Noise regulation by quorum sensing in low mRNA copy number systems

We present a stochastic model of gene expression that accounts for the main biochemical processes that describe the QS mechanism close to its activation threshold. Within that framework we study, both numerically and analytically, the role that diffusion plays in the regulation of the dynamics and the fluctuations of signaling molecules. In addition, we unveil the contribution of different sources of noise, intrinsic and transcriptional, in the QS mechanism.The interplay between noisy sources and the communication process produces a repertoire of dynamics that depends on the diffusion rate. Importantly, the total noise shows a non-monotonic behavior as a function of the diffusion rate. QS systems seems to avoid values of the diffusion that maximize the total noise. These results point towards the direction that bacteria have adapted their communication mechanisms in order to improve the signal-to-noise ratio.Gene regulation at the transcriptional level is one of the corner stones of molecular and cellular biology [1]. Recent studies in prokaryotes have revealed the existence of antisense and alternative transcripts and multiple regulators per gene that imply a highly dynamic transcriptome more similar to that of eukaryotes than first thought [2]. Still, prokaryotic gene regulation mainly relies on the binding of regulatory proteins that attach to DNA for either stimulating or repressing transcription. These binding/unbinding events are intrinsically probabilistic because of the significance of thermal fluctuations at that scale and the low number of molecules involved in the process. In this regard, over the past years a growing number of experiments have indeed characterized not only the levels of randomness in cellular biochemical processes but also their functionality [3-8].Technical advances such as the use of fluorescent tags in single-cell experiments have allowed for quantitative measurements of the noise in protein concentration and have shed light on the me