A second nitrogenase-like enzyme for bacteriochlorophyll biosynthesis: reconstitution of chlorophyllide a reductase with purified X-protein (BchX) and YZ-protein (BchY-BchZ) from Rhodobacter capsulatus

A nitrogenase-like enzyme is involved in the novel anaerobic assimilation pathway of a sulfonate, isethionate, in the photosynthetic bacterium Rhodobacter capsulatus

Prokaryotes contribute to the global sulfur cycle by using diverse sulfur compounds as sulfur sources or electron acceptors. In this study, we report that a nitrogenase-like enzyme (NFL) and a radical SAM enzyme (RSE) are involved in the novel anaerobic assimilation pathway of a sulfonate, isethionate, in the photosynthetic bacterium Rhodobacter capsulatus. The nflHDK genes for NFL are localized at a locus containing genes for known sulfonate metabolism in the genome. A gene nflB encoding an RSE is present just upstream of nflH, forming a small gene cluster nflBHDK. Mutants lacking any nflBHDK genes are incapable of growing with isethionate as the sole sulfur source under anaerobic photosynthetic conditions, indicating that all four NflBHDK proteins are essential for the isethionate assimilation pathway. Heterologous expression of the islAB genes encoding a known isethionate lyase that degrades isethionate to sulfite and acetaldehyde restored the isethionate-dependent growth of a mutant lacking nflDK, indicating that the enzyme encoding nflBHDK is involved in an isethionate assimilation reaction to release sulfite. Furthermore, the heterologous expression of nflBHDK and ssuCAB encoding an isethionate transporter in the closely related species R. sphaeroides, which does not have nflBHDK and cannot grow with isethionate as the sole sulfur source, conferred isethionate-dependent growth ability to this species. We propose to rename nflBHDK as isrBHDK (isethionate reductase). The isrBHDK genes are widely distributed among various prokaryote phyla. Discovery of the isethionate assimilation pathway by IsrBHDK provides a missing piece for the anaerobic sulfur cycle and for understanding the evolution of ancient sulfur metabolism.IMPORTANCENitrogenase is an important enzyme found in prokaryotes that reduces atmospheric nitrogen to ammonia and plays a fundamental role in the global nitrogen cycle. It has been noted that nitrogenase-like enzymes (NFLs), which share an evolutionary origin with nitrogenase, have evolved to catalyze diverse reactions such as chlorophyll biosynthesis (photosynthesis), coenzyme F430 biosynthesis (methanogenesis), and methionine biosynthesis. In this study, we discovered that an NFL with unknown function in the photosynthetic bacterium Rhodobacter capsulatus is a novel isethionate reductase (Isr), which catalyzes the assimilatory degradation of isethionate, a sulfonate, releasing sulfite used as the sulfur source under anaerobic conditions. Isr is widely distributed among various bacterial phyla, including intestinal bacteria, and is presumed to play an important role in sulfur metabolism in anaerobic environments such as animal guts and microbial mats. This finding provides a clue for understanding ancient metabolism that evolved under anaerobic environments at the dawn of life.

Characterization of the iron-sulfur clusters in the nitrogenase-like reductase CfbC/D required for coenzyme F430 biosynthesis

Coenzyme F430 is a nickel-containing tetrapyrrole, serving as the prosthetic group of methyl-coenzyme M reductase in methanogenic and methanotrophic archaea. During coenzyme F430 biosynthesis, the tetrapyrrole macrocycle is reduced by the nitrogenase-like CfbC/D system consisting of the reductase component CfbC and the catalytic component CfbD. Both components are homodimeric proteins, each carrying a [4Fe-4S] cluster. Here, the ligands of the [4Fe-4S] clusters of CfbC2 and CfbD2 were identified revealing an all cysteine ligation of both clusters. Moreover, the midpoint potentials of the [4Fe-4S] clusters were determined to be -256 mV for CfbC2 and -407 mV for CfbD2. These midpoint potentials indicate that the consecutive thermodynamically unfavorable 6 individual "up-hill" electron transfers to the organic moiety of the Ni2+-sirohydrochlorin a,c-diamide substrate require an intricate interplay of ATP-binding, hydrolysis, protein complex formation and release to drive product formation, which is a common theme in nitrogenase-like systems.

Illuminating the coevolution of photosynthesis and Bacteria

Life harnessing light energy transformed the relationship between biology and Earth-bringing a massive flux of organic carbon and oxidants to Earth's surface that gave way to today's organotrophy- and respiration-dominated biosphere. However, our understanding of how life drove this transition has largely relied on the geological record; much remains unresolved due to the complexity and paucity of the genetic record tied to photosynthesis. Here, through holistic phylogenetic comparison of the bacterial domain and all photosynthetic machinery (totally spanning >10,000 genomes), we identify evolutionary congruence between three independent biological systems-bacteria, (bacterio)chlorophyll-mediated light metabolism (chlorophototrophy), and carbon fixation-and uncover their intertwined history. Our analyses uniformly mapped progenitors of extant light-metabolizing machinery (reaction centers, [bacterio]chlorophyll synthases, and magnesium-chelatases) and enzymes facilitating the Calvin-Benson-Bassham cycle (form I RuBisCO and phosphoribulokinase) to the same ancient Terrabacteria organism near the base of the bacterial domain. These phylogenies consistently showed that extant phototrophs ultimately derived light metabolism from this bacterium, the last phototroph common ancestor (LPCA). LPCA was a non-oxygen-generating (anoxygenic) phototroph that already possessed carbon fixation and two reaction centers, a type I analogous to extant forms and a primitive type II. Analyses also indicate chlorophototrophy originated before LPCA. We further reconstructed evolution of chlorophototrophs/chlorophototrophy post-LPCA, including vertical inheritance in Terrabacteria, the rise of oxygen-generating chlorophototrophy in one descendant branch near the Great Oxidation Event, and subsequent emergence of Cyanobacteria. These collectively unveil a detailed view of the coevolution of light metabolism and Bacteria having clear congruence with the geological record.

Nitrogen-fixing bacterial communities differ between perennial agroecosystem crops

Biocrusts, common in natural ecosystems, are specific assemblages of microorganisms at or on the soil surface with associated microorganisms extending into the top centimeter of soil. Agroecosystem biocrusts have similar rates of nitrogen (N) fixation as those in natural ecosystems, but it is unclear how agricultural management influences their composition and function. This study examined the total bacterial and diazotrophic communities of biocrusts in a citrus orchard and a vineyard that shared a similar climate and soil type but differed in management. To contrast climate and soil type, these biocrusts were also compared with those from an apple orchard. Unlike natural ecosystem biocrusts, these agroecosystem biocrusts were dominated by proteobacteria and had a lower abundance of cyanobacteria. All of the examined agroecosystem biocrust diazotroph communities were dominated by N-fixing cyanobacteria from the Nostocales order, similar to natural ecosystem cyanobacterial biocrusts. Lower irrigation and fertilizer in the vineyard compared with the citrus orchard could have contributed to biocrust microbial composition, whereas soil type and climate could have differentiated the apple orchard biocrust. Season did not influence the bacterial and diazotrophic community composition of any of these agroecosystem biocrusts. Overall, agricultural management and climatic and edaphic factors potentially influenced the community composition and function of these biocrusts.

