An RNA thermosensor controls expression of virulence genes in Listeria monocytogenes

In Listeria monocytogenes, virulence genes are maximally expressed at 37degreesC, almost silent at 30degreesC and controlled by PrfA, a transcriptional activator whose expression is thermoregulated. Here, we show that the untranslated mRNA (UTR) preceding prfA, forms a secondary structure, which masks the ribosome binding region. Mutations predicted to destabilize this structure led to virulence gene expression and invasion of mammalian cells at 30degreesC. Chemical probing, native gel electrophoresis, in vitro translation, and "compensatory" and "increased stability" mutations demonstrated that the UTR switches between a structure active at high temperatures, and another inactive at low temperatures. Strikingly, when the DNA corresponding to the UTR was fused to gfp in E. coli, bacteria became fluorescent at 37degreesC, but not at 30degreesC. This mechanism of posttranscriptional thermoregulation may have important applications.