CrtJ bound to distant binding sites interacts cooperatively to aerobically repress photopigment biosynthesis and light harvesting II gene expression in Rhodobacter capsulatus

Structural Analyses of CrtJ and Its B12-Binding Co-Regulators SAerR and LAerR from the Purple Photosynthetic Bacterium Rhodobacter capsulatus

Among purple photosynthetic bacteria, the transcription factor CrtJ is a major regulator of photosystem gene expression. Depending on growing conditions, CrtJ can function as an aerobic repressor or an anaerobic activator of photosystem genes. Recently, CrtJ's activity was shown to be modulated by two size variants of a B12 binding co-regulator called SAerR and LAerR in Rhodobacter capsulatus. The short form, SAerR, promotes CrtJ repression, while the longer variant, LAerR, converts CrtJ into an activator. In this study, we solved the crystal structure of R. capsulatus SAerR at a 2.25 Å resolution. Hydroxycobalamin bound to SAerR is sandwiched between a 4-helix bundle cap, and a Rossman fold. This structure is similar to a AerR-like domain present in CarH from Thermus termophilus, which is a combined photoreceptor/transcription regulator. We also utilized AlphaFold software to predict structures for the LAerR, CrtJ, SAerR-CrtJ and LAerR-CrtJ co-complexes. These structures provide insights into the role of B12 and an LAerR N-terminal extension in regulating the activity of CrtJ.

Differing isoforms of the cobalamin binding photoreceptor AerR oppositely regulate photosystem expression

Phototrophic microorganisms adjust photosystem synthesis in response to changes in light intensity and wavelength. A variety of different photoreceptors regulate this process. Purple photosynthetic bacteria synthesize a novel photoreceptor AerR that uses cobalamin (B₁₂) as a blue-light absorbing chromophore to control photosystem synthesis. AerR directly interacts with the redox responding transcription factor CrtJ, affecting CrtJ's interaction with photosystem promoters. In this study, we show that AerR is translated as two isoforms that differ by 41 amino acids at the amino terminus. The ratio of these isoforms was affected by light and cell growth phase with the long variant predominating during photosynthetic exponential growth and the short variant predominating in dark conditions and/or stationary phase. Pigmentation and transcriptomic analyses show that the short AerR variant represses, while long variant activates, photosynthesis genes. The long form of AerR also activates many genes involved in cellular metabolism and motility.

The Vitamin B12-Dependent Photoreceptor AerR Relieves Photosystem Gene Repression by Extending the Interaction of CrtJ with Photosystem Promoters

Purple nonsulfur bacteria adapt their physiology to a wide variety of environmental conditions often through the control of transcription. One of the main transcription factors involved in controlling expression of the Rhodobacter capsulatus photosystem is CrtJ, which functions as an aerobic repressor of photosystem genes. Recently, we reported that a vitamin B₁₂ binding antirepressor of CrtJ called AerR is required for anaerobic expression of the photosystem. However, the mechanism whereby AerR regulates CrtJ activity is unclear. In this study, we used a combination of next-generation sequencing and biochemical methods to globally identify genes under control of CrtJ and the role of AerR in controlling this regulation. Our results indicate that CrtJ has a much larger regulon than previously known, with a surprising regulatory function under both aerobic and anaerobic photosynthetic growth conditions. A combination of in vivo chromatin immunoprecipitation-DNA sequencing (ChIP-seq) and ChIP-seq and exonuclease digestion (ChIP-exo) studies and in vitro biochemical studies demonstrate that AerR forms a 1:2 complex with CrtJ (AerR-CrtJ₂) and that this complex binds to many promoters under photosynthetic conditions. The results of in vitro and in vivo DNA binding studies indicate that AerR-CrtJ₂ anaerobically forms an extended interaction with the bacteriochlorophyll bchC promoter to relieve repression by CrtJ. This is contrasted by aerobic growth conditions where CrtJ alone functions as an aerobic repressor of bchC expression. These results indicate that the DNA binding activity of CrtJ is modified by interacting with AerR in a redox-regulated manner and that this interaction alters CrtJ’s function.

Photoreceptors control a wide range of physiology often by regulating downstream gene expression in response to light absorption via a bound chromophore. Different photoreceptors are known to utilize a number of different compounds for light absorption, including the use of such compounds as flavins, linearized tetrapyrroles (bilins), and carotenoids. Recently, a novel class of photoreceptors that use vitamin B₁₂ (cobalamin) as a blue-light-absorbing chromophore have been described. In this study, we analyzed the mechanism by which the vitamin B₁₂ binding photoreceptor AerR controls the DNA binding activity of the photosystem regulator CrtJ. This study shows that a direct interaction between the vitamin B₁₂ binding photoreceptor AerR with CrtJ modulates CrtJ binding to DNA and importantly, the regulatory outcome of gene expression, as shown here with photosystem promoters.

Members of the PpaA/AerR Antirepressor Family Bind Cobalamin

PpaA from Rhodobacter sphaeroides is a member of a family of proteins that are thought to function as antirepressors of PpsR, a widely disseminated repressor of photosystem genes in purple photosynthetic bacteria. PpaA family members exhibit sequence similarity to a previously defined SCHIC (sensor containing heme instead of cobalamin) domain; however, the tetrapyrrole-binding specificity of PpaA family members has been unclear, as R. sphaeroides PpaA has been reported to bind heme while the Rhodobacter capsulatus homolog has been reported to bind cobalamin. In this study, we reinvestigated tetrapyrrole binding of PpaA from R. sphaeroides and show that it is not a heme-binding protein but is instead a cobalamin-binding protein. We also use bacterial two-hybrid analysis to show that PpaA is able to interact with PpsR and activate the expression of photosynthesis genes in vivo. Mutations in PpaA that cause loss of cobalamin binding also disrupt PpaA antirepressor activity in vivo. We also tested a number of PpaA homologs from other purple bacterial species and found that cobalamin binding is a conserved feature among members of this family of proteins.

