Abstract
Francisella tularensis is a highly infectious pathogen that infects animals and humans to cause the disease tularemia. The primary targets of this bacterium are macrophages, in which it replicates in the cytoplasm after escaping the initial phagosomal compartment. The ability to replicate within macrophages relies on the tightly regulated expression of a series of genes. One of the most commonly used means of coordinating the regulation of multiple genes in bacteria consists of the association of dedicated alternative sigma factors with the core of the RNA polymerase (RNAP). In silico analysis of the F. tularensis LVS genome led us to identify, in addition to the genes encoding the RNAP core (comprising the α1, α2, β, β' and ω subunits), one gene (designated rpoD) encoding the major sigma factor σ70, and a unique gene (FTL_0851) encoding a putative alternative sigma factor homologue of the σ32 heat-shock family (designated rpoH). Hence, F. tularensis represents one of the minority of bacterial species that possess only one or no alternative sigma factor in addition to the main factor σ70. In the present work, we show that FTL_0851 encodes a genuine σ32 factor. Transcriptomic analyses of the F. tularensis LVS heat-stress response allowed the identification of a series of orthologues of known heat-shock genes (including those for Hsp40, GroEL, GroES, DnaK, DnaJ, GrpE, ClpB and ClpP) and a number of genes implicated in Francisella virulence. A bioinformatic analysis was used to identify genes preceded by a putative σ32-binding site, revealing both similarities to and differences from RpoH-mediated gene expression in Escherichia coli. Our results suggest that RpoH is an essential protein of F. tularensis, and positively regulates a subset of genes involved in the heat-shock response.