Intensification of harmful cyanobacterial blooms in a eutrophic, temperate lake caused by nitrogen, temperature, and CO2

Warmer temperatures can significantly increase the intensity of cyanobacterial harmful algal blooms (CHABs) in eutrophic freshwater ecosystems. However, few studies have examined the effects of CO2 enrichment in tandem with elevated temperature and/or nutrients on cyanobacterial taxa in freshwater ecosystems. Here, we observed changes in the biomass of cyanobacteria, nutrients, pH, and carbonate chemistry over a two-year period in a shallow, eutrophic freshwater lake and performed experiments to examine the effects and co-effects of CO2, temperature, and nutrient enrichment on cyanobacterial and N2-fixing (diazotrophic) communities assessed via high throughput sequencing of the 16S rRNA and nifH genes, respectively. During both years, there were significant CHABs (50-500 μg cyanobacterial chlorophyll-a L-1) and lake CO2 levels were undersaturated (≤300 μatm pCO2). NH4+ significantly increased the net growth rates of cyanobacteria as well as the biomass of the diazotrophic cyanobacterial order Nostocales under elevated and ambient CO2 conditions. In a fall experiment, the N2 fixation rates of Nostocales were significantly higher when populations were enriched with CO2 and P, relative to CO2-enriched populations that were not amended with P. During a summer experiment, N2 fixation rates increased significantly under N and CO2 - enriched conditions relative to N-enriched and ambient CO2 conditions. Nostocales dominated the diazotrophic communities of both experiments, achieving the highest relative abundance under CO2-enriched conditions when N was added in the first experiment and when CO2 and temperature were elevated in the second experiment, when N2 fixation rates also increased significantly. Collectively, this study indicates that N promotes cyanobacterial blooms including those formed by Dolichospermum and that the biomass and N2 fixation rates of diazotrophic cyanobacterial taxa may benefit from enhanced CO2 levels in eutrophic lakes.

Globally distributed Myxococcota with photosynthesis gene clusters illuminate the origin and evolution of a potentially chimeric lifestyle

Photosynthesis is a fundamental biogeochemical process, thought to be restricted to a few bacterial and eukaryotic phyla. However, understanding the origin and evolution of phototrophic organisms can be impeded and biased by the difficulties of cultivation. Here, we analyzed metagenomic datasets and found potential photosynthetic abilities encoded in the genomes of uncultivated bacteria within the phylum Myxococcota. A putative photosynthesis gene cluster encoding a type-II reaction center appears in at least six Myxococcota families from three classes, suggesting vertical inheritance of these genes from an early common ancestor, with multiple independent losses in other lineages. Analysis of metatranscriptomic datasets indicate that the putative myxococcotal photosynthesis genes are actively expressed in various natural environments. Furthermore, heterologous expression of myxococcotal pigment biosynthesis genes in a purple bacterium supports that the genes can drive photosynthetic processes. Given that predatory abilities are thought to be widespread across Myxococcota, our results suggest the intriguing possibility of a chimeric lifestyle (combining predatory and photosynthetic abilities) in members of this phylum.

Engineering Chlorophyll, Bacteriochlorophyll, and Carotenoid Biosynthetic Pathways in Escherichia coli

The biosynthesis of chlorophylls (Chls) and bacteriochlorophylls (BChls) represents a key aspect of photosynthesis research. Our previous work assembled the complete pathway for the synthesis of Chl a in Escherichia coli; here we engineer the more complex BChl a pathway in the same heterotrophic host. Coexpression of 18 genes enabled E. coli to produce BChl a, verifying that we have identified the minimum set of genes for the BChl a biosynthesis pathway. The protochlorophyllide reduction step was mediated by the bchNBL genes, and this same module was used to modify the Chl a pathway previously constructed in E. coli, eliminating the need for the light-dependent protochlorophyllide reductase. Furthermore, we demonstrate the feasibility of synthesizing more than one family of photosynthetic pigments in one host by engineering E. coli strains that accumulate the carotenoids neurosporene and β-carotene in addition to BChl a.

The Photoactive Photosynthetic Reaction Center of a Rhodobacter sphaeroides Mutant Lacking 3-Vinyl (Bacterio)Chlorophyllide a Hydratase Contains 3-Vinyl Bacteriochlorophyll a

Rhodobacter sphaeroides mutant BF-lacking 3-vinyl (bacterio)chlorophyllide a hydratase (BchF)-accumulates chlorophyllide a (Chlide a) and 3-vinyl bacteriochlorophyllide a (3V-Bchlide a). BF synthesizes 3-vinyl bacteriochlorophyll a (3V-Bchl a) through prenylation of 3V-Bchlide a and assembles a novel reaction center (V-RC) using 3V-Bchl a and Mg-free 3-vinyl bacteriopheophytin a (3V-Bpheo a) at a molar ratio of 2:1. We aimed to verify whether a bchF-deleted R. sphaeroides mutant produces a photochemically active RC that facilitates photoheterotrophic growth. The mutant grew photoheterotrophically-implying a functional V-RC-as confirmed by the emergence of growth-competent suppressors of bchC-deleted mutant (BC) under irradiation. Suppressor mutations in BC were localized to bchF, which diminished BchF activity and caused 3V-Bchlide a accumulation. bchF expression carrying the suppressor mutations in trans resulted in the coproduction of V-RC and wild-type RC (WT-RC) in BF. The V-RC had a time constant (τ) for electron transfer from the primary electron donor P (a dimer of 3V-Bchl a) to the A-side containing 3V-Bpheo a (HA) similar to that of the WT-RC and a 60% higher τ for electron transfer from HA to quinone A (QA). Thus, the electron transfer from HA to QA in the V-RC should be slower than that in the WT-RC. Furthermore, the midpoint redox potential of P/P+ of the V-RC was 33 mV more positive than that of the WT-RC. R. sphaeroides, thus, synthesizes the V-RC when 3V-Bchlide a accumulates. The V-RC can support photoheterotrophic growth; however, its photochemical activity is inferior to that of the WT-RC. IMPORTANCE 3V-Bchlide a is an intermediate in the bacteriochlorophyll a (Bchl a)-specific biosynthetic branch and prenylated by bacteriochlorophyll synthase. R. sphaeroides synthesizes V-RC that absorbs light at short wavelengths. The V-RC was not previously discovered because 3V-Bchlide a does not accumulate during the growth of WT cells synthesizing Bchl a. The levels of reactive oxygen species increased with the onset of photoheterotrophic growth in BF, resulting in a long lag period. Although the inhibitor of BchF is unknown, the V-RC may act as a substitute for the WT-RC when BchF is completely inhibited. Alternatively, it may act synergistically with WT-RC at low levels of BchF activity. The V-RC may broaden the absorption spectra of R. sphaeroides and supplement its photosynthetic ability at various wavelengths of visible light to a greater extent than that by the WT-RC alone.

Multiple sources of aerobic methane production in aquatic ecosystems include bacterial photosynthesis

Aquatic ecosystems are globally significant sources of the greenhouse gas methane to the atmosphere. Until recently, methane production was thought to be a strictly anaerobic process confined primarily to anoxic sediments. However, supersaturation of methane in oxygenated waters has been consistently observed in lakes and the ocean (termed the 'methane paradox'), indicating that methane can be produced under oxic conditions through unclear mechanisms. Here we show aerobic methane production from multiple sources in freshwater incubation experiments under different treatments and based on biogeochemical, metagenomic, and metatranscriptomic data. We find that aerobic methane production appears to be associated with (bacterio)chlorophyll metabolism and photosynthesis, as well as with Proteobacterial degradation of methylphosphonate. Genes encoding pathways for putative photosynthetic- and methylphosphonate-based methane production also co-occur in Proteobacterial metagenome-assembled genomes. Our findings provide insight into known mechanisms of aerobic methane production, and suggest a potential co-occurring mechanism associated with bacterial photosynthesis in aquatic ecosystems.

Origin and evolution of nitrogen fixation in prokaryotes

The origin of nitrogen fixation is an important issue in evolutionary biology. While nitrogen is required by all living organisms, only a small fraction of bacteria and archaea can fix nitrogen. The prevailing view is that nitrogen fixation first evolved in archaea and was later transferred to bacteria. However, nitrogen-fixing (Nif) bacteria are far larger in number and far more diverse in ecological niches than Nif archaea. We, therefore, propose the bacteria-first hypothesis, which postulates that nitrogen fixation first evolved in bacteria and was later transferred to archaea. As >30,000 prokaryotic genomes have been sequenced, we conduct an in-depth comparison of the two hypotheses. We first identify the six genes involved in nitrogen fixation in all sequenced prokaryotic genomes and then reconstruct phylogenetic trees using the six Nif proteins individually or in combination. In each of these trees, the earliest lineages are bacterial Nif protein sequences and in the oldest clade (group) the archaeal sequences are all nested inside bacterial sequences, suggesting that the Nif proteins first evolved in bacteria. The bacteria-first hypothesis is further supported by the observation that the majority of Nif archaea carry the major bacterial Mo (molybdenum) transporter (ModABC) rather than the archaeal Mo transporter (WtpABC). Moreover, in our phylogeny of all available ModA and WtpA protein sequences, the earliest lineages are bacterial sequences while archaeal sequences are nested inside bacterial sequences. Furthermore, the bacteria-first hypothesis is supported by available isotopic data. In conclusion, our study strongly supports the bacteria-first hypothesis.