IMPORTANCE

Cobalamin (vitamin B12) has only recently been recognized as a cofactor that affects gene expression by interacting in a light-dependent manner with transcription factors. A group of related antirepressors known as the AppA/PpaA/AerR family are known to control the expression of photosynthesis genes in part by interacting with either heme or cobalamin. The specificity of which tetrapyrroles that members of this family interact with has, however, remained cloudy. In this study, we address the tetrapyrrole-binding specificity of the PpaA/AerR subgroup and establish that it preferentially binds cobalamin over heme.

Redox and light control the heme-sensing activity of AppA

The DNA binding activity of the photosystem-specific repressor PpsR is known to be repressed by the antirepressor AppA. AppA contains a blue-light-absorbing BLUF domain and a heme-binding SCHIC domain that controls the interaction of AppA with PpsR in response to light and heme availability. In this study, we have solved the structure of the SCHIC domain and identified the histidine residue that is critical for heme binding. We also demonstrate that dark-adapted AppA binds heme better than light-excited AppA does and that heme bound to the SCHIC domain significantly reduces the length of the BLUF photocycle. We further show that heme binding to the SCHIC domain is affected by the redox state of a disulfide bridge located in the Cys-rich carboxyl-terminal region. These results demonstrate that light, redox, and heme are integrated inputs that control AppA’s ability to disrupt the DNA binding activity of PpsR.

Photosynthetic bacteria must coordinate synthesis of the tetrapyrroles cobalamin, heme, and bacteriochlorophyll, as overproduction of the latter two is toxic to cells. A key regulator controlling tetrapyrrole biosynthesis is PpsR, and the activity of PpsR is controlled by the heme-binding and light-regulated antirepressor AppA. We show that AppA binds heme only under dark conditions and that heme binding significantly affects the length of the AppA photocycle. Since AppA interacts with PpsR only in the dark, bound heme thus stimulates the antirepressor activity of PpsR. This causes the redirection of tetrapyrrole biosynthesis away from heme into the bacteriochlorophyll branch.

Controlling the delicate balance of tetrapyrrole biosynthesis

Tetrapyrroles are a family of compounds that contain four pyrrole rings. They are involved in many fundamental biological processes such as photoreception, electron transport, gas transport and also as cofactors for enzymatic reactions. As regulators of protein activity, tetrapyrroles mediate cellular response to light, oxygen and nutrient levels in the surrounding environment. Biosynthesis of haem tetrapyrroles shares, conserved pathways and enzymes among all three domains of life. This is contrasted by chlorophyll biosynthesis that is only present in eubacteria and chloroplasts, or cobalamin biosynthesis that is only present in eubacteria and archaea. This implicates haem as the most ancient, and chlorophyll as the most recent, of the common tetrapyrroles that are currently synthesized by existing organisms. Haem and chlorophyll are both toxic when synthesized in excess over apo-proteins that bind these tetrapyrroles. Accordingly, the synthesis of these tetrapyrroles has to be tightly regulated and coordinated with apo-protein production. The mechanism of regulating haem and chlorophyll synthesis has been studied intensively in Rhodobacter species and will be discussed.

Activity of the tetrapyrrole regulator CrtJ is controlled by oxidation of a redox active cysteine located in the DNA binding domain

CrtJ from Rhodobacter capsulatus is a regulator of genes involved in the biosynthesis of haem, bacteriochlorophyll, carotenoids as well as structural proteins of the light harvesting-II complex. Fluorescence anisotropy-based DNA-binding analysis demonstrates that oxidized CrtJ exhibits ~20-fold increase in binding affinity over that of reduced CrtJ. Liquid chromatography electrospray tandem ionization mass spectrometric analysis using DAz-2, a sulfenic acid (-SOH)-specific probe, demonstrates that exposure of CrtJ to oxygen or to hydrogen peroxide leads to significant accumulation of a sulfenic acid derivative of Cys420 which is located in the helix-turn-helix (HTH) motif. In vivo labelling with 4-(3-azidopropyl)cyclohexane-1,3-dione (DAz-2) shows that Cys420 also forms a sulfenic acid modification in vivo when cells are exposed to oxygen. Moreover, a Cys420 to Ala mutation leads to a ~60-fold reduction of DNA binding activity while a Cys to Ser substitution at position 420 that mimics a cysteine sulfenic acid results in a ~4-fold increase in DNA binding activity. These results provide the first example where sulfenic acid oxidation of a cysteine in a HTH-motif leads to differential effects on gene expression.

PpsR, a regulator of heme and bacteriochlorophyll biosynthesis, is a heme-sensing protein

Heme-mediated regulation, presented in many biological processes, is achieved in part with proteins containing heme regulatory motif. In this study, we demonstrate that FLAG-tagged PpsR isolated from Rhodobacter sphaeroides cells contains bound heme. In vitro heme binding studies with tagless apo-PpsR show that PpsR binds heme at a near one-to-one ratio with a micromolar binding constant. Mutational and spectral assays suggest that both the second Per-Arnt-Sim (PAS) and DNA binding domains of PpsR are involved in the heme binding. Furthermore, we show that heme changes the DNA binding patterns of PpsR and induces different responses of photosystem genes expression. Thus, PpsR functions as both a redox and heme sensor to coordinate the amount of heme, bacteriochlorophyll, and photosystem apoprotein synthesis thereby providing fine tune control to avoid excess free tetrapyrrole accumulation.

Diverse arrangement of photosynthetic gene clusters in aerobic anoxygenic phototrophic bacteria

Background

Aerobic anoxygenic photototrophic (AAP) bacteria represent an important group of marine microorganisms inhabiting the euphotic zone of the ocean. They harvest light using bacteriochlorophyll (BChl) a and are thought to be important players in carbon cycling in the ocean.

Methodology/Principal Findings

Aerobic anoxygenic phototrophic (AAP) bacteria represent an important part of marine microbial communities. Their photosynthetic apparatus is encoded by a number of genes organized in a so-called photosynthetic gene cluster (PGC). In this study, the organization of PGCs was analyzed in ten AAP species belonging to the orders Rhodobacterales, Sphingomonadales and the NOR5/OM60 clade. Sphingomonadales contained comparatively smaller PGCs with an approximately size of 39 kb whereas the average size of PGCs in Rhodobacterales and NOR5/OM60 clade was about 45 kb. The distribution of four arrangements, based on the permutation and combination of the two conserved regions bchFNBHLM-LhaA-puhABC and crtF-bchCXYZ, does not correspond to the phylogenetic affiliation of individual AAP bacterial species. While PGCs of all analyzed species contained the same set of genes for bacteriochlorophyll synthesis and assembly of photosynthetic centers, they differed largely in the carotenoid biosynthetic genes. Spheroidenone, spirilloxanthin, and zeaxanthin biosynthetic pathways were found in each clade respectively. All of the carotenoid biosynthetic genes were found in the PGCs of Rhodobacterales, however Sphingomonadales and NOR5/OM60 strains contained some of the carotenoid biosynthetic pathway genes outside of the PGC.

