Contributions of the Heme Coordinating Ligands of the Outer Membrane Receptor HasR to Extracellular Heme Sensing and Transport

exhibits a high requirement for iron, which it can acquire via several mechanisms, including the acquisition and utilization of heme. The genome encodes two heme uptake systems, the heme assimilation system (Has) and the heme utilization (Phu) system. Extracellular heme is sensed via the Has system, which encodes an extracytoplasmic function (ECF) σ factor system. Previous studies have shown that the transfer of heme from the extracellular hemophore HasAp to the outer membrane receptor HasR is required for activation of the σ factor HasI and upregulation of operon expression. Here, employing site-directed mutagenesis, allelic exchange, quantitative PCR analyses, immunoblotting, and C-heme uptake experiments, we delineated the differential contributions of the extracellular FRAP/PNPNL loop residue His-624 in HasR and of His-221 in its N-terminal plug domain required for heme capture to heme transport and signaling, respectively. Specifically, we show that substitution of the N-terminal plug His-221 disrupts both signaling and transport, leading to dysregulation of both the Has and Phu uptake systems. Our results are consistent with a model wherein heme release from HasAp to the N-terminal plug of HasR is required to initiate signaling, whereas His-624 is required for simultaneously closing off the heme transport channel from the extracellular medium and triggering heme transport. Our results provide critical insight into heme release, signaling, and transport in and suggest a functional link between the ECF σ factor and Phu heme uptake system.