Effects of glycine on cell growth and pigment biosynthesis in Rhodobacter azotoformans

The effect of exogenous glycine (a precursor for the biosynthesis of bacteriochlorophyll) on the cell growth and photopigment accumulation was investigated in phototrophic growing Rhodobacter azotoformans 134K20. The growth rate and the biomass of strain 134K20 were significantly inhibited by glycine addition when ammonium sulfate or glutamate were used as nitrogen sources and acetate or succinate as carbon sources. A characteristic absorption maximum at approximately 423 nm was present in the absorption spectra of glutamate cultures while it was absent by the addition of high-concentration glycine of 15 mM. The component account for the 423 nm peak was eventually identified as magnesium protoporphyrin IX monomethyl ester, a precursor of bacteriochlorophyll a (BChl a). Comparative analysis of pigment composition revealed that the amount of BChl a precursors was significantly decreased by the addition of 15-mM glycine while the BChl a accumulation was increased. Moreover, glycine changed the carotenoid compositions and stimulated the accumulation of spheroidene. The A₈₅₀ /A₈₇₅ in the growth-inhibited cultures was increased, indicating an increased level of the light-harvesting complex 2 compared to the reaction center. The exogenous glycine possibly played an important regulation role in photosynthesis of purple bacteria.

The monofunctional cobalamin biosynthesis enzyme precorrin-3B synthase (CobZRR) is essential for anaerobic photosynthesis in Rhodospirillum rubrum but not for aerobic dark metabolism

The in vivo physiological role of the gene cobZ, which encodes precorrin-3B synthase, which catalyzes the initial porphyrin ring contraction step of cobalamin biosynthesis via the cob pathway, has been demonstrated here for the first time. Cobalamin is known to be essential for an early step of bacteriochlorophyll biosynthesis in anoxygenic purple bacteria. The cobZ (cobZRR) gene of the purple bacterium Rhodospirillum rubrum was localized to a 23.5 kb insert of chromosomal DNA contained on the cosmid pSC4. pSC4 complemented several mutants of bacteriochlorophyll and carotenoid biosynthesis, due to the presence of the bchCX and crtCDEF genes at one end of the cosmid insert, flanking cobZRR. A second gene, citB/tcuB, immediately downstream of cobZRR, shows homologies to both a tricarballylate oxidoreductase (tcuB) and a gene (citB) involved in signal transduction during citrate uptake. CobZRR shows extensive homology to the N-terminal domain of the bifunctional CobZ from Rhodobacter capsulatus, and the R. rubrum citB/tcuB gene is homologous to the CobZ C-terminal domain. A mutant, SERGK25, containing a terminatorless kanamycin interposon inserted into cobZRR, could not grow by anaerobic photosynthesis, but grew normally under dark, aerobic and microaerophilic conditions with succinate and fructose as carbon sources. The anaerobic in vivo activity of CobZ indicates that it does not require oxygen as a substrate. The mutant excreted large amounts of protoporphyrin IX-monomethylester, a brown precursor of bacteriochlorophyll biosynthesis. The mutant was complemented either by the cobZRR gene in trans, or when exogenous cobalamin was added to the medium. A deletion mutant of tcuB/citB did not exhibit the cob phenotype. Thus, a role for tcuB/citB in cobalamin biosynthesis could not be confirmed.

C-C bond forming radical SAM enzymes involved in the construction of carbon skeletons of cofactors and natural products

Covering: up to the end of 2017 C-C bond formations are frequently the key steps in cofactor and natural product biosynthesis. Historically, C-C bond formations were thought to proceed by two electron mechanisms, represented by Claisen condensation in fatty acids and polyketide biosynthesis. These types of mechanisms require activated substrates to create a nucleophile and an electrophile. More recently, increasing number of C-C bond formations catalyzed by radical SAM enzymes are being identified. These free radical mediated reactions can proceed between almost any sp3 and sp2 carbon centers, allowing introduction of C-C bonds at unconventional positions in metabolites. Therefore, free radical mediated C-C bond formations are frequently found in the construction of structurally unique and complex metabolites. This review discusses our current understanding of the functions and mechanisms of C-C bond forming radical SAM enzymes and highlights their important roles in the biosynthesis of structurally complex, naturally occurring organic molecules. Mechanistic consideration of C-C bond formation by radical SAM enzymes identifies the significance of three key mechanistic factors: radical initiation, acceptor substrate activation and radical quenching. Understanding the functions and mechanisms of these characteristic enzymes will be important not only in promoting our understanding of radical SAM enzymes, but also for understanding natural product and cofactor biosynthesis.

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.

