherichia coli and Bacillus subtilis. In Pseudomonas, the Crc and Hfq proteins, CbrAB two-component systems and the CrcZ/CrcY small RNA are key components of CCR. The main focus of this review is to elucidate the mechanism of CCR and the accessories involved in regulation of preferred carbon source utilisation over non-preferred ones and how CCR influences the virulence, antibiotic resistance, bioremediation and plant growth promotion pathways. Furthermore, we have also tried to shed some light on the "omics" approaches which can provide deep mechanistic insights into the regulation of CCR. Understanding the mechanistic picture of key regulatory entities and mechanism responsible for metabolic flexibility will create opportunities for exploitation of these versatile prokaryotes in several biotechnological processes.