Bacterial cell-cell communication, also termed quorum sensing (QS) is a wide-spread process that coordinates multicellular behaviors such as virulence, biofilm formation, and nutrient acquisition in response to cell density, population structure and environmental viscosity. There has been an explosion in research directed at understanding the molecular mechanisms of QS, but there is a paucity of information on the ecophysiological implications and on the emergent properties of QS regulatory networks. The current project addresses this need by combining genetics, physiology, and systems biology in understanding QS in the model bacterium Pseudomonas aeruginosa. This bacterium communicates via diffusible acyl-homoserine lactone signals to control the expression of hundreds of genes. The particular focus is on two central properties of the P. aeruginosa QS network, antiactivation and co-regulation. Antiactivation, initially characterized in the plant pathogen Agrobacterium tumefaciens, inhibits the activity of cognate QS receptors through direct protein-protein interaction. Co-regulation permits the integration of other environmental signals into the quorum response. A key feature here is the starvation-dependent transcription of the main P. aeruginosa QS receptor, LasR. Because several QS-controlled products are costly extracellular enzymes involved in nutrient acquisition, co-regulation by starvation appears ecologically worthwhile. The roles of antiactivation and lasR regulation in modulating the quorum response and in preventing "short-circuiting" will be investigated. Short-circuiting, or self-induction, is a major unanswered question in bacterial QS: How is it that diffusible quorum-signals do not immediately bind to their cognate receptors in the same cell in which they are produced and activate gene expression independent of cell density? Based on recent modeling data, the PIs hypothesize that antiactivation and lasR regulation help prevent short-circuiting, and that the tight environmental control of lasR expression is key in modulating quorum responses that are either short-circuited, triggered by cell density, or triggered by starvation. The specific aims of the project, which integrate experimentation and computational modeling, are therefore to (1) directly observe short-circuiting of QS target gene expression in antiactivator-deficient and lasR overexpressing cells, (2) investigate the growth-rate dependence of QS gene induction and short-circuiting in wild-type cells, and (3) develop a model of the las QS network that, in addition to antiactivation and co-regulation, incorporates and evaluates key properties such as receptor-QS signal interaction, receptor dimerization, autoregulation, and active efflux.
Broader impacts Research. The research conducted by the PIs over the last decade, funded in part by NSF, has established P. aeruginosa QS as a global regulatory network, has provided insight into the function of the central QS regulator LasR, has demonstrated that QS is a cooperative behavior subject to social conflict, and has resulted in the first computational model of P. aeruginosa QS. The current project will incorporate and extend these findings to understand the basic design features of a QS network, including antiactivation and the integration of environmental cues. The work will broadly benefit and will find application in synthetic biology and biotechnology for the design of novel genetic response circuits. Education. The described project provides excellent educational opportunities for students. The PIs have and will continue to train graduate and undergraduate students. Dr. Schuster will also provide educational opportunities for high-school students through the Apprenticeship for Science and Engineering, an established summer internship program at Oregon State University. Many of the proposed experiments are conceptually and technically straight-forward and are particularly well suited for the engagement of high-school and undergraduate students in the scientific process.