Biomass and pigments production in photosynthetic bacteria wastewater treatment: effects of light sources

This study is aimed at enhancing biomass and pigments production together with pollution removal in photosynthetic bacteria (PSB) wastewater treatment via different light sources. Red, yellow, blue, white LED and incandescent lamp were used. Results showed different light sources had great effects on the PSB. PSB had the highest biomass production, COD removal and biomass yield with red LED. The corresponding biomass, COD removal and biomass yield reached 2580 mg/L, 88.6% and 0.49 mg-biomass/mg-COD-removal, respectively. The hydraulic retention time of wastewater treatment could be shortened to 72 h with red LED. Mechanism analysis showed higher ATP was produced with red LED than others. Light sources could significantly affect the pigments production. The pigments productions were greatly higher with LED than incandescent lamp. Yellow LED had the highest pigments production while red LED produced the highest carotenoid/bacteriochlorophyll ratio. Considering both efficiency and energy cost, red LED was the optimal light source.

[Influence of iron on siderophore and photosynthetic pigments biosynthesis by siderophore-producing Rhodopesudomonnas palustris]

OBJECTIVE

To explore the regulation of iron on siderophore production, cell growth and photosynthetic pigments biosynthesis by siderophore-producing anoxygenic phototrophic bacteria.

METHODS

Siderophore production was determined using Chrome Azurol S (CAS) assay. The siderophore types were determined by Arnow method, Csaky test and Shenker test. The compositions and contents of photosynthetic pigments were determined by spectrophotometry and HPLC analysis.

RESULTS

Rhodopseudomonas palustris (Rps. palustris) CQV97 was capable of producing hydroxamate-type of siderophore. Siderophore production reached the highest yield in the absence of ferric chloride. With increasing ferric chloride concentrations, the lag phase of cell growth was shortened, and the cell growth rate, final biomass and the total amounts of carotenoid and bacteriochlorophyll a were increased significantly. The characteristic absorption maxima of carotenoids from pigment extracts were blueshifted. Iron concentration had little effect on the compositions and relative contents of bacteriochlorophylls a, whereas predominately affected carotenoid compositions, rhodopin was present as major carotenoid component instead of spirillxanthin. Culture tends to accumulate the Cars having shorter conjugated double bonds at the expense of longer conjugated double bonds as the ferric chloride concentration increased. The changes in carotenoid composition were consistent with those of the blue shift of absorption spectra of pigment extracts.

CONCLUSION

Rps. palustris CQV97 can produce siderophore and the changes in microbial growth, siderophore production and photosynthetic pigments accumulation of anoxygenic phototrophic bacteria are related to the iron concentration in the medium.

A tale of two reductases: extending the bacteriochlorophyll biosynthetic pathway in E. coli

The creation of a synthetic microbe that can harvest energy from sunlight to drive its metabolic processes is an attractive approach to the economically viable biosynthetic production of target compounds. Our aim is to design and engineer a genetically tractable non-photosynthetic microbe to produce light-harvesting molecules. Previously we created a modular, multienzyme system for the heterologous production of intermediates of the bacteriochlorophyll (BChl) pathway in E. coli. In this report we extend this pathway to include a substrate promiscuous 8-vinyl reductase that can accept multiple intermediates of BChl biosynthesis. We present an informative comparative analysis of homologues of 8-vinyl reductase from the model photosynthetic organisms Rhodobacter sphaeroides and Chlorobaculum tepidum. The first purification of the enzymes leads to their detailed biochemical and biophysical characterization. The data obtained reveal that the two 8-vinyl reductases are substrate promiscuous, capable of reducing the C8-vinyl group of Mg protoporphyrin IX, Mg protoporphyrin IX methylester, and divinyl protochlorophyllide. However, activity is dependent upon the presence of chelated Mg²⁺ in the porphyrin ring, with no activity against non-Mg²⁺ chelated intermediates observed. Additionally, CD analyses reveal that the two 8-vinyl reductases appear to bind the same substrate in a different fashion. Furthermore, we discover that the different rates of reaction of the two 8-vinyl reductases both in vitro, and in vivo as part of our engineered system, results in the suitability of only one of the homologues for our BChl pathway in E. coli. Our results offer the first insights into the different functionalities of homologous 8-vinyl reductases. This study also takes us one step closer to the creation of a nonphotosynthetic microbe that is capable of harvesting energy from sunlight for the biosynthesis of molecules of choice.

