RESERACH SUMMARY

To be competitive in the environments they colonize, bacteria should optimize metabolism by attaining maximum gain from available nutrients at a minimum energetic cost. Not all potential carbon sources are equally effective in this respect. Probably for this reason, when confronted by a mixture of potentially assimilable compounds at sufficient concentrations, many bacteria preferentially use one of them, the non-preferred compounds being ignored until the preferred one is consumed.

This selection implies a complex regulatory process generally known as catabolite repression control. Unravelling the molecular mechanisms underlying these regulatory events helps to understand how bacteria coordinate their metabolism and their gene expression programs. In addition, it has implications in the design and optimization of biotechnological processes and is important for learning how bacteria degrade compounds in nature.

This is particularly true in the case of compounds that are difficult to degrade and that tend to accumulate in the environment, creating pollution problems. Hydrocarbons, which frequently pose important pollution problems, are a particularly relevant example of non-preferred compounds for most bacteria. The influence of catabolite repression goes beyond the optimization of metabolism, since it also affects virulence and antibiotic resistance in pathogenic bacteria.

Our aim is to characterize the global regulation networks responsible for catabolite repression, identifying their components, the signals to which they respond, and the molecular mechanisms by which they regulate gene expression. The regulatory proteins involved in these networks are different in distinct microorganisms.

We use Pseudomonas putida as an experimental model because it is metabolically very versatile; it colonizes very diverse habitats, and is widely used in biotechnology. We are currently focused on two catabolite repression networks.

One relies on the Crc protein. Our work has shown that Crc binds to an unpaired A-rich sequence located at the translation initiation region of some mRNAs, thereby inhibiting their translation. Many Crc targets are found at genes involved in the uptake and assimilation of diverse compounds, but targets can be found as well in genes implicated in other cellular processes.

The other regulatory network under study receives signals from the Cyo terminal oxidase, a component of the electron transport chain, thereby coordinating respiration with metabolic needs.

Finally, we are analyzing the influence of growth temperature on catabolite repression. This can be relevant for several biotechnological applications. We found that, at low temperatures, repression is relieved at some genes, but not at other ones. The reasons for this are currently being investigated.