Small regulatory RNAs control the multi-cellular adhesive lifestyle of Escherichia coli

Small regulatory RNA molecules have recently been recognized as important regulatory elements of developmental processes in both eukaryotes and bacteria. We here describe a striking example in Escherichia coli that can switch between a single-cell motile lifestyle and a multi-cellular, sessile and adhesive state that enables biofilm formation on surfaces. For this, the bacterium needs to reprogramme its gene expression, and in many E. coli and Salmonella strains the lifestyle shift relies on control cascades that inhibit flagellar expression and activate the synthesis of curli, extracellular adhesive fibres important for co-aggregation of cells and adhesion to biotic and abiotic surfaces. By combining bioinformatics, genetic and biochemical analysis we identified three small RNAs that act by an antisense mechanism to downregulate translation of CsgD, the master regulator of curli synthesis. Our demonstration that basal expression of each of the three RNA species is sufficient to downregulate CsgD synthesis and prevent curli formation indicates that all play a prominent role in the curli regulatory network. Our findings provide the first clue as to how the Rcs signalling pathway negatively regulates curli synthesis and increase the number of small regulatory RNAs that act directly on the csgD mRNA to five.