Effects of multiple cell regulators on curli gene expression in <i>Escherichia coli</i>

Curli amyloid fibers are the key protenacious component of the extracellular matrix in <i>Escherichia coli</i>. The regulation of curli expression is highly complex and depends on multiple environmental responses. While many genetic determinants of curli production are known, previous studies have been conducted under various conditions and using different strains of <i>E. coli</i> or <i>Salmonella</i>. Furthermore, while the expression of curli genes in an <i>E. coli</i> population is known to be bimodal, the origins of this bimodality are only partially understood. Here, we systematically investigated the role of various cellular factors in the expression of the curli structural genes <i>csgBA</i> at the single-cell level in planktonic <i>E. coli</i> culture. We observed that multiple factors involved in the regulation of stress response, cell motility, cell physiology and metabolism, maintenance of DNA architecture, and second messenger signaling either promote or repress the expression of <i>csgBA</i> genes. We further elucidated which regulators act upstream of the master transcription factor CsgD and which are crucial for the bimodality of curli gene expression. Overall, this study provides an overview of the regulation of curli gene expression in planktonic <i>E. coli</i> culture in the absence of any microenvironmental gradients.IMPORTANCEThe transition from a solitary planktonic lifestyle to a multicellular biofilm is a complex developmental process involving multiple changes in bacterial cell physiology. For <i>Enterobacteriaceae</i>, a critical step in this process is the production of curli amyloid fibers, the main component of their extracellular matrix. A commitment to express curli genes already occurs in a subpopulation of planktonically growing <i>Escherichia coli</i> cells. Here, we investigated how this activation depends on multiple stress response factors, global regulators of gene expression, and the second messenger cyclic-di-GMP. We demonstrated that bimodal expression of curli structural genes in planktonic cultures requires an interplay between several transcription factors and chromosome-organizing proteins but not second messenger signaling.