Expression of the phytochrome operon in Pseudomonas aeruginosa is dependent on the alternative sigma factor RpoS

Phytochromes are red/far-red light photoreceptors found in plants, cyanobacteria and heterotrophic bacteria. Biochemical analyses have established that the genes bphO and bphP (PA4116 and PA4117) of Pseudomonas aeruginosa encode both phytochrome components: BphO, a heme oxygenase that produces the linear tetrapyrrole chromophore biliverdin IXα, and BphP, the apo-phytochrome. Reverse transcription-PCR established that both genes form a bicistronic operon. Expression of the bphOP operon was induced in the stationary phase, indicating an involvement of the P. aeruginosa quorum-sensing system and/or the stationary-phase alternative sigma factor RpoS. Bioinformatic analyses of the promoter region revealed a potential binding site for the quorum sensing regulators LasR and/or RhlR. While a direct involvement of the quorum-sensing system could be ruled out, the dependence of bphOP expression on RpoS was clearly demonstrated. Chromosomal knock-out mutants showed identical growth behavior as a wild type under various conditions but increased levels of pyocyanin were detected in the ΔbphO strain. Additionally, this strain showed decreased heat tolerance in the stationary phase, indicating a potential protective role of the BphO reaction product biliverdin. Therefore, BphO might have an additional function besides providing the chromophore for BphP and both proteins are likely to fulfill a task in the stationary phase.