Regulatory interplay between RNase III and asRNAs in E. coli; the case of AsflhD and the master regulator of motility, flhDC
In order to respond to ever-changing environmental cues, bacteria have evolved resilient regulatory mechanisms controlling gene expression. At the post-transcriptional level, this is achieved by a combination of RNA-binding proteins, such as ribonucleases (RNases) and RNA chaperones, and regulatory RNAs including antisense RNAs (asRNAs). AsRNAs bound to their complementary mRNA are primary targets for the double-strand-specific endoribonuclease, RNase III. By comparing primary and processed transcripts in an rnc strain, mutated for RNase III, and its isogenic wild type strain, we detected several asRNAs. We confirmed the existence of RNase III-sensitive asRNA for crp, ompR, phoP and flhD genes, encoding master regulators of gene expression. AsflhD, the asRNA to the master regulator of motility flhDC, is slightly induced under heat-shock conditions in a sigma24 (RpoE)-dependent manner. We demonstrate that expression of AsflhD asRNA is involved in the transcriptional attenuation of flhD and thus participates in the control of the whole motility cascade. This study demonstrates that AsflhD and RNase III are additional players in the complex regulation ensuring a tight control of flagella synthesis and motility. ImportanceThe importance of asRNAs in the regulation of gene expression has long been underestimated. Here, we confirm that asRNAs can be part of layered regulatory networks since some are found opposite to genes encoding global regulators. In particular, we show how an antisense RNA (AsflhD) to the gene expressing a transcription factor serving as the primary regulator of bacterial swimming motility (FlhD4C2) is involved in the transcriptional attenuation of flhD, which in turn impacts the expression of other genes of the motility cascade. The role of AsflhD highlights the importance of discrete fine-tuning mechanisms in the control of complex regulatory networks.