is an opportunistic pathogen causing life-threatening infections. Previously, we showed that elevated calcium (Ca) levels increase the production of virulence factors in In the effort to characterize the Ca regulatory network, we identified a – egulated β- ropeller protein, CarP, and showed that expression of the encoding gene is controlled by the Ca-regulated two-component system CarSR. Here, by using a model, we showed that CarP plays a role in regulating virulence. By using RNA-Seq, RT-PCR, RT-qPCR, and promoter fusions, we determined that is transcribed into at least two transcripts and regulated by several bacterial and host factors. The transcription of is elevated in response to Ca in cystic fibrosis isolates and PAO1 laboratory strain. Elevated Fe also induces Simultaneous addition of Ca and Fe increased the promoter activity synergistically, which requires the presence of CarR. analysis of the intergenic sequence upstream of predicted recognition sites of RhlR/LasR, OxyR, and LexA, suggesting regulation by quorum sensing (QS) and oxidative stress. In agreement, the promoter was activated in response to stationary-phase PAO1 supernatant and required the presence of elevated Ca and CarR, but remained silent in the triple mutant lacking and synthases. We also showed that transcription is regulated by oxidative stress and that CarP contributes to Ca-dependent HO tolerance. The multifactorial regulation of suggests that CarP plays an important role in adaptations to host environments. is a human pathogen causing life-threatening infections. It is particularly notorious for its ability to adapt to diverse environments within the host. Understanding the signals and the signaling pathways enabling adaptation is imperative for developing effective therapies to treat infections caused by this organism. One host signal of particular importance is calcium. Previously, we identified a component of calcium-signaling network, CarP, whose expression is induced by elevated levels of calcium. Here we show that plays an important role in virulence and is up-regulated in strains isolated from sputa of patients with cystic fibrosis. We also identified several bacterial and host factors that regulate the transcription of Such multifactorial regulation highlights the interconnectedness between regulatory circuits and together with the pleotropic effect of CarP on virulence suggests the importance of this protein in adaptations to the host.
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