Conclusions/Significance

Our investigations shed light on the evolution and functional implications in PGCs of marine aerobic anoxygenic phototrophs, and support the notion that AAP are a heterogenous physiological group phylogenetically scattered among Proteobacteria.

The tetrapyrrole biosynthetic pathway and its regulation in Rhodobacter capsulatus

The purple anoxygenic photosynthetic bacterium Rhodobacter capsulatus is capable of growing in aerobic or anaerobic conditions, in the dark or using light, etc. Achieving versatile metabolic adaptations from respiration to photosynthesis requires the use of tetrapyrroles such as heme and bacteriochlorophyll, in order to carry oxygen, to transfer electrons, and to harvest light energy. A third tetrapyrrole, cobalamin (vitamin B(12)), is synthesized and used as a cofactor for many enzymes. Heme, bacteriochlorophyll, and vitamin B(12) constitute three major end products of the tetrapyrrole biosynthetic pathway in purple bacteria. Their respective synthesis involves a plethora of enzymes, several that have been characterized and several that are uncharacterized, as described in this review. To respond to changes in metabolic requirements, the pathway undergoes complex regulation to direct the flow of tetrapyrrole intermediates into a specific branch(s) at the expense of other branches of the pathway. Transcriptional regulation of the tetrapyrrole synthesizing enzymes by redox conditions and pathway intermediates is reviewed. In addition, we discuss the involvement of several transcription factors (RegA, CrtJ, FnrL, AerR, HbrL, Irr) as well as the role of riboswitches. Finally, the interdependence of the tetrapyrrole branches on each other synthesis is discussed.

Regulation of aerobic photosystem synthesis in the purple bacterium Rhodospirillum centenum by CrtJ and AerR

Genes coding for putative CrtJ and AerR homologs were identified and characterized in the purple photosynthetic bacterium Rhodospirillum centenum (also known as Rhodocista centenaria), an organism that synthesizes photopigments even under highly aerated conditions. Mutational analysis indicated that in Rsp. centenum, gene crtJ codes for a repressor for photosynthesis gene expression as in Rhodobacter capsulatus, which exhibits a high level of oxygen repression of photosystem synthesis. In contrast to Rba. capsulatus, AerR in Rsp. centenum appears to be an aerobic activator; an aerR mutation resulted in significantly reduced levels of photopigment synthesis. Both aerR and crtJ mutants retained essentially normal levels of photosystem synthesis under anaerobic conditions, indicating that their activities are specific for aerobic photosystem synthesis. The readthrough transcript from crtE promoter, which is regulated by AerR and CrtJ, seems to be significant in maintaining the expression levels of the light harvesting I (puf) genes in Rsp. centenum. We suggest that AerR and CrtJ regulate aerobic photosystem synthesis primarily through controlling activity of the transcriptional readthrough.

Bacteriophytochromes in anoxygenic photosynthetic bacteria

Since the first discovery of a bacteriophytochrome in Rhodospirillum centenum, numerous bacteriophytochromes have been identified and characterized in other anoxygenic photosynthetic bacteria. This review is focused on the biochemical and biophysical properties of bacteriophytochromes with a special emphasis on their roles in the synthesis of the photosynthetic apparatus.

RegA control of bacteriochlorophyll and carotenoid synthesis in Rhodobacter capsulatus

We provide in vivo genetic and in vitro biochemical evidence that RegA directly regulates bacteriochlorophyll and carotenoid biosynthesis in Rhodobacter capsulatus. beta-Galactosidase expression assays with a RegA-disrupted strain containing reporter plasmids for Mg-protoporphyrin IX monomethyl ester oxidative cyclase (bchE), Mg-protoporphyrin IX chelatase (bchD), and phytoene dehydrogenase (crtI) demonstrate RegA is responsible for fourfold anaerobic induction of bchE, threefold induction of bchD, and twofold induction of crtI. Promoter mapping studies, coupled with DNase I protection assays, map the region of RegA binding to three sites in the bchE promoter region. Similar studies at the crtA and crtI promoters indicate that RegA binds to a single region equidistant from these divergent promoters. These results demonstrate that RegA is directly responsible for anaerobic induction of bacteriochlorophyll biosynthesis genes bchE, bchD, bchJ, bchI, bchG, and bchP and carotenoid biosynthesis genes crtI, crtB, and crtA.

Rhodopseudomonas palustris CGA009 has two functional ppsR genes, each of which encodes a repressor of photosynthesis gene expression

The PpsR protein is a regulator of redox-dependent photosystem development in purple phototrophic bacteria. In contrast to most species, Rhodopseudomonas palustris contains two ppsR genes. We show that the inactivation of each of the R. palustris strain CGA009 ppsR genes results in an elevated level of formation of the photosystem under dark aerobic conditions. Absorption spectra of the two PpsR mutants revealed qualitative and quantitative differences in light-harvesting peak amplitude increases. A sequence difference in the helix-turn-helix DNA binding motif of PpsR2 (Arg 439 to Cys) between R. palustris strains CEA001 and CGA009 is shown to be a natural polymorphism that does not inactivate the repressor activity of the protein. To evaluate which photosynthesis genes are regulated by the two PpsR proteins, transcriptome profiles of the CGA009 and PpsR mutant strains were analyzed in microarray experiments. Transcription of most but not all photosystem genes was derepressed in the mutant strains to levels consistent with the in vivo absorption spectra, mathematical analyses of peak shapes and amplitudes, reaction center protein levels, and real-time PCR of selected mRNAs. Closely spaced PpsR binding motif repeats were identified 5' of genes that were derepressed in the transcriptome analysis of PpsR mutants. This work shows that both the PpsR1 and PpsR2 proteins from R. palustris strain CGA009 function as oxygen-responsive transcriptional repressors.

