Pseudomonas putida F1: a model for the study of chemotaxis by a soil bacterium

Pseudomonas putida F1, which was isolated by Dave Gibson from a polluted creek in Urbana, Illinois, has served a model organism for studies of aromatic hydrocarbon degradation and biocatalysis for decades. We have extended these studies to include an analysis of its complex chemosensory system for two reasons: 1) chemotaxis has been shown to enhance biodegradation, but a detailed understanding of responses to relevant chemicals is lacking; and 2) chemoreceptor proteins have the potential to be developed as biosensors for toxic chemicals. P. putida F1 is capable of growth on a diverse range of organic compounds, and we have found that it is capable of chemotaxis to most, if not all chemicals that serve as growth substrates, as well as many nonmetabolizable structural analogs. Using various techniques, including genome analysis, mutant construction, gene overexpression, and construction of chimeric receptors, we have identified the functions of 9 of the 27 putative chemoreceptors encoded in the P. putida F1 genome. Some aromatic acids, including phenylacetic, p-coumaric, and ferulic acids are indirectly sensed by a metabolism-dependent process via the energy taxis receptor Aer2. In contrast, chemotaxis to other aromatic compounds, including aromatic hydrocarbons and substituted benzoic acids is metabolism-independent, and these chemicals are sensed by specific chemoreceptors.  PcaY, for example, is a specific chemoreceptor that senses aromatic acids and hydroaromatic compounds that are degraded via the beta-ketoadipate pathway. We have successfully constructed a PcaY chimera that functions as a biosensor of aromatic acids in an Escherichia coli reporter strain.