Genomic Features of the Bundle-Forming Heliobacterium Heliophilum fasciatum

Eight species of heliobacteria have had their genomes sequenced. However, only two of these genomes have been analyzed in detail, those from the thermophilic Heliomicrobium (Hmi.) modesticaldum and the alkaliphilic Heliorestis (Hrs.) convoluta. Here we present analyses of the draft genome sequence of a species of heliobacterium that grows optimally at a moderate temperature and neutral pH. The organism, Heliophilum (Hph.) fasciatum, is phylogenetically unique among cultured heliobacteria and was isolated from rice soil, a common habitat for heliobacteria. The Hph. fasciatum genome contains 3.14 Mbp—similar to that of other reported heliobacteria—but has a G+C base ratio that lies between that of Hmi. modesticaldum and Hrs. convoluta. Many of the genomic features of Hmi. modesticaldum and Hrs. convoluta, such as the absence of genes encoding autotrophic pathways, the presence of a superoperonal cluster of photosynthesis-related genes, and genes encoding endospore-specific proteins, are also characteristic of the Hph. fasciatum genome. However, despite the fact that Hph. fasciatum is diazotrophic, classical nif genes encoding the alpha and beta subunits of dinitrogenase (nifDK) present in other heliobacteria could not be identified. Instead, genes encoding several highly divergent NifDK homologs were present, at least one of which likely encodes a functional dinitrogenase and another a methylthio-alkane reductase (MarDK) for sulfur assimilation. A classical NifH (dinitrogenase reductase) homolog was also absent in Hph. fasciatum, but a related protein was identified that likely carries out this function as well as electron delivery to MarDK. The N₂-fixing system of Hph. fasciatum is therefore distinct from that of other heliobacteria and may have unusual properties.

Incomplete Hydrogenation by Geranylgeranyl Reductase from a Proteobacterial Phototroph Halorhodospira halochloris, Resulting in the Production of Bacteriochlorophyll with a Tetrahydrogeranylgeranyl Tail

Light harvesting and charge separation are functions of chlorophyll and bacteriochlorophyll pigments. While most photosynthetic organisms use (bacterio)chlorophylls with a phytyl (2-phytenyl) group as the hydrophobic isoprenoid tail, Halorhodospira halochloris, an anoxygenic photosynthetic bacterium belonging to Gammaproteobacteria, produces bacteriochlorophylls with a unique 6,7,14,15-tetrahydrogeranylgeranyl (2,10-phytadienyl) tail. Geranylgeranyl reductase (GGR), encoded by the bchP gene, catalyzes hydrogenation at three unsaturated C=C bonds of a geranylgeranyl group, giving rise to the phytyl tail. In this study, we discovered that H. halochloris GGR exhibits only partial hydrogenation activities, resulting in the tetrahydrogeranylgeranyl tail formation. We hypothesized that the hydrogenation activity of H. halochloris GGR differed from that of Chlorobaculum tepidum GGR, which also produces a pigment with partially reduced hydrophobic tails (2,6-phytadienylated chlorophyll a). An engineered GGR was also constructed and demonstrated to perform only single hydrogenation, resulting in the dihydrogeranylgeranyl tail formation. H. halochloris original and variant GGRs shed light on GGR catalytic mechanisms and offer prospective bioengineering tools in the microbial production of isoprenoid compounds. IMPORTANCE Geranylgeranyl reductase (GGR) catalyzes the hydrogenation of carbon-carbon double bonds of unsaturated hydrocarbons of isoprenoid compounds, including α-tocopherols, phylloquinone, archaeal cell membranes, and (bacterio)chlorophyll pigments in various organisms. GGRs in photosynthetic organisms, including anoxygenic phototrophic bacteria, cyanobacteria, and plants perform successive triple hydrogenation to produce chlorophylls and bacteriochlorophylls with a phytyl chain. Here, we demonstrated that the GGR of a gammaproteobacterium Halorhodospira halochloris catalyzed unique double hydrogenation to produce bacteriochlorophylls with a tetrahydrogeranylgeranyl tail. We also constructed a variant enzyme derived from H. halochloris GGR that performs only single hydrogenation. The results of this study provide new insights into catalytic mechanisms of multiposition reductions by a single enzyme.

Coral-associated nitrogen fixation rates and diazotrophic diversity on a nutrient-replete equatorial reef

The role of diazotrophs in coral physiology and reef biogeochemistry remains poorly understood, in part because N2 fixation rates and diazotrophic community composition have only been jointly analyzed in the tissue of one tropical coral species. We performed field-based 15N2 tracer incubations during nutrient-replete conditions to measure diazotroph-derived nitrogen (DDN) assimilation into three species of scleractinian coral (Pocillopora acuta, Goniopora columna, Platygyra sinensis). Using multi-marker metabarcoding (16S rRNA, nifH, 18S rRNA), we analyzed DNA- and RNA-based communities in coral tissue and skeleton. Despite low N2 fixation rates, DDN assimilation supplied up to 6% of the holobiont's N demand. Active coral-associated diazotrophs were chiefly Cluster I (aerobes or facultative anaerobes), suggesting that oxygen may control coral-associated diazotrophy. Highest N2 fixation rates were observed in the endolithic community (0.20 µg N cm-2 per day). While the diazotrophic community was similar between the tissue and skeleton, RNA:DNA ratios indicate potential differences in relative diazotrophic activity between these compartments. In Pocillopora, DDN was found in endolithic, host, and symbiont compartments, while diazotrophic nifH sequences were only observed in the endolithic layer, suggesting a possible DDN exchange between the endolithic community and the overlying coral tissue. Our findings demonstrate that coral-associated diazotrophy is significant, even in nutrient-rich waters, and suggest that endolithic microbes are major contributors to coral nitrogen cycling on reefs.

A Rhizobium bacterium and its population dynamics under different culture conditions of its associated toxic dinoflagellate Gambierdiscus balechii

Rhizobium bacteria are known as symbionts of legumes for developing nodules on plant roots and fixing N₂ for the host plants but unknown for associations with dinoflagellates. Here, we detected, isolated, and characterized a Rhizobium species from the marine toxic dinoflagellate Gambierdiscus culture. Its 16S rRNA gene (rDNA) is 99% identical to that of Rhizobium rosettiformans, and the affiliation is supported by the phylogenetic placement of its cell wall hydrolase -encoding gene (cwh). Using quantitative PCR of 16S rDNA and cwh, we found that the abundance of this bacterium increased during the late exponential growth phase of Gambierdiscus and under nitrogen limitation, suggesting potential physiological interactions between the dinoflagellate and the bacterium. This is the first report of dinoflagellate-associated Rhizobium bacterium, and its prevalence and ecological roles in dinoflagellate-Rhizobium relationships remain to be investigated in the future.

Supplementary Information

The online version contains supplementary material available at 10.1007/s42995-021-00102-1.

