| Abstract | Many bacteria modify their behavior in response to the concentration of a diffusible signal molecule that they produce. This signal is commonly believed to be an indicator of local cell density, so the phenomenon is known as quorum sensing. In the soil dwelling bacterium Pseudomonas fluorescens, quorum sensing is regulated by the Gac/Rsm network of genes; similar networks are found in many other γ-proteobacteria. These systems are noteworthy in part because of the central role played by small untranslated RNAs, which are involved in the post-transcriptional regulation of target genes. I am presenting a model of the Gac/Rsm regulatory network, consisting of nonlinear differential equations for the cell population and the concentrations of proteins and RNAs in the system. The model can be parameterized to agree with published reporter protein data, despite the fact that aspects of the system are not yet well understood. I have investigated the signal-processing behavior of the network, which allows the bacteria to be hyper-sensitive to fluctuations in the signal concentration, while remaining robust to the noise inherent in gene regulation. This provides clues about the function of feedbacks within the rather complex network. Like other models of quorum sensing, this one predicts a hysteretic response to population density. However, hysteresis can be achieved by a much simpler positive feedback loop. By focusing on the effects of transient fluctuations in signal concentration, I show that the complex structure of quorum sensing systems may result from the conflicting demands of amplifying external signals while filtering out internally generated noise. |
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