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In the food-borne pathogen Listeria monocytogenes, virulence genes are expressed at 37°C, but not 30°C. This process is under the control of PrfA — a transcriptional activator thermoregulated by a mechanism that has remained unclear. In September 6 Cell, Jörgen Johansson and colleagues from Institut Pasteur, Paris, show that an untranslated mRNA (UTR) preceding prfA acts as a thermosensor and controls Listeria monocytogenes expression of virulence genes at 37°C (Cell 2002, 110:551–561).

Johansson et al. observed that the UTR preceding prfA formed a secondary structure, masking the ribosome-binding region. This structure could switch between an active form at high temperatures, and an inactive form at low temperatures. Mutations predicted to destabilize this structure led to virulence gene expression and invasion of mammalian cells at 30°C. In addition, when DNA corresponding to the UTR was fused to a fluorescent marker in E. coli, the bacteria...