In situ formation of photoactive B-ring reduced chlorophyll isomer in photosynthetic protein LH2

Natural chlorophylls have a D-ring reduced chlorin π-system; however, no naturally occurring photosynthetically active B-ring reduced chlorins have been reported. Here we report a B-ring reduced chlorin, 17,18-didehydro-bacteriochlorophyll (BChl) a, produced by in situ oxidation of B800 bacteriochlorophyll (BChl) a in a light-harvesting protein LH2 from a purple photosynthetic bacterium Phaeospirillum molischianum. The regioselective oxidation of the B-ring of B800 BChl a is rationalized by its molecular orientation in the protein matrix. The formation of 17,18-didehydro-BChl a produced no change in the local structures and circular arrangement of the LH2 protein. The B-ring reduced 17,18-didehydro-BChl a functions as an energy donor in the LH2 protein. The photoactive B-ring reduced Chl isomer in LH2 will be helpful for understanding the photofunction and evolution of photosynthetic cyclic tetrapyrrole pigments.

Substrate recognition induces sequential electron transfer across subunits in the nitrogenase-like DPOR complex

A key step in bacteriochlorophyll biosynthesis is the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide), catalyzed by dark-operative protochlorophyllide oxidoreductase (DPOR). DPOR is made of electron donor (BchL) and acceptor (BchNB) component proteins. BchNB is further composed of two subunits each of BchN and BchB arranged as an α₂β₂ heterotetramer with two active sites for substrate reduction. Such oligomeric architectures are found in several other electron transfer (ET) complexes, but how this architecture influences activity is unclear. Here, we describe allosteric communication between the two identical active sites in Rhodobacter sphaeroides BchNB that drives sequential and asymmetric ET. Pchlide binding to one BchNB active site initiates ET from the pre-reduced [4Fe-4S] cluster of BchNB, a process similar to the deficit spending mechanism observed in the structurally related nitrogenase complex. Pchlide binding in one active site is recognized in trans by an Asp-274 from the opposing half, which is positioned to serve as the initial proton donor. A D274A variant DPOR binds to two Pchlide molecules in the BchNB complex, but only one is bound productively, stalling Pchlide reduction in both active sites. A half-active complex combining one WT and one D274A monomer also stalled after one electron was transferred in the WT half. We propose that such sequential electron transfer in oligomeric enzymes serves as a regulatory mechanism to ensure binding and recognition of the correct substrate. The findings shed light on the functional advantages imparted by the oligomeric architecture found in many electron transfer enzymes.

A nitrogenase-like enzyme system catalyzes methionine, ethylene, and methane biogenesis

Bacterial production of gaseous hydrocarbons such as ethylene and methane affects soil environments and atmospheric climate. We demonstrate that biogenic methane and ethylene from terrestrial and freshwater bacteria are directly produced by a previously unknown methionine biosynthesis pathway. This pathway, present in numerous species, uses a nitrogenase-like reductase that is distinct from known nitrogenases and nitrogenase-like reductases and specifically functions in C-S bond breakage to reduce ubiquitous and appreciable volatile organic sulfur compounds such as dimethyl sulfide and (2-methylthio)ethanol. Liberated methanethiol serves as the immediate precursor to methionine, while ethylene or methane is released into the environment. Anaerobic ethylene production by this pathway apparently explains the long-standing observation of ethylene accumulation in oxygen-depleted soils. Methane production reveals an additional bacterial pathway distinct from archaeal methanogenesis.

Biosynthesis of the modified tetrapyrroles-the pigments of life

Modified tetrapyrroles are large macrocyclic compounds, consisting of diverse conjugation and metal chelation systems and imparting an array of colors to the biological structures that contain them. Tetrapyrroles represent some of the most complex small molecules synthesized by cells and are involved in many essential processes that are fundamental to life on Earth, including photosynthesis, respiration, and catalysis. These molecules are all derived from a common template through a series of enzyme-mediated transformations that alter the oxidation state of the macrocycle and also modify its size, its side-chain composition, and the nature of the centrally chelated metal ion. The different modified tetrapyrroles include chlorophylls, hemes, siroheme, corrins (including vitamin B12), coenzyme F430, heme d1, and bilins. After nearly a century of study, almost all of the more than 90 different enzymes that synthesize this family of compounds are now known, and expression of reconstructed operons in heterologous hosts has confirmed that most pathways are complete. Aside from the highly diverse nature of the chemical reactions catalyzed, an interesting aspect of comparative biochemistry is to see how different enzymes and even entire pathways have evolved to perform alternative chemical reactions to produce the same end products in the presence and absence of oxygen. Although there is still much to learn, our current understanding of tetrapyrrole biogenesis represents a remarkable biochemical milestone that is summarized in this review.

Chlorophyllide a oxidoreductase Preferentially Catalyzes 8-Vinyl Reduction over B-Ring Reduction of 8-Vinyl Chlorophyllide a in the Late Steps of Bacteriochlorophyll Biosynthesis

Bacteriochlorophyll a (BChl) is an essential pigment for anoxygenic photosynthesis. In late steps of the BChl biosynthesis of Rhodobacter capsulatus, the C8 vinyl group and C7=C8 double bond of 8-vinyl chlorophyllide a (8 V-Chlide) are reduced by a C8 vinyl reductase (8VR), BciA, and a nitrogenase-like enzyme, chlorophyllide a oxidoreductase (COR), respectively, to produce 3-vinyl-bacteriochlorphyllide a. Recently, we discovered 8VR activity in COR. However, the kinetic parameters of the COR 8VR activity remain unknown, while those of the COR C7=C8 reductase activity and BciA have been reported. Here, we determined the kinetic parameters of COR 8VR activity by using 8 V-Chlide. The K_m value for 8 V-Chlide was 1.4 μM, which is much lower than the 6.2 μM determined for the C7=C8 reduction of Chlide. The kinetic parameters of the dual activities of COR suggest that COR catalyzes the reduction of the C8 vinyl group of 8 V-Chlide preferentially over C7=C8 reduction when both substrates are supplied during BChl biosynthesis.

Divergent Members of the Nitrogenase Superfamily: Tetrapyrrole Biosynthesis and Beyond

The nitrogenase superfamily constitutes a large and diverse ensemble of two-component metalloenzymes. These systems couple the hydrolysis of ATP to the reduction of disparate substrates from diatomic gases (Mo and alternative nitrogenases) to photosynthetic pigments (protochlorophyllide and chlorophyllide oxidoreductases). Only very recently have the activities of the highly divergent and paraphyletic Group IV nitrogenases begun to be uncovered. This review highlights the first characterized member of this group, which was found to catalyze an unprecedented reaction in the coenzyme F430 biosynthetic pathway, and the catalytic potential of a superfamily that has yet to be fully explored.

Diazotroph Diversity and Nitrogen Fixation in Summer Active Perennial Grasses in a Mediterranean Region Agricultural Soil

Summer-growing perennial grasses such as Panicum coloratum L. cv. Bambatsi (Bambatsi panic), Chloris gayana Kunth cv. Katambora (Rhodes grass) and Digitaria eriantha Steud. cv. Premier (Premier digit grass) growing in the poor fertility sandy soils in the Mediterranean regions of southern Australia and western Australia mainly depend upon soil N and biological N inputs through diazotrophic (free living or associative) N fixation. We investigated the community composition and diversity (nifH-amplicon sequencing), abundance (qPCR) and functional capacity (15N incubation assay) of the endophytic diazotrophic community in the below and above ground plant parts of field grown and unfertilized grasses. Results showed a diverse and abundant diazotrophic community inside plant both above and below-ground and there was a distinct diazotrophic assemblage in the different plant parts in all the three grasses. There was a limited difference in the diversity between leaves, stems and roots except that Panicum grass roots harbored greater species richness. Nitrogen fixation potentials ranged between 0.24 and 5.9 mg N kg-1 day-1 and N fixation capacity was found in both the above and below ground plant parts. Results confirmed previous reports of plant species-based variation and that Alpha-Proteobacteria were the dominant group of nifH-harboring taxa both in the belowground and aboveground parts of the three grass species. Results also showed a well-structured nifH-harboring community in all plant parts, an example for a functional endophytic community. Overall, the variation in the number and identity of module hubs and connectors among the different plant parts suggests that co-occurrence patterns within the nifH-harboring community specific to individual compartments and local environments of the niches within each plant part may dictate the overall composition of diazotrophs within a plant.

