Pirin, a redox-sensitive modulator of beta-oxidation, generates hydroxyl radicals and interacts with CatR, the transcriptional repressor of the major vegetative catalase gene in Streptomyces.

Pirins are iron-containing proteins conserved throughout evolution, which have been implicated in diverse cellular processes, mostly associated with stress. In prokaryotes, Pirins are present in many taxonomic groups and can be present in multiple copies, and only a few of these proteins have been studied. In Streptomyces ambofaciens a Pirin-like protein, PirA, is a redox-sensitive negative modulator of AcdB, a very long-chain acyl-CoA dehydrogenase (vLCAD), which catalyzes the first committed step of the beta-oxidation pathway. In this study, we first classified the Pirins in different prokaryotic and non-prokaryotic taxa, and we found strong connections between the occurrence of Pirins and aerobic energy metabolism. We then studied whether the presence of Pirins is connected to the regulation of antioxidant systems, after observing that a pirA-defective mutant of S. ambofaciens accumulated large amounts of HO during the vegetative growth. In vitro experiments suggested that the accumulation of HO in the pirA mutant could be partially due to an increased vLCAD activity of AcdB, which releases HO as a byproduct of the reaction. However, the reduction in catalase and alkylhydroperoxidase expression levels in the pirA mutant, despite the increased amount of intracellular HO, also indicated a dysregulation of these antioxidant systems. Indeed, the gene catA encoding the major vegetative catalase and the adjacent regulatory gene catR were down-regulated in the pirA mutant. In Gram-positive bacteria, CatR/PerR family regulators repress transcription of genes coding for the major vegetative catalase. These repressors are irreversible inactivated by hydroxyl radicals and detach from the DNA, which leads to de-repression of catalase expression. We found that PirA generates hydroxyl radicals after reacting with HO, and binds CatR in vitro, suggesting that by generating hydroxyl radicals in response to HO exposure, PirA could promote CatR inactivation allowing catA transcription.