Specific gene bciD for C7-methyl oxidation in bacteriochlorophyll e biosynthesis of brown-colored green sulfur bacteria

The gene named bciD, which encodes the enzyme involved in C7-formylation in bacteriochlorophyll e biosynthesis, was found and investigated by insertional inactivation in the brown-colored green sulfur bacterium Chlorobaculum limnaeum (previously called Chlorobium phaeobacteroides). The bciD mutant cells were green in color, and accumulated bacteriochlorophyll c homologs bearing the 7-methyl group, compared to C7-formylated BChl e homologs in the wild type. BChl-c homolog compositions in the mutant were further different from those in Chlorobaculum tepidum which originally produced BChl c: (3(1) S)-8-isobutyl-12-ethyl-BChl c was unusually predominant.

In vitro synthesis and characterization of bacteriochlorophyll-f and its absence in bacteriochlorophyll-e producing organisms

Bacteriochlorophyll(BChl)-f which has not yet been found in natural phototrophs was prepared by chemically modifying chlorophyll-b. The retention time of reverse-phase high-performance liquid chromatography of the synthetic monomeric BChl-f as well as its visible absorption and fluorescence emission spectra in a solution were identified and compared with other naturally occurring chlorophyll pigments obtained from the main light-harvesting antenna systems of green sulfur bacteria, BChls-c/d/e. Based on the above data, BChl-f was below the level of detection in three strains of green photosynthetic bacteria producing BChl-e.

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.

Chlorophyll Biosynthesis in Bacteria: The Origins of Structural and Functional Diversity

The use of photochemical reaction centers to convert light energy into chemical energy, chlorophototrophy, occurs in organisms belonging to only five eubacterial phyla: Cyanobacteria, Proteobacteria, Chlorobi, Chloroflexi, and Firmicutes. All chlorophototrophs synthesize two types of pigments: (a) chlorophylls and bacteriochlorophylls, which function in both light harvesting and uniquely in photochemistry; and (b) carotenoids, which function primarily as photoprotective pigments but can also participate in light harvesting. Although hundreds of carotenoids have been identified, only 12 types of chlorophylls (Chl a, b, d; divinyl-Chl a and b; and 8(1)-hydroxy-Chl a) and bacteriochlorophylls (BChl a, b, c, d, e, and g) are currently known to occur in bacteria. This review summarizes recent progress in the identification of genes and enzymes in the biosynthetic pathways leading to Chls and BChls, the essential tetrapyrrole cofactors of photosynthesis, and addresses the mechanisms for generating functional diversity for solar energy capture and conversion in chlorophototrophs.

Candidatus Chloracidobacterium thermophilum: an aerobic phototrophic Acidobacterium

Only five bacterial phyla with members capable of chlorophyll (Chl)-based phototrophy are presently known. Metagenomic data from the phototrophic microbial mats of alkaline siliceous hot springs in Yellowstone National Park revealed the existence of a distinctive bacteriochlorophyll (BChl)-synthesizing, phototrophic bacterium. A highly enriched culture of this bacterium grew photoheterotrophically, synthesized BChls a and c under oxic conditions, and had chlorosomes and type 1 reaction centers. "Candidatus Chloracidobacterium thermophilum" is a BChl-producing member of the poorly characterized phylum Acidobacteria.

Rhodobacter capsulatus magnesium chelatase subunit BchH contains an oxygen sensitive iron-sulfur cluster

Magnesium chelatase is the first unique enzyme of the bacteriochlorophyll biosynthetic pathway. It consists of three subunits (BchI, BchD, and BchH). Amino acid sequence analysis of the Rhodobacter capsulatus BchH revealed a novel cysteine motif (393CX2CX3CX14C) that was found in only six other proteobacteria (CX2CX3CX11-14C). The cysteine motif is likely to coordinate an unprecedented [Fe-S] cluster. Purified BchH demonstrated absorbance in the 460 nm region. This absorbance was abolished in BchH proteins with alanine substitutions at positions Cys396 and Cys414. These modified proteins were also EPR silent. In contrast, wild type BchH protein in the reduced state showed EPR signals resembling those of a [4Fe-4S] cluster with rhombic symmetry and g values at 1.90, 1.93, and 2.09, superimposed with a [3Fe-4S] cluster centered at g = 2.02. The [3Fe-4S] signal was observed independently of the [4Fe-4S] signal under oxidizing conditions. Mg-chelatase activity assays showed that the cluster is not catalytic. We suggest that the [4Fe-4S] and [3Fe-4S] signals originate from a single coordination site on the monomeric BchH protein and that the [4Fe-4S] cluster is sensitive to oxidation. It is speculated that the cluster participates in the switching between aerobic and anaerobic life of the proteobacteria.

Recent advances in chlorophyll biosynthesis

The importance of chlorophyll (Chl) to the process of photosynthesis is obvious, and there is clear evidence that the regulation of Chl biosynthesis has a significant role in the regulation of assembly of the photosynthetic apparatus. The understanding of Chl biosynthesis has rapidly advanced in recent years. The identification of genetic loci associated with each of the biochemical steps has been accompanied by a greater appreciation of the role of Chl biosynthesis intermediates in intracellular signaling. The purpose of this review is to provide a source of information for all the steps in Chl and bacteriochlorophyll a biosynthesis, with an emphasis on steps that are believed to be key regulation points.

