The UzcRS two-component system integrates regulatory input from a diverse set of auxiliary regulators

The UzcRS two-component system in Caulobacter crescentus functions as a global activator of an extracytoplasmic stress response pathway and is induced by the metals U, Zn and Cu. Unexpectedly, a screen for mutations that affected the activity of the UzcR-regulated urcA promoter (PurcA) revealed four previously uncharacterized proteins whose inactivation led to metal-independent induction of PurcA. Using molecular genetics and functional genomics, we found that these auxiliary regulators regulate PurcA expression by modulating the activity of UzcRS through distinct mechanisms. The membrane protein UzcX, whose expression is positively regulated by UzcR, forms a regulatory complex with UzcS, providing a mechanism of feedback inhibition within the UzcRS circuit. UrtAP, an ABC transporter with a periplasmic metallo-aminopeptidase domain, negatively regulates UzcRS activity and antagonizes metal dependent stimulation by virtue of its enzymatic activity. Lastly, two MarR family transcription factors, MarR1 and MarR2, synergistically regulate UzcRS activity by repressing the expression of the membrane proteins UzcY and UzcZ, respectively, which function as UzcRS activators. In addition to modulating basal UzcRS activity, UzcY and UzcZ enhance the U/Zn/Cu sensitivity of UzcRS, shifting its responsiveness to more environmentally-relevant metal concentrations. Collectively, these data suggest that UzcRS functions as the core-signaling unit within a multicomponent signal transduction pathway that includes an unusually diverse set of auxiliary regulators. Gene expression profiles of ΔuzcR, ΔuzcX ΔuzcR, ΔurtAP ΔuzcR, ΔmarR1 ΔuzcR, ΔmarR2 ΔuzcR strains were determined in triplicate