Multiple regulatory inputs for hierarchical control of phenol catabolism by Pseudomonas putida

Metabolically versatile bacteria have evolved diverse strategies to adapt to different environmental niches and respond to fluctuating physico-chemical parameters. In order to survive in soil and water habitats, they employ specific and global regulatory circuits to integrate external and internal signals to counteract stress and optimise their energy status. One strategic endurance mechanism is the ability to choose the most energetically favourable carbon source amongst a number on offer. Pseudomonas putida strains possess large genomes that underlie much of their ability to use diverse carbon sources as growth substrates. Their metabolic potential is frequently expanded by possession of catabolic plasmids to include the ability to grow at the expense of seemingly obnoxious carbon sources such as phenols. However, this ability comes with a metabolic price tag. Carbon source repression is one of the main regulatory networks employed to subvert use of these expensive pathways in favour of alternative sources that provide a higher metabolic gain. This thesis identifies some of the key regulatory elements and factors used by P. putida to supress expression of plasmid-encoded enzymes for degradation of phenols until they are beneficial. I first present evidence for a newly identified DNA and RNA motif within the regulatory region of the gene encoding the master regulator of phenol catabolism – DmpR. The former of these motifs functions to decrease the number of transcripts originating from the dmpR promoter, while the latter mediates a regulatory checkpoint for translational repression by Crc – the carbon repression control protein of P. putida. The ability of Crc to form repressive riboprotein complexes with RNA is shown to be dependent on the RNA chaperone protein Hfq – a co-partnership demonstrated to be required for many previously identified Crc-targets implicated in hierarchical assimilation of different carbon sources in P. putida. Finally, I present evidence for a model in which Crc and Hfq co-target multiple RNA motifs to bring about a two-tiered regulation to subvert catabolism of phenols in the face of preferred substrates – one at the level of the regulator DmpR and another at the level of translation of the catabolic enzymes