Enzymatic Systems with Homology to Nitrogenase: Biosynthesis of Bacteriochlorophyll and Coenzyme F430

Enzymes with homology to nitrogenase are essential for the reduction of chemically stable double bonds within the biosynthetic pathways of bacteriochlorophyll and coenzyme F₄₃₀. These tetrapyrrole-based compounds are crucial for bacterial photosynthesis and the biogenesis of methane in methanogenic archaea. Formation of bacteriochlorophyll requires the unique ATP-dependent enzyme chlorophyllide oxidoreductase (COR) for the two-electron reduction of chlorophyllide to bacteriochlorophyllide. COR catalysis is based on the homodimeric protein subunit BchX₂, which facilitates the transfer of electrons to the corresponding heterotetrameric catalytic subunit (BchY/BchZ)₂. By analogy to the nitrogenase system, the dynamic switch protein BchX₂ contains a [4Fe-4S] cluster that triggers the ATP-driven transfer of electrons onto a second [4Fe-4S] cluster located in (BchY/BchZ)₂. The subsequent substrate reduction and protonation is unrelated to nitrogenase catalysis, with no further involvement of a molybdenum-containing cofactor. The biosynthesis of the nickel-containing coenzyme F₄₃₀ includes the six-electron reduction of the tetrapyrrole macrocycle of Ni²⁺-sirohydrochlorin a,c-diamide to Ni²⁺-hexahydrosirohydrochlorin a,c-diamide catalyzed by CfbC/D. The homodimeric CfbC₂ subunit carrying a [4Fe-4S] cluster shows close homology to BchX₂. Accordingly, parallelism for the initial ATP-driven electron transfer steps of CfbC/D was proposed. Electrons are received by the dimeric catalytic subunit CfbD₂, which contains a second [4Fe-4S] cluster and carries out the saturation of an overall of three double bonds in a highly orchestrated spatial and regioselective process. Following a short introduction to nitrogenase catalysis, this chapter will focus on the recent progress toward the understanding of the nitrogenase-like enzymes COR and CfbC/D, with special emphasis on the underlying enzymatic mechanism(s).

Unusual features in the photosynthetic machinery of Halorhodospira halochloris DSM 1059 revealed by complete genome sequencing

Halorhodospira halochloris is an anaerobic, halophilic, purple photosynthetic bacterium belonging to γ-Proteobacteria. H. halochloris is also characteristic as a thermophilic phototrophic isolate producing bacteriochlorophyll (BChl) b. Here, we report the complete genome sequence of H. halochloris DSM 1059. The genetic arrangement for this bacterium's photosynthetic apparatus is of particular interest; its genome contains two sets of puf operons encoding the reaction center and core light-harvesting 1 (LH1) complexes having almost identical nucleotide sequences (e.g., 98.8-99.9% of nucleotide identities between two sets of pufLM genes, but 100% of deduced amino acid sequence identities). This duplication of photosynthetic genes may provide a glimpse at natural selection in action. The β-polypeptides of the LH1 complex in purple bacteria usually contain two histidine residues to bind BChl a; however, those of H. halochloris were revealed to have four histidine residues, indicating unusual pigment organization in the LH1 complex of this species. Like in other BChl b-producing phototrophs, the genome of H. halochloris lacks the divinyl reductase genes bciA and bciB. The phylogeny of chlorophyllide a oxidoreductase, which catalyzes committed steps in the synthesis of BChl a and BChl b, indicates that evolution toward BChl b production is convergent. Geranylgeranyl reductase (BchP) of H. halochloris has an insertion region in its primary structure, which could be important for its unusual sequential reduction reactions.

Evolution of light-independent protochlorophyllide oxidoreductase

The nonhomologous enzymes, the light-independent protochlorophyllide reductase (DPOR) and the light-dependent protochlorophyllide oxidoreductase (LPOR), catalyze the reduction of protochlorophyllide (Pchlide) to chlorophyllide (Chlide) in the penultimate step of biosynthesis of chlorophyll (Chl) required for photosynthetic light absorption and energy conversion. The two enzymes differ with respect to the requirement of light for catalysis and oxygen sensitivity. DPOR and LPOR initially evolved in the ancestral prokaryotic genome perhaps at different times. DPOR originated in the anoxygenic environment of the Earth from nitrogenase-like enzyme of methanogenic archaea. Due to the transition from anoxygenic to oxygenic photosynthesis in the prokaryote, the DPOR was mostly inactivated in the daytime by photosynthetic O₂ leading to the evolution of oxygen-insensitive LPOR that could function in the light. The primary endosymbiotic event transferred the DPOR and LPOR genes to the eukaryotic phototroph; the DPOR remained in the genome of the ancestor that turned into the plastid, whereas LPOR was transferred to the host nuclear genome. From an evolutionary point of view, several compelling theories that explain the disappearance of DPOR from several species cutting across different phyla are as follows: (i) pressure of the oxygenic environment; (ii) change in the light conditions and temperature; and (iii) lineage-specific gene losses, RNA editing, and nonsynonymous substitution. Certain primary amino acid sequence and the physiochemical properties of the ChlL subunit of DPOR have similarity with that of LPOR suggesting a convergence of these two enzymes in certain evolutionary event. The newly obtained sequence data from different phototrophs will further enhance the width of the phylogenetic information on DPOR.

Expression, Purification, and Activity Analysis of Chlorophyllide Oxidoreductase and Ni2+-Sirohydrochlorin a,c-Diamide Reductase

Nitrogenase-like enzymes play a vital role in the reduction of the conjugated ring systems of diverse tetrapyrrole molecules. The biosynthesis of all bacteriochlorophylls involves the two-electron reduction of the C7-C8 double bond of the green pigment chlorophyllide, which is catalyzed by the nitrogenase-like two-component metalloenzyme chlorophyllide oxidoreductase (COR); whereas in all methanogenic microbes, another nitrogenase-like system, CfbC/D, is responsible for the sophisticated six-electron reduction of Ni²⁺-sirohydrochlorin a,c-diamide in the course of coenzyme F₄₃₀ biosynthesis. The first part of this chapter describes the production and purification of the COR components (BchY/BchZ)₂ and BchX₂, the measurement of COR activity, and the trapping of the ternary COR complex; and the second part describes the strategy for obtaining homogenous and catalytically active preparations of CfbC₂ and CfbD₂ and a suitable method for extracting the reaction product Ni²⁺-hexahydrosirohydrochlorin a,c-diamide.

Diazotrophic microbial community and abundance in acidic subtropical natural and re-vegetated forest soils revealed by high-throughput sequencing of nifH gene

Biological nitrogen fixation (BNF) is an important natural biochemical process converting the inert dinitrogen gas (N₂) in the atmosphere to ammonia (NH₃) in the N cycle. In this study, the nifH gene was chosen to detect the diazotrophic microorganisms with high-throughput sequencing from five acidic forest soils, including three natural forests and two re-vegetated forests. Soil samples were taken in two seasons (summer and winter) at two depth layers (surface and lower depths). A dataset of 179,600 reads obtained from 20 samples were analyzed to provide the microbial community structure, diversity, abundance, and relationship with physiochemical parameters. Both archaea and bacteria were detected in these samples and diazotrophic bacteria were the dominant members contributing to the biological dinitrogen fixation in the acidic forest soils. Cyanobacteria, Firmicutes, Proteobacteria, Spirocheates, and Verrucomicrobia were observed, especially the Proteobacteria as the most abundant phylum. The core genera were Bradyrhizobium and Methylobacterium from α-Proteobacteia, and Desulfovibrio from δ-Proteobacteia in the phylum of Proteobacteia of these samples. The diversity indices and the gene abundances of all samples were higher in the surface layer than the lower layer. Diversity was apparently higher in re-vegetated forests than the natural forests. Significant positive correlation to the organic matter and nitrogen-related parameters was observed, but there was no significant seasonal variation on the community structure and diversity in these samples between the summer and winter. The application of high-throughput sequencing method provides a better understanding and more comprehensive information of diazotrophs in acidic forest soils than conventional and PCR-based ones.

