The two-component system PrIS/PrIR of Brucella melitensis is required for persistence in mice and appears to respond to ionic strength

Bacterial adaptation to environmental conditions is essential to ensure maximal fitness in the face of several stresses. In this context, two-component systems (TCSs) represent a predominant signal transduction mechanism, allowing an appropriate response to be mounted when a stimulus is sensed. As facultative intracellular pathogens, Brucella spp. face various environmental conditions, and an adequate response is required for a successful infection process. Recently, bioinformatic analysis of Brucella genomes predicted a set of 15 bona fide TCS pairs, among which some have been previously investigated. In this report, we characterized a new TCS locus called prlSIR, for probable proline sensor regulator. It encodes a hybrid histidine kinase (PrIS) with an unusual Na+/solute symporter N-terminal domain and a transcriptional regulator (belonging to the LuxR family) (PrIR). In vitro, Brucella spp. with a functional PrIR/S system form bacterial aggregates, which seems to be an adaptive response to a hypersaline environment, while a prlSIR mutant does not. We identified ionic strength as a possible signal sensed by this TCS. Finally, this work correlates the absence of a functional PrIR/S system with the lack of hypersaline-induced aggregation in particular marine Brucella spp.