RyhB, an iron-responsive small RNA molecule, regulates Shigella dysenteriae virulence.

Regulation of bacterial gene expression by small RNA (sRNA) molecules is an increasingly recognized phenomenon but one that is not yet fully understood. We show that the sRNA RyhB suppresses several virulence-associated phenotypes of Shigella dysenteriae, a causative agent of bacillary dysentery in humans. The virulence genes repressed by S. dysenteriae RyhB include those encoding the type III secretion apparatus, its secreted effectors, and specific chaperones. Suppression of Shigella virulence occurs via RyhB-dependent repression of the transcriptional activator VirB, leading to reduced expression of genes within the VirB regulon. Efficient repression of virB is mediated by a single-stranded region of RyhB that is distinct from the region required for repression of Shigella sodB. Regulation of virB by RyhB implicates iron as an environmental factor contributing to the complex regulation of Shigella virulence determinants. Shigella species, which are closely related to Escherichia coli, cause bacillary dysentery, a disease associated with invasion of the colonic epithelium and provocation of an intense inflammatory response (12). Using cultured epithelial cells to measure bacterial invasion (invasion assay) (8) and intercellular spread (plaque assay) (22), investigators have identified a number of the genes required for Shigella pathogenesis. Many Shigella virulence-associated genes, including those encoding the type III secretion apparatus (mxi and spa genes), its secreted effectors (ipaA-D, ipgD, icsB, and virA), and specific chaperones (ipgC, ipgA, ipgE, and spa15), map to a 220-kbp virulence plasmid. Expression of these Shigella virulence determinants is highly regulated in response to environmental signals, such as temperature, osmolarity, and pH (12). This complex regulation is accomplished primarily by the transcriptional activators VirF and VirB (1). Transcription of many Shigella virulence-associated genes, including those encoding the type III secretion system (TTSS), is positively regulated by direct binding of VirB to the regulated promoters. Transcription of virB, in turn, is controlled primarily by the opposing activities of VirF, a transcriptional activator, and HNS, a transcriptional repressor. (For a review of the regulation of Shigella virulence gene expression, see reference 6.) VirF and VirB are both required to induce expression of virulence determinants that allow efficient invasion of epithelial cells by Shigella, an essential step in disease initiation. While the regulation of Shigella virulence has been the focus of intense investigation, the identification and characterization of all contributing environmental factors and regulatory elements have not yet been achieved. Noncoding RNA molecules (ncRNAs) control diverse cellular functions in organisms ranging from bacteria to humans. Although it is known that ncRNAs employ a variety of mechanisms, including methylation of rRNA, inhibition of translation or transcription, and sequestration of regulatory proteins (11, 13, 21), the full spectrum of ncRNAs, their mechanism of action, and their impact on cellular activities remain to be