Crystal structures of BchU, a methyltransferase involved in bacteriochlorophyll c biosynthesis, and its complex with S-adenosylhomocysteine: implications for reaction mechanism

BchU plays a role in bacteriochlorophyll c biosynthesis by catalyzing methylation at the C-20 position of cyclic tetrapyrrole chlorin using S-adenosylmethionine (SAM) as a methyl source. This methylation causes red-shifts of the electronic absorption spectrum of the light-harvesting pigment, allowing green photosynthetic bacteria to adapt to low-light environments. We have determined the crystal structures of BchU and its complex with S-adenosylhomocysteine (SAH). BchU forms a dimer and each subunit consists of two domains, an N-terminal domain and a C-terminal domain. Dimerization occurs through interactions between the N-terminal domains and the residues responsible for the catalytic reaction are in the C-terminal domain. The binding site of SAH is located in a large cavity between the two domains, where SAH is specifically recognized by many hydrogen bonds and a salt-bridge. The electron density map of BchU in complex with an analog of bacteriochlorophyll c located its central metal near the SAH-binding site, but the tetrapyrrole ring was invisible, suggesting that binding of the ring to BchU is loose and/or occupancy of the ring is low. It is likely that His290 acts as a ligand for the central metal of the substrate. The orientation of the substrate was predicted by simulation, and allows us to propose a mechanism for the BchU directed methylation: the strictly conserved Tyr246 residue acts catalytically in the direct transfer of the methyl group from SAM to the substrate through an S(N)2-like mechanism.

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

In most photosynthetic organisms, the chlorin ring structure of chlorophyll a is formed by the reduction of the porphyrin D-ring by the dark-operative nitrogenase-like enzyme, protochlorophyllide reductase (DPOR). Subsequently, the chlorin B-ring is reduced in bacteriochlorophyll biosynthesis to form a bacteriochlorin ring structure. Phenotypic analysis of mutants lacking one of three genes, bchX, bchY, or bchZ, which show significant sequence similarity to the structural genes of nitrogenase, suggests that a second nitrogenase-like enzyme is involved in the chlorin B-ring reduction. However, there is no biochemical evidence for this. Here, we report the reconstitution of chlorophyllide a reductase (COR) with purified proteins. Two Rhodobacter capsulatus strains that overexpressed Strep-tagged BchX and BchY were isolated. Strep-tagged BchX was purified as a single polypeptide, and BchZ was co-purified with Strep-tagged BchY. When BchX and BchY-BchZ components were incubated with chlorophyllide a, ATP, and dithionite under anaerobic conditions, chlorophyllide a was converted to a new pigment with a Qy band of longer wavelength at 734 nm (P734) in 80% acetone. The formation of P734 was dependent on ATP and dithionite. High performance liquid chromatography and mass spectroscopic analysis indicated that P734 is 3-vinyl bacteriochlorophyllide a, which is formed by the B-ring reduction of chlorophyllide a. These results demonstrate that the B-ring of chlorin is reduced by a second nitrogenase-like enzyme and that the sequential actions of two nitrogenase-like enzymes, DPOR and COR, convert porphyrin to bacteriochlorin. The evolutionary implications of nitrogenase-like enzymes to determine the ring structure of (bacterio)chlorophyll pigments are discussed.

Stappia marina sp. nov., a marine bacterium isolated from the Yellow Sea

A Gram-negative, aerobic and halophilic bacterium designated strain mano18T was isolated from a tidal flat area of Dae-Chun, Chung-Nam, Korea. This strain was motile by means of polar flagella, occasionally forming rosette-like aggregates, reduced nitrate to nitrite, required sodium ions for growth, exhibited catalase and oxidase activities and contained Q-10 as the major quinone and C(18 : 1)omega7c as the dominant cellular fatty acid. Analysis of the 16S rRNA gene sequence revealed that this strain is affiliated with a cluster within the Alphaproteobacteria. Strain mano18T synthesized bacteriochlorophyll under aerobic conditions. The 16S rRNA gene sequence similarity between strain mano18T and the most closely related species, Stappia aggregata DSM 13394T, was 98.5 %. Levels of DNA-DNA relatedness between strain mano18T and the type strains of S. aggregata and Stappia stellulata were respectively 6.2-11.2 and 3.3-7.6 %. Strain mano18T, like other Stappia strains, possesses carbon monoxide dehydrogenase genes. The results of DNA-DNA hybridization and the polyphasic data confirmed that strain mano18T can be considered to represent a novel taxon in the genus Stappia. The name Stappia marina sp. nov. is proposed for the tidal flat isolate; the type strain is strain mano18T (= KCTC 12288T = DSM 17023T).

