Chemotaxis is the ability of motile bacteria to detect and respond to specific chemicals in the environment, moving up gradients of attractant compounds and away from repellents. Although diverse bacteria appear to have conserved chemotaxis signal transduction systems, soil bacteria seem to have more complex chemosensory systems than the well-studied enteric bacteria. For example, enteric bacteria such as Escherichia coli have a relatively small number of methyl-accepting chemotaxis proteins (MCPs), which is likely to reflect the range of available carbon sources present in the ecological niche in which they reside. In contrast, the available sequenced genomes of members of the genus Pseudomonas have between 26 and 49 annotated MCP-like proteins. Soil bacteria such as the pseudomonads are known for their catabolic diversity and with such a large number of MCPs, they appear to have equally broad chemosensory systems. However, few studies have attempted to characterize the range of chemotactic responses or identify specific chemoreceptors in soil bacteria. This research focuses on the relationship between the biodegradation of environmental pollutants and the ability of degradative bacteria to sense these toxic chemicals by using specific chemotaxis systems. The overall goal of this project is to characterize the chemotactic responses of a biodegradative bacterium to aromatic hydrocarbons and related pollutants. The catabolically versatile aromatic hydrocarbon-degrading bacterium Pseudomonas putida F1 will be used as the model organism for these studies. P. putida F1 has the ability to sense and respond chemotactically to several aromatic hydrocarbons including toluene, benzene, ethylbenzene, and p-cymene, all of which serve as carbon and energy sources for the strain. In addition, this strain is also attracted to the chlorinated hydrocarbon pollutants trichloroethylene, cis-dichloroethylene, and perchloroethylene. The responses to these compounds are inducible--current data from the Principal Investigator's laboratory indicate that P. putida F1 contains at least two chemoreceptors to mediate these responses. At this time, however, nothing further is known about the specific receptors involved in chemotaxis to toluene and related pollutants in P. putida F1. The project will investigate the relationship between aromatic hydrocarbon metabolism, chemotaxis, and gene expression in P. putida F1, focusing primarily on the toluene and p-cymene chemotactic responses. This work will be facilitated by the availability of the complete genome sequence of P. putida F1. The results obtained in this study will increase our understanding of the breadth of bacterial chemosensory systems and how bacteria detect and respond to toxic aromatic hydrocarbons and man-made chlorinated alkenes.
Broader impacts: This research project will contribute to the education and training of high school, undergraduate, and graduate students, providing them with valuable research experience. At the University of California-Davis, the project will provide research experience for two graduate students, one of whom will focus primarily on toluene chemotaxis, and the other on p-cymene chemotaxis. Part-time undergraduate researchers will work on the project at UC Davis during the academic year, and high school students in the Young Scholars Program and undergraduates in the Summer Undergraduate Research Program at UC Davis will also participate in the research. At the University of St. Thomas (UST) research will be integrated into the Biology curriculum in a new Bioinformatics course in which students will actively annotate the P. putida F1 genome while they carry out wet laboratory research projects with this organism. Summer UST students will carry out full time research on chemotaxis in P. putida F1, and one student per summer will travel to UC Davis to work for one month with graduate students working on the project.
Certain bacteria are known to be chemotactic, that is they are physically attracted to specific chemicals in the environment. The attraction is usually directed towards chemicals that the organism is able to metabolize and harness as sources of energy. This process is controlled by protein receptors located on the surface of the cell that recognize specific chemicals that are of particular interest. Once a chemical is detected, these receptors initiate a series of biochemical reactions that lead the organism towards that chemical. Most bacteria are attracted to compounds like sugars and amino acids; however, certain soil bacteria like Pseudomonas putida F1 display chemotactic behavior towards toxic compounds such as benzene and toluene. What is also unique about these bacteria is that they are able to utilize these toxic compounds as a food source. The ultimate goal of these studies is to determine whether chemotaxis plays a role in the bacterial breakdown of pollutants. We characterized the chemotactic responses of bacteria to various man-made pollutants and similar nontoxic chemicals. We also determined which receptors are important for the ability to detect these compounds in the environment, and identified the specific genes that encode the relevant cell surface receptors. In several cases, genes encoding the degradation of a chemical and the receptor that senses that particular chemical were controlled by the same mechanism. Some receptors sensed a particular chemical or set of related chemicals directly, others sensed an intermediate in the chemical's breakdown, while others sensed the energy gained by the cell during the chemical breakdown. These findings have provided a better understanding of how bacteria sense and respond to chemicals and may result in new solutions for the clean up of chemical toxins in the environment. This project contributed to the education and training of high school, undergraduate, and graduate students at the University of California Davis and the University of St. Thomas, providing them with valuable research experience. Research at the University of St. Thomas was integrated into the Biology curriculum in a new Bioinformatics course in which students used computer programs to hypothesize the functions of uncharacterized receptor genes in Pseudomonas putida F1. They then tested their hypotheses in the laboratory to determine whether the genes of interest encoded proteins involved in the chemotactic response of this organism to various chemical attractants. In addition to the students in the bioinformatics course, three graduate students, twenty-two undergraduates and six high school students participated in the research described here. Three of the University of St. Thomas undergraduates also carried out summer research projects at the University of California Davis.