The AppA and PpsR proteins from Rhodobacter sphaeroides can establish a redox-dependent signal chain but fail to transmit blue-light signals in other bacteria

The AppA protein of Rhodobacter sphaeroides has the unique ability to sense and transmit redox and light signals. In response to decreasing oxygen tension, AppA antagonizes the transcriptional regulator PpsR, which represses the expression of photosynthesis genes, including the puc operon. This mechanism, which is based on direct protein-protein interaction, is prevented by blue-light absorption of the BLUF domain located in the N-terminal part of AppA. In order to test whether AppA and PpsR are sufficient to transmit redox and light signals, we expressed these proteins in three different bacterial species and monitored oxygen- and blue-light-dependent puc expression either directly or by using a luciferase-based reporter construct. The AppA/PpsR system could mediate redox-dependent gene expression in the alphaproteobacteria Rhodobacter capsulatus and Paracoccus denitrificans but not in the gammaproteobacterium Escherichia coli. Analysis of a prrA mutant strain of R. sphaeroides strongly suggests that light-dependent gene expression requires a balanced interplay of the AppA/PpsR system with the PrrA response regulator. Therefore, the AppA/PpsR system was unable to establish light signaling in other bacteria. Based on our data, we present a model for the interdependence of AppA/PpsR signaling and the PrrA transcriptional activator.

Redox properties of the Rhodobacter sphaeroides transcriptional regulatory proteins PpsR and AppA

Redox properties of the photosynthetic gene repressor PpsR and the blue-light photoreceptor/antirepressor AppA from Rhodobacter sphaeroides have been characterized. Redox titrations of PpsR reveal the presence of a two-electron couple, with an E (m) value of -320 mV at pH 7.0, which is likely to arise from the reversible conversion of two cysteine thiols to a disulfide. This E (m) value is very much more negative than the E (m) = -180 mV value measured previously at pH 7.0 for the disulfide/dithiol couple in CrtJ, the homolog for PpsR in the closely related bacterium Rhodobacter capsulatus. AppA, a flavin-containing blue-light receptor that is also involved in the regulation of gene expression in R. sphaeroides, contains multiple cysteines in its C-terminal region, two of which function as a redox-active dithiol/disulfide couple with an E (m) value of -325 mV at pH 7.0 in the dark. Titrations of this dithiol/disulfide couple in illuminated samples of AppA indicate that the E (m) value of this disulfide/dithiol couple is -315 mV at pH 7.0, identical to the value obtained for AppA in the dark within the combined experimental uncertainties of the two measurements. The E (m) values of AppA and PpsR demonstrate that these proteins are thermodynamically capable of electron transfer for their activity as an anti-repressor/repressor in R. sphaeroides.

Light-dependent regulation of photosynthesis genes in Rhodobacter sphaeroides 2.4.1 is coordinately controlled by photosynthetic electron transport via the PrrBA two-component system and the photoreceptor AppA

Formation of the photosynthetic apparatus in Rhodobacter is regulated by oxygen tension and light intensity. Here we show that in anaerobically grown Rhodobacter cells a light-dependent increase in expression of the puc and puf operons encoding structural proteins of the photosynthetic complexes requires an active photosynthetic electron transport. The redox-sensitive CrtJ/PpsR repressor of photosynthesis genes, which was suggested to mediate electron transport-dependent signals, is not involved in this light-dependent signal chain. Our data reveal that the signal initiated in the photosynthetic reaction centre is transmitted via components of the electron transport chain and the PrrB/PrrA two-component system in Rhodobacter sphaeroides. Under blue light illumination in the absence of oxygen this signal leads to activation of photosynthesis genes and interferes with a blue-light repression mediated by the AppA photoreceptor and the PpsR transcriptional repressor in R. sphaeroides. Thus, light either sensed by a photoreceptor or initiating photosynthetic electron transport has opposite effects on the transcription of photosynthesis genes. Both signalling pathways involve redox-dependent steps that finally determine the effect of light on gene expression.

Biogenesis, molecular regulation and function of plant isoprenoids

Isoprenoids represent the oldest class of known low molecular-mass natural products synthesized by plants. Their biogenesis in plastids, mitochondria and the endoplasmic reticulum-cytosol proceed invariably from the C5 building blocks, isopentenyl diphosphate and/or dimethylallyl diphosphate according to complex and reiterated mechanisms. Compounds derived from the pathway exhibit a diverse spectrum of biological functions. This review centers on advances obtained in the field based on combined use of biochemical, molecular biology and genetic approaches. The function and evolutionary implications of this metabolism are discussed in relation with seminal informations gathered from distantly but related organisms.

PpsR: a multifaceted regulator of photosynthesis gene expression in purple bacteria

Purple bacteria control the level of expression and the composition of their photosystem according to light and redox conditions. This control involves several regulatory systems that have been now well characterized. Among them, the PpsR regulator plays a central role, because it directly or indirectly controls the synthesis of all of the different components of the photosystem. In this review, we report our knowledge of the PpsR protein, highlighting the diversity of its mode of action and focusing on the proteins identified in four model purple bacteria (Rhodobacter capsulatus, Rhodobacter sphaeroides, Rubrivivax gelatinosus, Bradyrhizobium ORS278). This regulator exhibits unique regulatory features in each bacterium: it can activate and/or repress the expression of photosynthesis genes, its activity can be modulated or not by the redox conditions, it can interact with other specific regulators and therefore be involved differently in light and/or redox regulatory circuits.