Biogenesis of chlorophyll-binding proteins under iron stress in Synechocystis sp. PCC 6803

The biogenesis of chlorophyll-binding proteins under iron stress has been investigated in vivo in a chlN deletion mutant of Synechocystis sp. PCC 6803. The chlN gene encodes one subunit of the light-independent protochlorophyllide reductase. The mutant is unable to synthesis chlorophyll in darkness, causing chlorophyll biosynthesis to become light dependent. When the mutant was propagated in darkness, essentially no chlorophyll and photosystems were detected. Upon return of the chlN deletion mutant to light, 77 K fluorescence emission spectra and oxygen evolution of greening cells under iron-sufficient or -deficient conditions were measured. The gradual blue shift of the photosystem I (PS I) peak upon greening under iron stress suggested the structural alteration of newly synthesized PS I. Furthermore, the rate of biogenesis of PS II was delayed under iron stress, which might be due to the presence of IsiA.

Novel water-soluble bacteriochlorophyll derivatives for vascular-targeted photodynamic therapy: synthesis, solubility, phototoxicity and the effect of serum proteins

New negatively charged water-soluble bacteriochlorophyll (Bchl) derivatives were developed in our laboratory for vascular-targeted photodynamic therapy (VTP). Here we focused on the synthesis, characterization and interaction of the new candidates with serum proteins and particularly on the effect of serum albumin on the photocytotoxicity of WST11, a representative compound of the new derivatives. Using several approaches, we found that aminolysis of the isocyclic ring with negatively charged residues markedly increases the hydrophilicity of the Bchl sensitizers, decreases their self-association constant and selectively increases their affinity to serum albumin, compared with other serum proteins. The photocytotoxicity of the new candidates in endothelial cell culture largely depends on the concentration of the serum albumin. Importantly, after incubation with physiological concentrations of serum albumin (500-600 microM), WST11 was found to be poorly photocytotoxic (>80% endothelial cell survival in cell cultures). However, in a recent publication (Mazor, O. et al. [2005] Photochem. Photobiol. 81, 342-351) we showed that VTP of M2R melanoma xenografts with a similar WST11 concentration resulted in approximately 100% tumor flattening and >70% cure rate. We therefore propose that the two studies collectively suggest that the antitumor activity of WST11 and probably of other similar candidates does not depend on direct photointoxication of individual endothelial cells but on the vascular tissue response to the VTP insult.

Crystallization and preliminary X-ray diffraction study of BchU, a methyltransferase from Chlorobium tepidum involved in bacteriochlorophyll c biosynthesis

The S-adenosylmethionine-dependent methyltransferase BchU is an enzyme involved in the bacteriochlorophyll c biosynthetic pathway and catalyzes methylation at the C-20 position of the chlorin moiety. Recombinant Chlorobium tepidum BchU overproduced in Escherichia coli was purified and crystallized by the hanging-drop vapour-diffusion method using ammonium sulfate as a precipitant. The crystals belonged to the hexagonal space group P6(1)22 or P6(5)22, with unit-cell parameters a = b = 81.5, c = 250.7 A. A native data set was collected to 2.27 A resolution using synchrotron radiation at SPring-8.

The puhE gene of Rhodobacter capsulatus is needed for optimal transition from aerobic to photosynthetic growth and encodes a putative negative modulator of bacteriochlorophyll production

A conserved orf of previously unknown function (herein designated as puhE) is located 3' of the reaction centre H (puhA) gene in purple photosynthetic bacteria, in the order puhABCE in Rhodobacter capsulatus. Disruptions of R. capsulatus puhE resulted in a long lag in the growth of photosynthetic cultures inoculated with cells grown under high aeration, and increased the level of the peripheral antenna, light-harvesting complex 2 (LH2). The amount of the photosynthetic reaction centre (RC) and its core antenna, light-harvesting complex 1 (LH1), was reduced; however, there was no decrease in expression of a lacZ reporter fused to the puf (RC and LH1) promoter, in RC assembly in the absence of LH1, or in LH1 assembly in the absence of the RC. In strains that lack LH2, disruption of puhE increased the in vivo absorption at 780 nm, which we attribute to excess bacteriochlorophyll a (BChl) pigment production. This effect was seen in the presence and absence of PufQ, a protein that stimulates BChl biosynthesis. Expression of puhE from a plasmid reduced A(780) production in puhE mutants. We suggest that PuhE modulates BChl biosynthesis independently of PufQ, and that the presence of excess BChl in PuhE(-)LH2(+) strains results in excess LH2 assembly and also interferes with the adaptation of cells during the transition from aerobic respiratory to anaerobic photosynthetic growth.