Inhibition of bacteriochlorophyll biosynthesis in the purple phototrophic bacteria Rhodospirillumrubrum and Rhodobacter capsulatus grown in the presence of a toxic concentration of selenite

Background In many works, the chemical composition of bacterially-produced elemental selenium nanoparticles (Se0-nanoparticles) was investigated using electron dispersive X-ray analysis. The results suggest that these particles should be associated with organic compounds. However, a complete analysis of their chemical composition is still missing. Aiming at identifying organic compounds associated with the Se0-nanoparticles produced by the purple phototrophic bacteria Rhodospirillum rubrum and Rhodobacter capsulatus (α group of the proteobacteria), we used MALDI-TOF spectrometry.Results This technic revealed that numerous signals obtained from particles produced by both species of bacteria were from metabolites of the photosynthetic system. Furthermore, not only bacteriochlorophyll a, bacteriopheophytin a, and bacteriopheophorbide a, which are known to accumulate in stationary phase cultures of these bacteria grown phototrophically in the absence of selenite, were identified. The particles were also associated with intermediary metabolites of the bacteriochlorophyll a biosynthesis pathway such as protoporphyrin IX, protoporphyrin IX monomethyl ester, bacteriochlorophyllide a and, most likely, Mg-protoporphyrin IX-monomethyl ester, as well as with oxidation products of the substrates of protochlorophyllide reductase and chlorin reductase.Conclusion Accumulation of intermediary metabolites of the bacteriochlorophyll biosynthesis pathway in these purple phototrophic bacteria was attributed to inhibition of oxygen-sensitive enzymes involved in this pathway. Consistent with this interpretation it has been reported that these bacteria reduce selenite intracellularly, that they contain high levels of glutathione and that the reduction of selenite with glutathione is a very fast reaction accompanied by the production of reactive oxygen species. As many enzymes involved in the biosynthesis of bacteriochlorophyll contain [Fe-S] clusters in their active site, which are known to be degraded in the presence of reactive oxygen species as well as in the presence of molecular oxygen, we concluded that the substrates of these enzymes accumulate in cells during selenite reduction.Association of metabolites of bacteriochlorophyll biosynthesis and degradation with the Se0-nanoparticles produced by Rhodospirillum rubrum and Rhodobacter capsulatus is proposed to result from coating of the nanoparticles with the intracytoplasmic membrane of these bacteria, where the photochemical apparatus is concentrated.

Semi-synthesis and HPLC analysis of (bacterio)chlorophyllides possessing a propionic acid residue at the C17-position

Various chlorophyll and bacteriochlorophyll derivatives possessing a magnesium or zinc atom at the central position and a free carboxylic acid group at the C17[Formula: see text]-position, also known as (bacterio)chlorophyllides, were synthesized through a combination of organic synthesis techniques and enzymatic steps. The semi-synthetic (bacterio)chlorophyllides were purified and analyzed using reversed-phase high-performance liquid chromatography with UV-vis spectroscopy and mass spectrometry. These free propionic acid-containing chlorophyllous pigments can be useful research materials for the study of (bacterio)chlorophyll metabolisms.

A physiological perspective on the origin and evolution of photosynthesis

The origin and early evolution of photosynthesis are reviewed from an ecophysiological perspective. Earth's first ecosystems were chemotrophic, fueled by geological H2 at hydrothermal vents and, required flavin-based electron bifurcation to reduce ferredoxin for CO2 fixation. Chlorophyll-based phototrophy (chlorophototrophy) allowed autotrophs to generate reduced ferredoxin without electron bifurcation, providing them access to reductants other than H2. Because high-intensity, short-wavelength electromagnetic radiation at Earth's surface would have been damaging for the first chlorophyll (Chl)-containing cells, photosynthesis probably arose at hydrothermal vents under low-intensity, long-wavelength geothermal light. The first photochemically active pigments were possibly Zn-tetrapyrroles. We suggest that (i) after the evolution of red-absorbing Chl-like pigments, the first light-driven electron transport chains reduced ferredoxin via a type-1 reaction center (RC) progenitor with electrons from H2S; (ii) photothioautotrophy, first with one RC and then with two, was the bridge between H2-dependent chemolithoautotrophy and water-splitting photosynthesis; (iii) photothiotrophy sustained primary production in the photic zone of Archean oceans; (iv) photosynthesis arose in an anoxygenic cyanobacterial progenitor; (v) Chl a is the ancestral Chl; and (vi), anoxygenic chlorophototrophic lineages characterized so far acquired, by horizontal gene transfer, RCs and Chl biosynthesis with or without autotrophy, from the architects of chlorophototrophy-the cyanobacterial lineage.

Engineered biosynthesis of bacteriochlorophyll gF in Rhodobacter sphaeroides

Engineering photosynthetic bacteria to utilize a heterologous reaction center that contains a different (bacterio) chlorophyll could improve solar energy conversion efficiency by allowing cells to absorb a broader range of the solar spectrum. One promising candidate is the homodimeric type I reaction center from Heliobacterium modesticaldum. It is the simplest known reaction center and uses bacteriochlorophyll (BChl) g, which absorbs in the near-infrared region of the spectrum. Like the more common BChls a and b, BChl g is a true bacteriochlorin. It carries characteristic C3-vinyl and C8-ethylidene groups, the latter shared with BChl b. The purple phototrophic bacterium Rhodobacter (Rba.) sphaeroides was chosen as the platform into which the engineered production of BChl g_F, where F is farnesyl, was attempted. Using a strain of Rba. sphaeroides that produces BChl b_P, where P is phytyl, rather than the native BChl a_P, we deleted bchF, a gene that encodes an enzyme responsible for the hydration of the C3-vinyl group of a precursor of BChls. This led to the production of BChl g_P. Next, the crtE gene was deleted, thereby producing BChl g carrying a THF (tetrahydrofarnesol) moiety. Additionally, the bchG^Rs gene from Rba. sphaeroides was replaced with bchG^Hm from Hba. modesticaldum. To prevent reduction of the tail, bchP was deleted, which yielded BChl g_F. The construction of a strain producing BChl g_F validates the biosynthetic pathway established for its synthesis and satisfies a precondition for assembling the simplest reaction center in a heterologous organism, namely the biosynthesis of its native pigment, BChl g_F.

Diazotroph Community Characterization via a High-Throughput nifH Amplicon Sequencing and Analysis Pipeline

The dinitrogenase reductase gene (nifH) is the most widely established molecular marker for the study of nitrogen-fixing prokaryotes in nature. A large number of PCR primer sets have been developed for nifH amplification, and the effective deployment of these approaches should be guided by a rapid, easy-to-use analysis protocol. Bioinformatic analysis of marker gene sequences also requires considerable expertise. In this study, we advance the state of the art for nifH analysis by evaluating nifH primer set performance, developing an improved amplicon sequencing workflow, and implementing a user-friendly bioinformatics pipeline. The developed amplicon sequencing workflow is a three-stage PCR-based approach that uses established technologies for incorporating sample-specific barcode sequences and sequencing adapters. Based on our primer evaluation, we recommend the Ando primer set be used with a modified annealing temperature of 58°C, as this approach captured the largest diversity of nifH templates, including paralog cluster IV/V sequences. To improve nifH sequence analysis, we developed a computational pipeline which infers taxonomy and optionally filters out paralog sequences. In addition, we employed an empirical model to derive optimal operational taxonomic unit (OTU) cutoffs for the nifH gene at the species, genus, and family levels. A comprehensive workflow script named TaxADivA (TAXonomy Assignment and DIVersity Assessment) is provided to ease processing and analysis of nifH amplicons. Our approach is then validated through characterization of diazotroph communities across environmental gradients in beach sands impacted by the Deepwater Horizon oil spill in the Gulf of Mexico, in a peat moss-dominated wetland, and in various plant compartments of a sugarcane field.IMPORTANCE Nitrogen availability often limits ecosystem productivity, and nitrogen fixation, exclusive to prokaryotes, comprises a major source of nitrogen input that sustains food webs. The nifH gene, which codes for the iron protein of the nitrogenase enzyme, is the most widely established molecular marker for the study of nitrogen-fixing microorganisms (diazotrophs) in nature. In this study, a flexible sequencing/analysis pipeline, named TaxADivA, was developed for nifH amplicons produced by Illumina paired-end sequencing, and it enables an inference of taxonomy, performs clustering, and produces output in formats that may be used by programs that facilitate data exploration and analysis. Diazotroph diversity and community composition are linked to ecosystem functioning, and our results advance the phylogenetic characterization of diazotroph communities by providing empirically derived nifH similarity cutoffs for species, genus, and family levels. The utility of our pipeline is validated for diazotroph communities in a variety of ecosystems, including contaminated beach sands, peatland ecosystems, living plant tissues, and rhizosphere soil.