Transcriptome analysis of the Rhodobacter sphaeroides PpsR regulon: PpsR as a master regulator of photosystem development

PpsR from the anoxygenic phototrophic bacterium Rhodobacter sphaeroides has been known as an oxygen- and light-dependent repressor of bacteriochlorophyll and carotenoid biosynthesis genes and puc operons involved in photosystem development. However, the putative PpsR-binding sites, TGTN12ACA, are also located upstream of numerous nonphotosystem genes, thus raising the possibility that the role of PpsR is broader. To characterize the PpsR regulon, transcriptome profiling was performed on the wild-type strain grown at high and low oxygen tensions, on the strain overproducing PpsR, and on the ppsR mutant. Transcriptome analysis showed that PpsR primarily regulates photosystem genes; the consensus PpsR binding sequence is TGTcN10gACA (lowercase letters indicate lesser conservation); the presence of two binding sites is required for repression in vivo. These findings explain why numerous single TGTN12ACA sequences are nonfunctional. In addition to photosystem genes, the hemC and hemE genes involved in the early steps of tetrapyrrole biosynthesis were identified as new direct targets of PpsR repression. Unexpectedly, PpsR was found to indirectly repress the puf and puhA operons encoding photosystem core proteins. The upstream regions of these operons contain no PpsR binding sites. Involvement in regulation of these operons suggests that PpsR functions as a master regulator of photosystem development. Upregulation of the puf and puhA operons that resulted from ppsR inactivation was sufficient to restore the ability to grow phototrophically to the prrA mutant. PrrA, the global redox-dependent activator, was previously considered indispensable for phototrophic growth. It is revealed that the PrrBA and AppA-PpsR systems, believed to work independently, in fact interact and coordinately regulate photosystem development.

The PpsR regulator family

Under aerobic conditions, phototrophic bacteria repress the formation of pigments to protect cells in the presence of light from the damaging effects of reactive oxygen species and consume oxygen through respiratory complexes. Members of the PpsR family regulate the transcription of bch, crt, puc, and hem genes in respond to redox or light conditions. This mini-review focuses on the function and distribution of PpsR proteins.

Regulation of hem gene expression in Rhodobacter capsulatus by redox and photosystem regulators RegA, CrtJ, FnrL, and AerR

Biosynthetic pathways for heme and chlorophyll share common intermediates from 5-aminolevulinic acid through protoporphyrin IX. To obtain a better understanding of how photosynthetic organisms coordinate heme and chlorophyll biosynthesis, we have undertaken detailed analysis of the expression pattern of numerous heme biosynthesis genes in the purple photosynthetic bacterium Rhodobacter capsulatus. beta-Galactosidase reporter assays demonstrated that expression of hemA, hemB, hemC, hemE and hemZ genes is elevated under conditions that give rise to elevated bacteriochlorophyll synthesis. Heme gene expression is shown to be affected by mutations in previously identified transcriptional regulators RegA, FnrL, CrtJ, and AerR, which also control expression of genes involved in bacteriochlorophyll and carotenoid synthesis, and synthesis of the apoprotein subunits of the photosynthetic and electron transport apparatus. High-resolution primer extension analysis of hem mRNA reveals the presence of numerous putative RegA, FnrL and CrtJ binding sites in several hem promoter regions.

Regulators of nonsulfur purple phototrophic bacteria and the interactive control of CO2 assimilation, nitrogen fixation, hydrogen metabolism and energy generation

For the metabolically diverse nonsulfur purple phototrophic bacteria, maintaining redox homeostasis requires balancing the activities of energy supplying and energy-utilizing pathways, often in the face of drastic changes in environmental conditions. These organisms, members of the class Alphaproteobacteria, primarily use CO2 as an electron sink to achieve redox homeostasis. After noting the consequences of inactivating the capacity for CO2 reduction through the Calvin-Benson-Bassham (CBB) pathway, it was shown that the molecular control of many additional important biological processes catalyzed by nonsulfur purple bacteria is linked to expression of the CBB genes. Several regulator proteins are involved, with the two component Reg/Prr regulatory system playing a major role in maintaining redox poise in these organisms. Reg/Prr was shown to be a global regulator involved in the coordinate control of a number of metabolic processes including CO2 assimilation, nitrogen fixation, hydrogen metabolism and energy-generation pathways. Accumulating evidence suggests that the Reg/Prr system senses the oxidation/reduction state of the cell by monitoring a signal associated with electron transport. The response regulator RegA/PrrA activates or represses gene expression through direct interaction with target gene promoters where it often works in concert with other regulators that can be either global or specific. For the key CO2 reduction pathway, which clearly triggers whether other redox balancing mechanisms are employed, the ability to activate or inactivate the specific regulator CbbR is of paramount importance. From these studies, it is apparent that a detailed understanding of how diverse regulatory elements integrate and control metabolism will eventually be achieved.

Light and Redox Control of Photosynthesis Gene Expression in Bradyrhizobium

The two closely related bacteria Bradyrhizobium and Rhodopseudomonas palustris show an unusual mechanism of regulation of photosystem formation by light thanks to a bacteriophytochrome that antirepresses the regulator PpsR. In these two bacteria, we found out, unexpectedly, that two ppsR genes are present. We show that the two Bradyrhizobium PpsR proteins exert antagonistic effects in the regulation of photosystem formation with a classical repressor role for PpsR2 and an unexpected activator role for PpsR1. DNase I footprint analysis show that both PpsR bind to the same DNA TGTN12ACA motif that is present in tandem in the bchC promoter and the crtED intergenic region. Interestingly, the cycA and aerR promoter regions that contain only one conserved palindrome are recognized by PpsR2, but not PpsR1. Further biochemical analyses indicate that PpsR1 only is redox sensitive through the formation of an intermolecular disulfide bond, which changes its oligomerization state from a tetramer to an octamer under oxidizing conditions. Moreover, PpsR1 presents a higher DNA affinity under its reduced form in contrast to what has been previously found for PpsR or its homolog CrtJ from the Rhodobacter species. These results suggest that regulation of photosystem synthesis in Bradyrhizobium involves two PpsR competing for the binding to the same photosynthesis genes and this competition might be modulated by two factors: light via the antagonistic action of a bacteriophytochrome on PpsR2 and redox potential via the switch of PpsR1 oligomerization state.