In vitro activity of C-20 methyltransferase, BchU, involved in bacteriochlorophyllcbiosynthetic pathway in green sulfur bacteria

The activity of a methyltransferase, BchU, which catalyzes methylation at the C-20 position of chlorin ring in the biosynthetic pathway of bacteriochlorophyll c, was investigated in vitro. The bchU gene derived from the photosynthetic green sulfur bacterium, Chlorobium tepidum, was overexpressed in Escherichia coli as a His-tagged protein (His(6)-BchU), and the enzyme was purified. In the presence of S-adenosylmethionine, His(6)-BchU methylated zinc bacteriopheophorbide d at the C-20 position to give zinc bacteriopheophorbide c. Metal-free bacteriopheophorbide d could not be methylated by the BchU, indicating that the central metal in the chlorin should be required for the recognition by the BchU.

Seeing green bacteria in a new light: genomics-enabled studies of the photosynthetic apparatus in green sulfur bacteria and filamentous anoxygenic phototrophic bacteria

Based upon their photosynthetic nature and the presence of a unique light-harvesting antenna structure, the chlorosome, the photosynthetic green bacteria are defined as a distinctive group in the Bacteria. However, members of the two taxa that comprise this group, the green sulfur bacteria (Chlorobi) and the filamentous anoxygenic phototrophic bacteria ("Chloroflexales"), are otherwise quite different, both physiologically and phylogenetically. This review summarizes how genome sequence information facilitated studies of the biosynthesis and function of the photosynthetic apparatus and the oxidation of inorganic sulfur compounds in two model organisms that represent these taxa, Chlorobium tepidum and Chloroflexus aurantiacus. The genes involved in bacteriochlorophyll (BChl) c and carotenoid biosynthesis in these two organisms were identified by sequence homology with known BChl a and carotenoid biosynthesis enzymes, gene cluster analysis in Cfx. aurantiacus, and gene inactivation studies in Chl. tepidum. Based on these results, BChl a and BChl c biosynthesis is similar in the two organisms, whereas carotenoid biosynthesis differs significantly. In agreement with its facultative anaerobic nature, Cfx. aurantiacus in some cases apparently produces structurally different enzymes for heme and BChl biosynthesis, in which one enzyme functions under anoxic conditions and the other performs the same reaction under oxic conditions. The Chl. tepidum mutants produced with modified BChl c and carotenoid species also allow the functions of these pigments to be studied in vivo.

Thioredoxin can influence gene expression by affecting gyrase activity

The expression of many genes of facultatively photosynthetic bacteria of the genus Rhodobacter is controlled by the oxygen tension. Among these are the genes of the puf and puc operons, which encode proteins of the photosynthetic apparatus. Previous results revealed that thioredoxins are involved in the regulated expression of these operons, but it remained unsolved as to the mechanisms by which thioredoxins affect puf and puc expression. Here we show that reduced TrxA of Rhodobacter capsulatus and Rhodobacter sphaeroides and oxidized TrxC of R.capsulatus interact with DNA gyrase and alter its DNA supercoiling activity. While TrxA enhances supercoiling, TrxC exerts a negative effect on this activity. Furthermore, inhibition of gyrase activity strongly reduces puf and puc expression. Our results reveal a new signaling pathway by which oxygen can affect the expression of bacterial genes.

Oceanibulbus indolifex gen. nov., sp. nov., a North Sea alphaproteobacterium that produces bioactive metabolites

A water sample from the North Sea was used to isolate the abundant heterotrophic bacteria that are able to grow on complex marine media. Isolation was by serial dilution and spread plating. Phylogenetic analysis of nearly complete 16S rRNA gene sequences revealed that one of the strains, HEL-45T, had 97·4 % sequence similarity to Sulfitobacter mediterraneus and 96·5 % sequence similarity to Staleya guttiformis. Strain HEL-45T is a Gram-negative, non-motile rod and obligate aerobe and requires sodium and 1–7 % sea salts for growth. It contains storage granules and does not produce bacteriochlorophyll. Optimal growth temperatures are 25–30 °C. The DNA base composition (G+C content) is 60·1 mol%. Strain HEL-45T has Q10 as the dominant respiratory quinone. The major polar lipids are phosphatidyl glycerol, diphosphatidyl glycerol, phosphatidyl choline, phosphatidyl ethanolamine and an aminolipid. The fatty acids comprise 18 : 1ω7c, 18 : 0, 16 : 1ω7c, 16 : 0, 3-OH 10 : 0, 3-OH 12 : 1 (or 3-oxo 12 : 0) and traces of an 18 : 2 fatty acid. Among the hydroxylated fatty acids only 3-OH 12 : 1 (or 3-oxo 12 : 0) appears to be amide linked, whereas 3-OH 10 : 0 appears to be ester linked. The minor fatty acid components (between 1 and 7 %) allow three subgroups to be distinguished in the Sulfitobacter/Staleya clade, placing HEL-45T into a separate lineage characterized by the presence of 3-OH 12 : 1 (or 3-oxo 12 : 0) and both ester- and amide-linked 16 : 1ω7c phospholipids. HEL-45T produces indole and derivatives thereof, several cyclic dipeptides and thryptanthrin. Phylogenetic analysis of 16S rRNA gene sequences and chemotaxonomic data support the description of a new genus and species, to include Oceanibulbus indolifex gen. nov., sp. nov., with the type strain HEL-45T (=DSM 14862T=NCIMB 13983T).

