Post-transcriptional regulation of the Pseudomonas aeruginosa heme assimilation system (Has) fine-tunes extracellular heme sensing.

Pseudomonas aeruginosa is an opportunistic pathogen that utilizes heme as a primary iron source within the host. Extracellular heme is sensed via a heme assimilation system (has) that encodes an extracytoplasmic function (ECF) σ factor system. Herein, using has deletion mutants, quantitative PCR analyses, and immunoblotting, we show that the activation of the σ factor HasI requires heme release from the hemophore HasAp to the outer-membrane receptor HasR. Using RT-PCR and 5'-RACE, we observed that following transcriptional activation of the co-transcribed hasRAp, it is further processed into specific mRNAs varying in stability. We noted that the processing and variation in stability of the hasAp and hasR mRNAs in response to heme provide a mechanism for differential expression from co-transcribed genes. The multiple layers of post-transcriptional regulation of the ECF signaling cascade, including the previously reported post-transcriptional regulation of HasAp by the heme metabolites biliverdin IXβ and IXδ, allow fine-tuning of the cell-surface signaling system in response to extracellular heme levels. We hypothesize that the complex post-transcriptional regulation of the Has system provides P. aeruginosa an advantage in colonizing a variety of physiological niches in the host.