RegB/RegA, a highly conserved redox-responding global two-component regulatory system

The Reg regulon from Rhodobacter capsulatus and Rhodobacter sphaeroides encodes proteins involved in numerous energy-generating and energy-utilizing processes such as photosynthesis, carbon fixation, nitrogen fixation, hydrogen utilization, aerobic and anaerobic respiration, denitrification, electron transport, and aerotaxis. The redox signal that is detected by the membrane-bound sensor kinase, RegB, appears to originate from the aerobic respiratory chain, given that mutations in cytochrome c oxidase result in constitutive RegB autophosphorylation. Regulation of RegB autophosphorylation also involves a redox-active cysteine that is present in the cytosolic region of RegB. Both phosphorylated and unphosphorylated forms of the cognate response regulator RegA are capable of activating or repressing a variety of genes in the regulon. Highly conserved homologues of RegB and RegA have been found in a wide number of photosynthetic and nonphotosynthetic bacteria, with evidence suggesting that RegB/RegA plays a fundamental role in the transcription of redox-regulated genes in many bacterial species.

Regulation of photosynthesis genes in Rubrivivax gelatinosus: transcription factor PpsR is involved in both negative and positive control

Induction of biosynthesis of the photosystem in anoxygenic photosynthetic bacteria occurs when the oxygen concentration drops. Control of this induction takes place primarily at the transcriptional level, with photosynthesis genes expressed preferentially under anaerobic conditions. Here, we report analysis of the transcriptional control of two photosynthesis promoters, pucBA and crtI, by the PpsR factor in Rubrivivax gelatinosus. This was accomplished by analyzing the photosystem production in the wild type and in the PPSRK (ppsR::Km) mutant grown under anaerobic and semiaerobic conditions and by assessing the beta-galactosidase activity of lacZ transcriptionally fused to promoters possessing the putative PpsR-binding consensus sequences. It was found that under semiaerobic conditions, inactivation of the ppsR gene resulted in overproduction of carotenoid and bacteriochlorophyll pigments, while the production of LH2 was drastically reduced. The beta-galactosidase activity showed that, in contrast to what has been found previously for Rhodobacter species, PpsR acts in R. gelatinosus as an aerobic repressor of the crtI gene while it acts as an activator for the expression of pucBA. Inspection of the putative PpsR-binding consensus sequences revealed significant differences that may explain the different levels of expression of the two genes studied.

Nitrogen-fixing symbiosis between photosynthetic bacteria and legumes

Rhizobia having photosynthetic systems form nitrogen-fixing nodules on the stem and/or root of some species of the legumes Aeschynomene and Lotononis. This review is focused on the recent knowledge about the physiology, genetics and role of the photosystem in these bacteria. Photosynthetic electron transport seems to involve reaction centers, soluble cytochrome c2 and cytochrome bc1. Anaerobically, the electron transport system becomes over-reduced. The photosynthesis genes have been partially characterized; their organization is classical but their regulation is unusual as it is activated by far-red light via a bacteriophytochrome. This original mechanism of regulation seems well adapted to promote photosynthesis during stem symbiosis. Photosynthesis plays a major role in the efficiency of stem nodulation. It is also observed that infrared light stimulates nitrogen fixation in nodules containing photosynthetic bacteroids, suggesting that photosynthesis may additionally provides energy for nitrogen fixation, allowing for more efficient plant growth. Other aspects of these bacteria are discussed, in particular their taxonomic position and nodulation ability, the role of carotenoids and the potential for application of photosynthetic rhizobia in rice culture.

Redox and light regulation of gene expression in photosynthetic prokaryotes

All photosynthetic organisms control expression of photosynthesis genes in response to alterations in light intensity as well as to changes in cellular redox potential. Light regulation in plants involves a well-defined set of red- and blue-light absorbing photoreceptors called phytochrome and cryptochrome. Less understood are the factors that control synthesis of the plant photosystem in response to changes in cellular redox. Among a diverse set of photosynthetic bacteria the best understood regulatory systems are those synthesized by the photosynthetic bacterium Rhodobacter capsulatus. This species uses the global two-component signal transduction cascade, RegB and RegA, to anaerobically de-repress anaerobic gene expression. Under reducing conditions, the phosphate on RegB is transferred to RegA, which then activates genes involved in photosynthesis, nitrogen fixation, carbon fixation, respiration and electron transport. In the presence of oxygen, there is a second regulator known as CrtJ, which is responsible for repressing photosynthesis gene expression. CrtJ responds to redox by forming an intramolecular disulphide bond under oxidizing, but not reducing, growth conditions. The presence of the disulphide bond stimulates DNA binding activity of the repressor. There is also a flavoprotein that functions as a blue-light absorbing anti-repressor of CrtJ in the related bacterial species Rhodobacter sphaeroides called AppA. AppA exhibits a novel long-lived photocycle that is initiated by blue-light absorption by the flavin. Once excited, AppA binds to CrtJ thereby inhibiting the repressor activity of CrtJ. Various mechanistic aspects of this photocycle will be discussed.

AppA is a blue light photoreceptor that antirepresses photosynthesis gene expression in Rhodobacter sphaeroides

Photosynthetic bacteria regulate photosystem synthesis in response to alterations in oxygen tension and light intensity. In this study we show that the PpsR repressor from Rhodobacter sphaeroides binds to DNA in a redox-dependent manner through the formation/breakage of an intramolecular disulfide bond. We also demonstrate that PpsR is antagonized by the flavin-containing antirepressor, AppA, that is capable of breaking the disulfide bond in oxidized PpsR as well as forming a stable AppA-PpsR(2) antirepressor-repressor complex. Blue light excitation of AppA induces a photocycle that is characterized by a long-lived red-shifted absorbance of the flavin. Light-excited AppA was found to be incapable of forming the AppA-PpsR(2) antirepressor complex. These results establish AppA as a transcription factor that controls both redox and blue light repression of photosystem gene expression by mediating DNA binding activity of PpsR.

Coordination of ubiquinol oxidase and cytochrome cbb(3) oxidase expression by multiple regulators in Rhodobacter capsulatus

Rhodobacter capsulatus utilizes two terminal oxidases for aerobic respiration, cytochrome cbb(3) and ubiquinol oxidase. To determine the transcription factors involved in terminal oxidase expression, ccoN-lacZ and cydA-lacZ protein fusions were assayed in a variety of regulatory mutants. The results of this and previous studies indicate that cytochrome cbb(3) expression is controlled by regB-regA, fnrL, and hvrA and that ubiquinol oxidase expression is controlled by regB-regA, fnrL, hvrA, crtJ, and aerR.