The bchU gene of Chlorobium tepidum encodes the c-20 methyltransferase in bacteriochlorophyll c biosynthesis

Bacteriochlorophylls (BChls) c and d, two of the major light-harvesting pigments in photosynthetic green sulfur bacteria, differ only by the presence of a methyl group at the C-20 methine bridge position in BChl c. A gene potentially encoding the C-20 methyltransferase, bchU, was identified by comparative analysis of the Chlorobium tepidum and Chloroflexus aurantiacus genome sequences. Homologs of this gene were amplified and sequenced from Chlorobium phaeobacteroides strain 1549, Chlorobium vibrioforme strain 8327d, and C. vibrioforme strain 8327c, which produce BChls e, d, and c, respectively. A single nucleotide insertion in the bchU gene of C. vibrioforme strain 8327d was found to cause a premature, in-frame stop codon and thus the formation of a truncated, nonfunctional gene product. The spontaneous mutant of this strain that produces BChl c (strain 8327c) has a second frameshift mutation that restores the correct reading frame in bchU. The bchU gene was inactivated in C. tepidum, a BChl c-producing species, and the resulting mutant produced only BChl d. Growth rate measurements showed that BChl c- and d-producing strains of the same organism (C. tepidum or C. vibrioforme) have similar growth rates at high and intermediate light intensities but that strains producing BChl c grow faster than those with BChl d at low light intensities. Thus, the bchU gene encodes the C-20 methyltransferase for BChl c biosynthesis in Chlorobium species, and methylation at the C-20 position to produce BChl c rather than BChl d confers a significant competitive advantage to green sulfur bacteria living at limiting red and near-infrared light intensities.

Nine mutants of Chlorobium tepidum each unable to synthesize a different chlorosome protein still assemble functional chlorosomes

Chlorosomes of the green sulfur bacterium Chlorobium tepidum comprise mostly bacteriochlorophyll c (BChl c), small amounts of BChl a, carotenoids, and quinones surrounded by a lipid-protein envelope. These structures contain 10 different protein species (CsmA, CsmB, CsmC, CsmD, CsmE, CsmF, CsmH, CsmI, CsmJ, and CsmX) but contain relatively little total protein compared to other photosynthetic antenna complexes. Except for CsmA, which has been suggested to bind BChl a, the functions of the chlorosome proteins are not known. Nine mutants in which a single csm gene was inactivated were created; these mutants included genes encoding all chlorosome proteins except CsmA. All mutants had BChl c contents similar to that of the wild-type strain and had growth rates indistinguishable from or within approximately 90% (CsmC(-) and CsmJ(-)) of those of the wild-type strain. Chlorosomes isolated from the mutants lacked only the protein whose gene had been inactivated and were generally similar to those from the wild-type strain with respect to size, shape, and BChl c, BChl a, and carotenoid contents. However, chlorosomes from the csmC mutant were about 25% shorter than those from the wild-type strain, and the BChl c absorbance maximum was blue-shifted about 8 nm, indicating that the structure of the BChl c aggregates in these chlorosomes is altered. The results of the present study establish that, except with CsmA, when the known chlorosome proteins are eliminated individually, none of them are essential for the biogenesis, light harvesting, or structural organization of BChl c and BChl a within the chlorosome. These results demonstrate that chlorosomes are remarkably robust structures that can tolerate considerable changes in protein composition.

Biosynthesis of chlorophylls from protoporphyrin IX

A review of the biosynthesis of chlorophylls and bacteriochlorophylls from protoporphyrin IX with 235 references. The literature on the enzymes magnesium chelatase, S-adenosyl-L-methionine:magnesium protoporphyrin IX O-methyltransferase, magnesium-protoporphyrin IX monomethyl ester oxidative cyclase, protochlorophyllide oxidoreductase, chlorophyll synthase, bacteriochlorophyll synthase, protochlorophyllide 8-vinyl reductase and chlorophyll a oxidase from 1989 is discussed.