PufQ regulates porphyrin flux at the haem/bacteriochlorophyll branchpoint of tetrapyrrole biosynthesis via interactions with ferrochelatase

Facultative phototrophs such as Rhodobacter sphaeroides can switch between heterotrophic and photosynthetic growth. This transition is governed by oxygen tension and involves the large-scale production of bacteriochlorophyll, which shares a biosynthetic pathway with haem up to protoporphyrin IX. Here, the pathways diverge with the insertion of Fe2+ or Mg2+ into protoporphyrin by ferrochelatase or magnesium chelatase, respectively. Tight regulation of this branchpoint is essential, but the mechanisms for switching between respiratory and photosynthetic growth are poorly understood. We show that PufQ governs the haem/bacteriochlorophyll switch; pufQ is found within the oxygen-regulated pufQBALMX operon encoding the reaction centre-light-harvesting photosystem complex. A pufQ deletion strain synthesises low levels of bacteriochlorophyll and accumulates the biosynthetic precursor coproporphyrinogen III; a suppressor mutant of this strain harbours a mutation in the hemH gene encoding ferrochelatase, substantially reducing ferrochelatase activity and increasing cellular bacteriochlorophyll levels. FLAG-immunoprecipitation experiments retrieve a ferrochelatase-PufQ-carotenoid complex, proposed to regulate the haem/bacteriochlorophyll branchpoint by directing porphyrin flux toward bacteriochlorophyll production under oxygen-limiting conditions. The co-location of pufQ and the photosystem genes in the same operon ensures that switching of tetrapyrrole metabolism toward bacteriochlorophyll is coordinated with the production of reaction centre and light-harvesting polypeptides.

In vitro stereospecific hydration activities of the 3-vinyl group of chlorophyll derivatives by BchF and BchV enzymes involved in bacteriochlorophyll c biosynthesis of green sulfur bacteria

The photosynthetic green sulfur bacterium Chlorobaculum (Cba.) tepidum produces bacteriochlorophyll (BChl) c pigments bearing a chiral 1-hydroxyethyl group at the 3-position, which self-aggregate to construct main light-harvesting antenna complexes, chlorosomes. The secondary alcoholic hydroxy group is requisite for chlorosomal aggregation and biosynthesized by hydrating the 3-vinyl group of their precursors. Using recombinant proteins of Cba. tepidum BchF and BchV, we examined in vitro enzymatic hydration of some 3-vinyl-chlorophyll derivatives. Both the enzymes catalyzed stereoselective hydration of zinc 3-vinyl-8-ethyl-12-methyl-bacteriopheophorbide c or d to the zinc 3¹ R-bacteriopheophorbide c or d homolog, respectively, with a slight amount of the 3¹ S-epimric species. A similar R-stereoselectivity was observed in the BchF-hydration of zinc 3-vinyl-8-ethyl- and propyl-12-ethyl-bacteriopheophorbides c, while their BchV-hydration gave a relatively larger amount of the 3¹ S-epimers. The in vitro stereoselective hydration confirmed the in vivo production of the S-epimeric species by BchV. The enzymatic hydration for the above 8-propylated substrate proceeded more slowly than that for the 8-ethylated, and the 8-isobutylated substrate was no longer hydrated. Based on these results, biosynthetic pathways of BChl c homologs and epimers are proposed.

Broadened Substrate Specificity of 3-Hydroxyethyl Bacteriochlorophyllide a Dehydrogenase (BchC) Indicates a New Route for the Biosynthesis of Bacteriochlorophyll a

Bacteriochlorophyll a biosynthesis requires formation of a 3-hydroxyethyl group on pyrrole ring A that gets subsequently converted into a 3-acetyl group by 3-vinyl bacteriochlorophyllide a hydratase (BchF) followed by 3-hydroxyethyl bacteriochlorophyllide a dehydrogenase (BchC). Heterologous overproduction of Chlorobaculum tepidum BchF revealed an integral transmembrane protein that was efficiently isolated by detergent solubilization. Recombinant C. tepidum BchC was purified as a soluble protein-NAD(+) complex. Substrate recognition of BchC was investigated using six artificial substrate molecules. Modification of the isocyclic E ring, omission of the central magnesium ion, zinc as an alternative metal ion, and a non-reduced B ring system were tolerated by BchC. According to this broadened in vitro activity, the chlorin 3-hydroxyethyl chlorophyllide a was newly identified as a natural substrate of BchC in a reconstituted pathway consisting of dark-operative protochlorophyllide oxidoreductase, BchF, and BchC. The established reaction sequence would allow for an additional new branching point for the synthesis of bacteriochlorophyll a. Biochemical and site-directed mutagenesis analyses revealed, in contrast to theoretical predictions, a zinc-independent BchC catalysis that requires NAD(+) as a cofactor. Based on these results, we are designating a new medium-chain dehydrogenase/reductase family (MDR057 BchC) as theoretically proposed from a recent bioinformatics analysis.

Identification of the chlE gene encoding oxygen-independent Mg-protoporphyrin IX monomethyl ester cyclase in cyanobacteria

The fifth ring (E-ring) of chlorophyll (Chl) a is produced by Mg-protoporphyrin IX monomethyl ester (MPE) cyclase. There are two evolutionarily unrelated MPE cyclases: oxygen-independent (BchE) and oxygen-dependent (ChlA/AcsF) MPE cyclases. Although ChlA is the sole MPE cyclase in Synechocystis PCC 6803, it is yet unclear whether BchE exists in cyanobacteria. A BLAST search suggests that only few cyanobacteria possess bchE. Here, we report that two bchE candidate genes from Cyanothece strains PCC 7425 and PCC 7822 restore the photosynthetic growth and bacteriochlorophyll production in a bchE-lacking mutant of Rhodobacter capsulatus. We termed these cyanobacterial bchE orthologs "chlE."

Rhodobacter sphaeroides mutants overexpressing chlorophyllide a oxidoreductase of Blastochloris viridis elucidate functions of enzymes in late bacteriochlorophyll biosynthetic pathways

In previous studies we have demonstrated that chlorophyllide a oxidoreductases (CORs) from bacteriochlorophyll (BChl) a-producing Rhodobacter species and BChl b-producing Blastochloris viridis show distinct substrate recognition and different catalytic hydrogenation reactions, and that these two types of CORs therefore cause committed steps for BChls a and b biosynthesis. In this study, COR genes from B. viridis were incorporated and overexpressed in a series of Rhodobacter sphaeroides mutants. We found that the following two factors are essential in making R. sphaeroides produce BChl b: the loss of functions of both intrinsic COR and 8-vinyl reductase (BciA) in the host R. sphaeroides strain; and expression of the BchYZ catalytic components of COR from B. viridis, not the complete set of COR (BchXYZ), in the host strain. In addition, we incorporated bchYZ of B. viridis into the R. sphaeroides mutant lacking BchJ and BciA, resulting in the strain accumulating both BChl a and BChl b. This is the first example of an anoxygenic photosynthetic bacterium producing BChls a and b together. The results suggest that BchJ enhances activity of the intrinsic COR. The physiological significance of BchJ in pigment biosynthetic pathways will be discussed.