AerR, a second aerobic repressor of photosynthesis gene expression in Rhodobacter capsulatus

Open reading frame orf192, which is located immediately upstream of the aerobic repressor gene crtJ, was genetically and biochemically demonstrated to code for a second aerobic repressor (AerR) of photosynthesis gene expression in Rhodobacter capsulatus. Promoter-mapping studies indicate that crtJ has its own promoter but that a significant proportion of crtJ expression is promoted by read-through transcription of orf192 (aerR) transcripts through crtJ. Disruption of aerR resulted in increased photopigment biosynthesis during aerobic growth to a level similar to that of disruption of crtJ. Like that reported for CrtJ, beta-galactosidase assays of reporter gene expression indicated that disruption of aerR resulted in a two- to threefold increase in aerobic expression of the crtI and pucB operons. However, unlike CrtJ, AerR aerobically represses puf operon expression and does not aerobically repress bchC expression. Gel mobility shift analysis with purified AerR indicates that AerR does not bind to a bchC promoter probe but does bind to the crtI, puc, and puf promoter probes. These results indicate that AerR is a DNA-binding protein that targets genes partially overlapping a subset of genes that are also controlled by CrtJ. We also provide evidence for cooperative binding of AerR and CrtJ to the puc promoter region.

The RegB/RegA two-component regulatory system controls synthesis of photosynthesis and respiratory electron transfer components in Rhodobacter capsulatus

Recently, we demonstrated that the RegB/RegA two-component regulatory system from Rhodobacter capsulatus functions as a global regulator of metabolic processes that either generate or consume reducing equivalents. For example, the RegB/RegA system controls expression of such energy generating processes as photosynthesis and hydrogen utilization. In addition, RegB/RegA also control nitrogen and carbon fixation pathways that utilize reducing equivalents. Here, we use a combination of DNase I protection and plasmid-based reporter expression studies to demonstrate that RegA directly controls synthesis of cytochrome cbb3 and ubiquinol oxidases that function as terminal electron acceptors in a branched respiratory chain. We also demonstrate that RegA controls expression of cytochromes c2, c(y) and the cytochrome bc1 complex that are involved in both photosynthetic and respiratory electron transfer events. These data provide evidence that the RegB/RegA two-component system has a major role in controlling the synthesis of numerous processes that affect reducing equivalents in Rhodobacter capsulatus.

Interaction of CbbR and RegA* transcription regulators with the Rhodobacter sphaeroides cbbIPromoter-operator region

The form I (cbb(I)) Calvin-Benson-Bassham (CBB) reductive pentose phosphate cycle operon of Rhodobacter sphaeroides is regulated by both the transcriptional activator CbbR and the RegA/PrrA (RegB/PrrB) two-component signal transduction system. DNase I footprint analyses indicated that R. sphaeroides CbbR binds to the cbb(I) promoter between -10 and -70 base pairs (bp) relative to the cbb(I) transcription start. A cosmid carrying the R. capsulatus reg locus was capable of complementing an R. sphaeroides regA-deficient mutant to phototrophic growth with restored regulated synthesis of both photopigments and ribulose-bisphosphate carboxylase/oxygenase (Rubisco). DNase I footprint analyses, using R. capsulatus RegA*, a constitutively active mutant version of RegA, detected four RegA* binding sites within the cbb(I) promoter. Two sites were found within a previously identified cbb(I) promoter proximal regulatory region from -61 to -110 bp. One of these proximal RegA* binding sites overlapped that of CbbR. Two sites were within a previously identified promoter distal positive regulatory region between -301 and -415 bp. Expression from promoter insertion mutants showed that the function of the promoter distal regulatory region was helical phase-dependent. These results indicated that RegA exerts its regulatory affect on cbb(I) expression through direct interaction with the cbb(I) promoter.

Expression of uptake hydrogenase and molybdenum nitrogenase in Rhodobacter capsulatus is coregulated by the RegB-RegA two-component regulatory system

Purple photosynthetic bacteria are capable of generating cellular energy from several sources, including photosynthesis, respiration, and H(2) oxidation. Under nutrient-limiting conditions, cellular energy can be used to assimilate carbon and nitrogen. This study provides the first evidence of a molecular link for the coregulation of nitrogenase and hydrogenase biosynthesis in an anoxygenic photosynthetic bacterium. We demonstrated that molybdenum nitrogenase biosynthesis is under the control of the RegB-RegA two-component regulatory system in Rhodobacter capsulatus. Footprint analyses and in vivo transcription studies showed that RegA indirectly activates nitrogenase synthesis by binding to and activating the expression of nifA2, which encodes one of the two functional copies of the nif-specific transcriptional activator, NifA. Expression of nifA2 but not nifA1 is reduced in the reg mutants up to eightfold under derepressing conditions and is also reduced under repressing conditions. Thus, although NtrC is absolutely required for nifA2 expression, RegA acts as a coactivator of nifA2. We also demonstrated that in reg mutants, [NiFe]hydrogenase synthesis and activity are increased up to sixfold. RegA binds to the promoter of the hydrogenase gene operon and therefore directly represses its expression. Thus, the RegB-RegA system controls such diverse processes as energy-generating photosynthesis and H(2) oxidation, as well as the energy-demanding processes of N(2) fixation and CO(2) assimilation.

