Metabolite-driven regulation of heme uptake by the biliverdin IXβ/δ-selective heme oxygenase (HemO) of Pseudomonas aeruginosa

Pseudomonas aeruginosa acquires extracellular heme via the Phu (Pseudomonas heme uptake) and Has (heme assimilation system) systems. We have previously shown the catalytic actions of heme oxygenase (HemO) along with the cytoplasmic heme transport protein PhuS control heme flux into the cell. To further investigate the role of the PhuS-HemO couple in modulating heme uptake, we have characterized two HemO variants, one that is catalytically inactive (HemO H26A/K34A/K132A or HemO_in) and one that has altered regioselectivity (HemO N19K/K34A/F117Y/K132A or HemO_α), producing biliverdin IXα (BVIXα). HemO_α similar to wild type was able to interact and acquire heme from holo-PhuS. In contrast, the HemO_in variant did not interact with holo-PhuS and showed no enzymatic activity. Complementation of a hemO deletion strain with the hemO_in or hemO_α variants in combination with [¹³C]heme isotopic labeling experiments revealed that the absence of BVIXβ and BVIXδ leads to a decrease in extracellular levels of hemophore HasA. We propose BVIXβ and/or BVIXδ transcriptionally or post-transcriptionally regulates HasA. Thus, coupling the PhuS-dependent flux of heme through HemO to feedback regulation of the cell surface signaling system through HasA allows P. aeruginosa to rapidly respond to fluctuating extracellular heme levels independent of the iron status of the cell.