A quorum-sensing regulator controls expression of both the type IV secretion system and the flagellar apparatus of Brucella melitensis

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Abstract
Both a type IV secretion system and a flagellum have been described in Brucella melitensis. These two multimolecular surface appendages share several features. Their expression in bacteriological medium is growth curve dependent, both are induced intracellularly and are required for full virulence in a mouse model of infection. Here we report the identification of VjbR, a quorum sensing-related transcriptional regulator. A vjbR mutant has a downregulated expression of both virB operon and flagellar genes either during vegetative growth or during intracellular infection. In a cellular model, the vacuoles containing the vjbR mutant or a virB mutant are decorated with the same markers at similar times post infection. The vjbR mutant is also strongly attenuated in a mouse model of infection. As C12-homoserine lactone pheromone is known to be involved in virB repression, we postulated that VjbR is mediating this effect. In agreement with this hypothesis, we observed that, as virB operon, flagellar genes are controlled by the pheromone. All together these data support a model in which VjbR acts as a major regulator of virulence factors in Brucella.