Domain structure, oligomeric state, and mutational analysis of PpsR, the Rhodobacter sphaeroides repressor of photosystem gene expression

The transcription factor PpsR from the facultative photoheterotroph Rhodobacter sphaeroides is involved in repression of photosystem gene expression under aerobic growth conditions. We have isolated a number of spontaneous mutations as well as constructed directed mutations and deletions in ppsR. Repressor activities and the oligomeric state of the wild-type and mutant proteins were assayed. Our results suggest that the wild-type PpsR exists in cell extracts as a tetramer. Analysis of the PpsR mutants confirmed that the carboxy-terminal region of PpsR (residues 400 to 464) is involved in DNA binding. The central region of the protein (residues 150 to 400) was found to contain two PAS domains (residues 161 to 259 and 279 to 367). PAS domains are ubiquitous protein modules involved in sensory transduction as well as in protein-protein interactions. All spontaneously isolated mutations, which significantly impaired repressor activity and which mapped outside the DNA binding region, were positioned in the PAS domains. None of these, however, affected the overall oligomeric state. This implies that the conformation of the PAS domains within the tetramer is critical for repressor activity. Upstream of the first PAS domain resides a putative glutamine-rich hinge (residues 127 to 136) that connects the first PAS domain to the amino-terminal region (residues 1 to 135). The role of the amino terminus of PpsR is not obvious; however, extended deletions within this region abolish repressor activity, thus suggesting that the amino terminus is essential for structural integrity of the protein. We present a model of the domain architecture of the PpsR protein according to which PpsR is comprised of three regions: the carboxy terminus responsible for DNA binding, the central region primarily involved in protein oligomerization and possibly signal sensing, and the amino terminus of unknown function. This model may prove useful for determining the mode of PpsR action.

Mechanisms for redox control of gene expression

This review discusses various mechanisms that regulatory proteins use to control gene expression in response to alterations in redox. The transcription factor SoxR contains stable [2Fe-2S] centers that promote transcription activation when oxidized. FNR contains [4Fe-4S] centers that disassemble under oxidizing conditions, which affects DNA-binding activity. FixL is a histidine sensor kinase that utilizes heme as a cofactor to bind oxygen, which affects its autophosphorylation activity. NifL is a flavoprotein that contains FAD as a redox responsive cofactor. Under oxidizing conditions, NifL binds and inactivates NifA, the transcriptional activator of the nitrogen fixation genes. OxyR is a transcription factor that responds to redox by breaking or forming disulfide bonds that affect its DNA-binding activity. The ability of the histidine sensor kinase ArcB to promote phosphorylation of the response regulator ArcA is affected by multiple factors such as anaerobic metabolites and the redox state of the membrane. The global regulator of anaerobic gene expression in alpha-purple proteobacteria, RegB, appears to directly monitor respiratory activity of cytochrome oxidase. The aerobic repressor of photopigment synthesis, CrtJ, seems to contain a redox responsive cysteine. Finally, oxygen-sensitive rhizobial NifA proteins presumably bind a metal cofactor that senses redox. The functional variability of these regulatory proteins demonstrates that prokaryotes apply many different mechanisms to sense and respond to alterations in redox.

In vitro activation and repression of photosynthesis gene transcription in Rhodobacter capsulatus

It has been known for over half a century that anoxygenic photosynthetic bacteria maximally synthesize their photosystems in the absence of oxygen. During the last decade, it has become clear that this regulation is largely at the transcriptional level, with photosynthesis genes expressed only under anaerobic conditions. We describe here in vitro reconstitution of activation and repression of three photosynthesis promoters, bch (bacteriochlorophyll biosynthesis), puc (light-harvesting II apoproteins) and puf (reaction centre and light-harvesting I apoproteins) using purified transcription factors and RNA polymerase from Rhodobacter capsulatus. Previous genetic results have indicated that each of these three promoters is differentially regulated by three key regulators: CrtJ acting as a repressor of bch and puc and the two-component regulators RegA/RegB, which are activators of puc and puf. These regulators are distinct from those that mediate oxygen control in enteric bacteria. Our in vitro studies show that these purified regulators directly control the expression of the housekeeping RNA polymerase at these promoters. High-level basal expression of the bch promoter is shown to be repressed by CrtJ. The puc promoter is activated by the RegB-phosphorylated RegA protein and additionally repressed by CrtJ. At the puc promoter, CrtJ effectively competes for promoter binding with RegA, while at the bch promoter, repression appears to be by competition for the RNA polymerase binding site. In contrast to what has been suggested previously, the RegA-activated puf promoter is demonstrated as being recognized by the housekeeping RNA polymerase. We also discuss evidence that RegA approximately P activation of the puc and puf promoters involves recruitment of RNA polymerase by different modes of protein-protein interaction.

Autophosphorylation, phosphotransfer, and DNA-binding properties of the RegB/RegA two-component regulatory system in Rhodobacter capsulatus

In the purple, photosynthetic bacterium, Rhodobacter capsulatus, the RegB/RegA two-component system is required for activation of several anaerobic processes, such as synthesis of the photosynthetic apparatus and assimilation of CO2 and N2. It is believed that RegB is an integral membrane histidine kinase that monitors the external environment. Under anaerobic growth conditions, it transduces a signal through phosphorylation of the response regulator, RegA, which then induces target gene expression. We used an in vitro assay to characterize the phosphorylation of wild-type RegA and a mutant variant (RegA*) that is responsible for abnormally high photosynthesis gene expression under both aerobic and anaerobic growth conditions. Phosphorylation assays indicate that phosphorylated RegA* (RegA* approximately P) is much more stable than RegA approximately P, indicating that it may be locked in a conformation that is resistant to dephosphorylation. DNase I footprint assays also indicate that unphosphorylated RegA* has a much higher affinity for specific DNA binding sites than the wild-type protein. Phosphorylation of RegA* increases DNA binding 2. 5-fold, whereas phosphorylation of RegA increases DNA binding more than 16-fold. Collectively, these results support the hypothesis that RegA* is a constitutively active variant that does not require phosphorylation to assume a structural conformation required to bind DNA.

Aerobic repression of the Rhodobacter capsulatus bchC promoter involves cooperative interactions between CrtJ bound to neighboring palindromes

Previous studies demonstrated that bacteriochlorophyll, carotenoid, and light harvesting gene expression in Rhodobacter capsulatus is repressed under aerobic growth conditions by the repressor CrtJ. Isolated CrtJ is known to bind to the palindrome TGTN12ACA, which is present in two copies in the bchC promoter, one of which spans the -35 and the other the -10 sigma-70 recognition sequences. In this study, we demonstrate that CrtJ binds to the two palindromic sites in the bchC promoter in a cooperative manner. The level of cooperativity of CrtJ binding to the -35 palindrome was shown to be 26-fold. A distance of 8 base pairs between the two palindromic sites was shown to be critical for cooperative binding, as evidenced by the disruption of binding that resulted when +6 and +11 base pairs were inserted between the palindromes.