SifR is an Rrf2-family quinone sensor associated with catechol iron uptake in Streptococcus pneumoniae D39
Yifan Zhang1, Julia E Martin2, Katherine A Edmonds3
1Department of Chemistry, Indiana University, Bloomington, Indiana, USA; Department of Molecular and Cellular Biochemistry, Indiana University, Bloomington, Indiana, USA.
Abstract:
Streptococcus pneumoniae (pneumococcus) is a Gram-positive commensal and human respiratory pathogen. How this bacterium satisfies its nutritional iron (Fe) requirement in the context of endogenously produced hydrogen peroxide is not well understood. Here, we characterize a novel virulence-associated Rrf2-family transcriptional repressor that we term SifR (streptococcal IscR-like family transcriptional repressor) encoded by spd_1448 and conserved in Streptococci. Global transcriptomic analysis of a ΔsifR strain defines the SifR regulon as genes encoding a candidate catechol dioxygenase CatE, an uncharacterized oxidoreductase YwnB, a candidate flavin-dependent ferric reductase YhdA, a candidate heme-based ferric reductase domain-containing protein and the Piu (pneumococcus iron uptake) Fe transporter (piuBCDA). Previous work established that membrane-anchored PiuA binds FeIII-bis-catechol or monocatechol complexes with high affinity, including the human catecholamine stress hormone, norepinephrine. We demonstrate that SifR senses quinone via a single conserved cysteine that represses its regulon when in the reduced form. Upon reaction with catechol-derived quinones, we show that SifR dissociates from the DNA leading to regulon derepression, allowing the pneumococcus to access a catechol-derived source of Fe while minimizing reactive electrophile stress induced by quinones. Consistent with this model, we show that CatE is an FeII-dependent 2,3-catechol dioxygenase with broad substrate specificity, YwnB is an NAD(P)H-dependent quinone reductase capable of reducing the oxidized and cyclized norepinephrine, adrenochrome, and YhdA is capable of reducing a number of FeIII complexes, including PiuA-binding transport substrates. These findings are consistent with a model where FeIII-catechol complexes serve as significant nutritional Fe sources in the host.
Insights
Streptococcus pneumoniae uses a novel repressor, SifR, to sense and utilize iron from host catechols. This mechanism helps the bacterium acquire iron while avoiding stress from quinones.
Area of Science:
- Microbiology
- Molecular Biology
- Biochemistry
Background:
- Streptococcus pneumoniae requires iron for survival and virulence.
- The mechanisms by which pneumococcus acquires iron in the host, especially under oxidative stress, are not fully understood.
Purpose of the Study:
- To characterize a novel transcriptional repressor, SifR, involved in iron acquisition in Streptococcus pneumoniae.
- To elucidate the regulatory network controlled by SifR and its role in utilizing host-derived iron sources.
Main Methods:
- Global transcriptomic analysis of a ΔsifR mutant.
- Biochemical characterization of SifR's interaction with quinones.
- Enzymatic assays for CatE, YwnB, and YhdA.
Main Results:
- SifR represses genes involved in iron uptake and catechol metabolism, including the Piu transporter.
- SifR senses catechol-derived quinones via a conserved cysteine residue.
- Upon quinone binding, SifR dissociates from DNA, leading to derepression and iron acquisition.
- CatE, YwnB, and YhdA enzymes facilitate the processing and reduction of iron-catechol complexes.
Conclusions:
- SifR acts as a quinone sensor, enabling Streptococcus pneumoniae to utilize Fe(III)-catechol complexes as an iron source.
- This regulatory system balances iron acquisition with protection against reactive electrophile stress from quinones.
- Fe(III)-catechol complexes are likely significant nutritional iron sources for pneumococcus in the host environment.
