Abstract
Isogenic populations of cells exhibit phenotypic variability that has specific physiological consequences. For example, individual bacteria can differ in their sensitivity to an antibiotic, but whether this variability is regulated or an unavoidable consequence of stochastic fluctuations is unclear. We observed that a bacterial stress response gene, the (p)ppGpp synthetase sasA, exhibits high levels of extrinsic noise in expression, suggestive of a regulatory process. We traced this variability to the convergence of two signaling systems that together control an event largely unexplored in bacteria, the multisite phosphorylation of a transcription factor. We found that this regulatory intersection is crucial for controlling the appearance of outliers, rare cells with unusually high levels of sasA expression. Additionally, by examining the full distributions of gene expression we calculated the importance of multisite phosphorylation in setting the relative abundance of cells with a given a level of SasA. We then created a predictive model for the probability of a given cell surviving antibiotic treatment as a function of sasA expression. Therefore, our data show that multisite phosphorylation can be used to strongly regulate bacterial physiology and sensitivity to antibiotic treatment.
