EF-hand calcium sensor, EfhP, controls transcriptional regulation of iron uptake by calcium in .

The human pathogen () poses a major risk for a range of severe infections, particularly lung infections in patients suffering from cystic fibrosis (CF). As previously reported, the virulent behavior of this pathogen is enhanced by elevated levels of Ca that are commonly present in CF nasal and lung fluids. In addition, a Ca-binding EF-hand protein, EfhP (PA4107), was partially characterized and shown to be critical for the Ca-regulated virulence in . Here, we describe the rapid (10 min, 60 min), and adaptive (12 h) transcriptional responses of PAO1 to elevated Ca detected by genome-wide RNA sequencing and show that deletion significantly hindered both rapid and adaptive Ca regulation. The most differentially regulated genes included multiple Fe sequestering mechanisms, a large number of extracytoplasmic function sigma factors (ECFσ), and several virulence factors, such as the production of pyocins. The Ca regulation of Fe uptake was also observed in CF clinical isolates and appeared to involve the global regulator Fur. In addition, we showed that the transcription is controlled by Ca and Fe, and this regulation required a Ca-dependent two-component regulatory system CarSR. Furthermore, the expression is significantly increased in CF clinical isolates and upon pathogen internalization into epithelial cells. Overall, the results established for the first time that Ca controls Fe sequestering mechanisms in and that EfhP plays a key role in the regulatory interconnectedness between Ca and Fe signaling pathways, the two distinct and important signaling pathways that guide the pathogen's adaptation to the host.IMPORTANCE () poses a major risk for severe infections, particularly in patients suffering from cystic fibrosis (CF). For the first time, kinetic RNA sequencing analysis identified rapid and adaptive transcriptional responses to Ca levels consistent with those present in CF respiratory fluids. The most highly upregulated processes include iron sequestering, iron starvation sigma factors, and self-lysis factors pyocins. An EF-hand Ca sensor, EfhP, is required for at least 1/3 of the Ca response, including the majority of the iron uptake mechanisms and the production of pyocins. Transcription of itself is regulated by Ca and Fe, and increases during interactions with host epithelial cells, suggesting the protein's important role in infections. The findings establish the regulatory interconnectedness between Ca and iron signaling pathways that shape transcriptional responses. Therefore, understanding Pa's transcriptional response to Ca and associated regulatory mechanisms will serve in the development of future therapeutics targeting 's dangerous infections.