Evolutionary Aspects and Regulation of Tetrapyrrole Biosynthesis in Cyanobacteria under Aerobic and Anaerobic Environments

Chlorophyll a (Chl) is a light-absorbing tetrapyrrole pigment that is essential for photosynthesis. The molecule is produced from glutamate via a complex biosynthetic pathway comprised of at least 15 enzymatic steps. The first half of the Chl pathway is shared with heme biosynthesis, and the latter half, called the Mg-branch, is specific to Mg-containing Chl a. Bilin pigments, such as phycocyanobilin, are additionally produced from heme, so these light-harvesting pigments also share many common biosynthetic steps with Chl biosynthesis. Some of these common steps in the biosynthetic pathways of heme, Chl and bilins require molecular oxygen for catalysis, such as oxygen-dependent coproporphyrinogen III oxidase. Cyanobacteria thrive in diverse environments in terms of oxygen levels. To cope with Chl deficiency caused by low-oxygen conditions, cyanobacteria have developed elaborate mechanisms to maintain Chl production, even under microoxic environments. The use of enzymes specialized for low-oxygen conditions, such as oxygen-independent coproporphyrinogen III oxidase, constitutes part of a mechanism adapted to low-oxygen conditions. Another mechanism adaptive to hypoxic conditions is mediated by the transcriptional regulator ChlR that senses low oxygen and subsequently activates the transcription of genes encoding enzymes that work under low-oxygen tension. In diazotrophic cyanobacteria, this multilayered regulation also contributes in Chl biosynthesis by supporting energy production for nitrogen fixation that also requires low-oxygen conditions. We will also discuss the evolutionary implications of cyanobacterial tetrapyrrole biosynthesis and regulation, because low oxygen-type enzymes also appear to be evolutionarily older than oxygen-dependent enzymes.

Iron-sulfur cluster-dependent catalysis of chlorophyllide a oxidoreductase from Roseobacter denitrificans

Bacteriochlorophyll a biosynthesis requires the stereo- and regiospecific two electron reduction of the C7-C8 double bond of chlorophyllide a by the nitrogenase-like multisubunit metalloenzyme, chlorophyllide a oxidoreductase (COR). ATP-dependent COR catalysis requires interaction of the protein subcomplex (BchX)2 with the catalytic (BchY/BchZ)2 protein to facilitate substrate reduction via two redox active iron-sulfur centers. The ternary COR enzyme holocomplex comprising subunits BchX, BchY, and BchZ from the purple bacterium Roseobacter denitrificans was trapped in the presence of the ATP transition state analog ADP·AlF4(-). Electron paramagnetic resonance experiments revealed a [4Fe-4S] cluster of subcomplex (BchX)2. A second [4Fe-4S] cluster was identified on (BchY/BchZ)2. Mutagenesis experiments indicated that the latter is ligated by four cysteines, which is in contrast to the three cysteine/one aspartate ligation pattern of the closely related dark-operative protochlorophyllide a oxidoreductase (DPOR). In subsequent mutagenesis experiments a DPOR-like aspartate ligation pattern was implemented for the catalytic [4Fe-4S] cluster of COR. Artificial cluster formation for this inactive COR variant was demonstrated spectroscopically. A series of chemically modified substrate molecules with altered substituents on the individual pyrrole rings and the isocyclic ring were tested as COR substrates. The COR enzyme was still able to reduce the B ring of substrates carrying modified substituents on ring systems A, C, and E. However, substrates with a modification of the distantly located propionate side chain were not accepted. A tentative substrate binding mode was concluded in analogy to the related DPOR system.

Nitrogenase and homologs

Nitrogenase catalyzes biological nitrogen fixation, a key step in the global nitrogen cycle. Three homologous nitrogenases have been identified to date, along with several structural and/or functional homologs of this enzyme that are involved in nitrogenase assembly, bacteriochlorophyll biosynthesis and methanogenic process, respectively. In this article, we provide an overview of the structures and functions of nitrogenase and its homologs, which highlights the similarity and disparity of this uniquely versatile group of enzymes.

Engineered biosynthesis of bacteriochlorophyll b in Rhodobacter sphaeroides

Bacteriochlorophyll b has the most red-shifted absorbance maximum of all naturally occurring photopigments. It has a characteristic ethylidene group at the C8 position in place of the more common ethyl group, the product of a C8-vinyl reductase, which is carried by the majority of chlorophylls and bacteriochlorophylls used in photosynthesis. The subsequent and first step exclusive to bacteriochlorophyll biosynthesis, the reduction of the C7 = C8 bond, is catalyzed by chlorophyllide oxidoreductase. It has been demonstrated that the enzyme from bacteriochlorophyll a-utilizing bacteria can catalyze the formation of compounds carrying an ethyl group at C8 from both ethyl- and vinyl-carrying substrates, indicating a surprising additional C8-vinyl reductase function, while the enzyme from organisms producing BChl b could only catalyze C7 = C8 reduction with a vinyl substrate, but this product carried an ethylidene group at the C8 position. We have replaced the native chlorophyllide oxidoreductase-encoding genes of Rhodobacter sphaeroides with those from Blastochloris viridis, but the switch from bacteriochlorophyll a to b biosynthesis is only detected when the native conventional C8-vinyl reductase is absent. We propose a non-enzymatic mechanism for ethylidene group formation based on the absence of cellular C8-vinyl reductase activity.

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We engineer the production of a foreign photopigment in Rhodobacter sphaeroides.

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Native COR-encoding genes are replaced with those from Blastochloris viridis.

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Bacteriochlorophyll b is produced upon additional deletion of conventional 8VR.

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We propose that loss of 8VR activity by COR leads to ethylidene bond formation.

Dark-operative protochlorophyllide oxidoreductase generates substrate radicals by an iron-sulphur cluster in bacteriochlorophyll biosynthesis

Photosynthesis converts solar energy to chemical energy using chlorophylls (Chls). In a late stage of biosynthesis of Chls, dark-operative protochlorophyllide (Pchlide) oxidoreductase (DPOR), a nitrogenase-like enzyme, reduces the C17 = C18 double bond of Pchlide and drastically changes the spectral properties suitable for photosynthesis forming the parental chlorin ring for Chl a. We previously proposed that the spatial arrangement of the proton donors determines the stereospecificity of the Pchlide reduction based on the recently resolved structure of the DPOR catalytic component, NB-protein. However, it was not clear how the two-electron and two-proton transfer events are coordinated in the reaction. In this study, we demonstrate that DPOR initiates a single electron transfer reaction from a [4Fe-4S]-cluster (NB-cluster) to Pchlide, generating Pchlide anion radicals followed by a single proton transfer, and then, further electron/proton transfer steps transform the anion radicals into chlorophyllide (Chlide). Thus, DPOR is a unique iron-sulphur enzyme to form substrate radicals followed by sequential proton- and electron-transfer steps with the protein folding very similar to that of nitrogenase. This novel radical-mediated reaction supports the biosynthesis of Chl in a wide variety of photosynthetic organisms.

Reconstitution of a sequential reaction of two nitrogenase-like enzymes in the bacteriochlorophyll biosynthetic pathway of Rhodobacter capsulatus

The parental structure of bacteriochlorophyll a, bacteriochlorin, is formed by a sequential operation of two nitrogenase-like enzymes, dark-operative protochlorophyllide oxidoreductase (DPOR) and chlorophyllide a oxidoreductase (COR). Both DPOR and COR consist of two components, Fe protein and MoFe protein cognates. Here we determined kinetic parameters of COR and established the reconstitution system for the formation of bacteriochlorin (3-vinyl bacteriochlorophyllide a) from porphyrin (protochlorophyllide) with purified components of DPOR and COR from Rhodobacter capsulatus. This reconstitution system confirmed the recent finding that COR catalyzes 8-vinyl reduction of 8-vinyl chlorophyllide a in addition to the known activity of C7C8 double bond reduction, and provides a promising model to investigate how two nitrogenase-like enzymes are coordinated in bacteriochlorophyll biosynthesis.