CONICET | Buscador de Institutos y Recursos Humanos

Caulobacter crescentus is a gram-negative bacterium that grows in diluteaquatic environments and is a member of the alpha-subdivision ofproteobacteria. Much attention has been given to the study of C. crescentus signaling pathways todescribe how they control cellular development and cell-cycle progression. A system-levelstudy of two-component systems (TCS) described that the gene that codes for theresponse regulator (RR) NtrX was conditionally essential because it was only possibleto obtain a deletion strain of this gene in a minimal medium. However, the signalto which NtrX responds and its role in C.crescentus biology remains elusive. On the other hand, NtrX and its cognatehistidine kinase NtrY have been extensively studied in the pathogen Brucella abortus, where it has beenreported that they participate in the bacterial adaptation to low oxygentension. Here, we showconditions required for NtrX expression in C.crescentus, as well as a signal that triggers NtrX phosphorylation, and wedescribe the relevant role of this RR during growth in minimal media. We foundthat NtrX expression is induced by high concentrations of phosphate, despitethe fact that the system does not respond to it. Instead, we demonstrate thatacidic pH leads to NtrX phosphorylation and that this signal is physiologicallyrelevant because C. crescentusproduces the acidification of the medium upon entry to stationary phase,causing NtrX phosphorylation at this stage of the growth curve. Besides, weshow that ntrX deletion produces adecreased viability at stationary phase and a reduced resistance to acidicstress. Finally, we prove that NtrX is also phosphorylated by acidic pH in B. abortus, pointing out to a potentialconserved role across the alphaproteobacteria class.