Sulfide-responsive transcriptional repressor SqrR functions as a master regulator of sulfide-dependent photosynthesis

Abstract

Sulfide was used as an electron donor early in the evolution of photosynthesis, with many extant photosynthetic bacteria still capable of using sulfur compounds such as hydrogen sulfide (H 2 S) as a photosynthetic electron donor. Although enzymes involved in H 2 S oxidation have been characterized, mechanisms of regulation of sulfide-dependent photosynthesis have not been elucidated. In this study, we have identified a sulfide-responsive transcriptional repressor, SqrR, that functions as a master regulator of sulfide-dependent gene expression in the purple photosynthetic bacterium Rhodobacter capsulatus. SqrR has three cysteine residues, two of which, C41 and C107, are conserved in SqrR homologs from other bacteria. Analysis with liquid chromatography coupled with an electrospray-interface tandem-mass spectrometer reveals that SqrR forms an intramolecular tetrasulfide bond between C41 and C107 when incubated with the sulfur donor glutathione persulfide. SqrR is oxidized in sulfidestressed cells, and tetrasulfide-cross-linked SqrR binds more weakly to a target promoter relative to unmodified SqrR. C41S and C107S R. capsulatus SqrRs lack the ability to respond to sulfide, and constitutively repress target gene expression in cells. These results establish that SqrR is a sensor of H 2 S-derived reactive sulfur species that maintain sulfide homeostasis in this photosynthetic bacterium and reveal the mechanism of sulfide-dependent transcriptional derepression of genes involved in sulfide metabolism.sulfide sensor | photosynthesis regulation | reactive sulfur species | purple bacteria | Rhodobacter T he discovery of ∼550 deep-sea hydrothermal vents more than 30 y ago (1) has led to the theory that energy metabolism in early ancestral organisms may have arisen from deep-sea hydrothermal vents where simple inorganic molecules such as hydrogen sulfide or hydrogen gas, as well as methane, exist (2-4). Such ancient energy metabolism has been assumed to be similar to that of extant chemolithotrophs, which obtain energy from these molecules. Indeed, various chemolithoautotrophic microbes thrive in deep-sea hydrothermal vents and are capable of oxidizing sulfides, methane, and/or hydrogen gas for use as energy sources and electron donors (5). Some photosynthetic bacteria have also been isolated from deep-sea hydrothermal vents that can grow photosynthetically using sulfide as an electron donor and geothermal radiation as an energy source instead of solar radiation (6), as hypothesized for ancestral phototrophs.Many purple photosynthetic bacteria have remarkable metabolic versatility required to meet the energy demands of sulfidedependent and -independent photosynthesis as well as aerobic and anaerobic respiration. These bacteria tightly control the synthesis of their electron transfer proteins involved in each growth mode in response to a specific electron donor, oxygen tension, and light intensity (7, 8). Among these regulatory systems, oxygen-and lightsensing mechanisms have been well-studied; however, mechanisms used to sen...

Entropy redistribution controls allostery in a metalloregulatory protein

“…Ensemble models of allostery provide a solution to this problem (1,19). For example, in the ArsR superfamily (25), CzrA senses Zn in the α5 ligand-binding site, CadC and CmtR sense Cd and Pb in the α4 and α3 sites, respectively, while sensing of reactive oxygen, sulfur (53) and electrophilic species (54) occurs in the α2 and α1 sites (Fig. 5C and Fig.…”

Metallochaperones and metalloregulation in bacteria

“…The C-terminal α5 site in ArsR repressors, represented by S. aureus CzrA [88], is composed of an all N/O-donor ligand set used to coordinate harder metals such as Zn II ; in some cases, the α5 site is further elaborated to bind Ni II and Co II with octahedral coordination geometries, achieved by the recruitment of N/O donors from the extreme N-terminal tail [89]. It is becoming increasingly clear that ArsRs are not restricted to metal sensing [90–98], with individual members now known to have evolved to sense ROS [93,94,98,99] and RSS [90,92] via a conserved cysteine pair in the α2 and α5 helices, while RES appear to be sensed by a cysteine in α1 helix [91]. It is important to point out here that redox-sensing ArsR family members are often misannotated as MarR family members due to known or anticipated functional similarities ( vide infra ).…”

“…This will not, however, be discussed further here and we direct the reader instead to a number of recent reviews on this subject [17,21–23]. We provide sufficient mechanistic detail to pique the interest of the reader, while drawing general conclusions from recent findings in this rapidly expanding field [3,20,90,101]. …”

A new player in bacterial sulfide‐inducible transcriptional regulation

“…The fact that three Cys are functionally required and conserved in most FisRs suggests the possibility of thiol‐disulfide exchange after initial formation of a non‐native disulfide, as pointed out by the authors (Li et al ., ). In fact, R. capsulatus SqrR harbors a third cysteine, which although not required in cells for RSS sensing, changes the scope of oxidative reaction products obtained upon incubation with glutathione persulfide (Shimizu et al ., ). Do organic or inorganic per‐ and polysulfide species (see Fig.…”

“…This ‘action at a distance’ is mediated by integration host factor (IHF), which introduces a bend in the DNA, positioning bound FisR in close proximity to the promotor‐bound RNA polymerase, with ATP hydrolysis stimulating open complex formation and transcription initiation (Hartman et al ., ). The striking similarity to previous work (Luebke et al ., ; Shimizu et al ., ) is that FisR contains three conserved Cys residues (C53, C64 and C71) in the R domain and reduced FisR is activated by exposure to glutathione persulfide and inorganic polysulfide RSS (see Fig. C).…”