Biosynthesis of chlorophylls from protoporphyrin IX

A review of the biosynthesis of chlorophylls and bacteriochlorophylls from protoporphyrin IX with 235 references. The literature on the enzymes magnesium chelatase, S-adenosyl-L-methionine:magnesium protoporphyrin IX O-methyltransferase, magnesium-protoporphyrin IX monomethyl ester oxidative cyclase, protochlorophyllide oxidoreductase, chlorophyll synthase, bacteriochlorophyll synthase, protochlorophyllide 8-vinyl reductase and chlorophyll a oxidase from 1989 is discussed.

A comparative study of bchG from green photosynthetic bacteria

The gene bchG, coding for bacteriochlorophyll a synthase from a variety of green sulfur bacteria and the filamentous anoxygenic phototrophic bacteria, Chloroflexus aurantiacus, Chloronema sp., and Roseiflexus castenholzii HL08, was partially sequenced and compared. The deduced amino acid consensus sequences for green sulfur bacteria and green filamentous anoxygenic phototrophic bacteria were found to belong to the UbiA enzyme family of polyprenyltransferases with the most similar sequences being those of photosynthetic organisms. All deduced amino acid sequences showed a highly conserved region, which includes the motif DRXXD, characteristic of polyprenyltransferases, which was extended to DREVDAINEP for green sulfur bacteria. Neighbor-joining analysis of a protein similitude matrix displayed a relatively high distance between green sulfur bacteria and the other groups. Sequences from green sulfur bacteria were more closely related to those of purple bacteria than to those of filamentous anoxygenic phototrophic bacteria. In addition, internal grouping within green sulfur bacteria was congruent regarding taxonomic features including cell shape, presence of gas vacuoles and NaCl requirement. In addition to bchlG, another gene encoding for a second chlorophyll synthetase, previously tentatively identified as chlG, was also found in Chlorobium tepidum, showing the highest similarities with polyprenyltransferases from chlorophyll- a-containing organisms.

Novel carotenoid glucoside esters from alkaliphilic heliobacteria

Pigments of three species of alkaliphilic heliobacteria of the genus Heliorestis, H. daurensis, H. baculata and an undescribed species Heliorestis strain HH, were identified using spectroscopic methods. In these species, bacteriochlorophyll g esterified with farnesol was present, as for other heliobacteria. The carotenoids consisted of 4,4'-diaponeurosporene, also found in other heliobacteria, plus the novel pigments OH-diaponeurosporene glucoside esters (C16:0 and C16:1). In addition, trace amounts of biosynthetic intermediates, OH-diaponeurosporene and OH-diaponeurosporene glucoside, were found. Trace amounts of a carotenoid with 20 carbon atoms, 8,8'-diapo-zeta-carotene, were also found in these species as well as in the non-alkaliphilic heliobacteria. The non-alkaliphilic species Heliophilum fasciatum also contained trace amounts of the two OH-diaponeurosporene glucoside esters. The results are used to predict the pathway of carotenoid biosynthesis in heliobacteria.

Dynamics of C2 toxin and chlorophyll-a formation in the dinoflagellate Alexandrium tamarense during large scale cultivation

The production of paralytic shellfish toxins (PSTs) by the dinoflagellate Alexandrium tamarense ATCI01, a toxigenic strain isolated from South China coastal waters, was studied in batch cultures in relatively large volumes (20l). Under nutrient-replete conditions, this strain produced C2 toxin (C2T) as a predominant PST. In a 15-day production culture, phosphate was depleted by day 4, the stationary phase began at day 6, and the toxin productivity peaked at day 10, in which the cell content of C2T reached 76 fmol per cell. Much of the toxin was produced after the depletion of phosphate in the medium suggesting that C2T is a secondary metabolite. Aeration with small bubbles was useful in increasing cell mass and toxin yield. Chlorophyll-a (Chl-a) was formed in algal cells until the culture entered the stationary phase, after which Chl-a began to disappear rapidly from the culture while the C2T content continued to rise. These results suggest a metabolic relationship between Chl-a and C2T.

Molecular evidence for the early evolution of photosynthesis

The origin and evolution of photosynthesis have long remained enigmatic due to a lack of sequence information of photosynthesis genes across the entire photosynthetic domain. To probe early evolutionary history of photosynthesis, we obtained new sequence information of a number of photosynthesis genes from the green sulfur bacterium Chlorobium tepidum and the green nonsulfur bacterium Chloroflexus aurantiacus. A total of 31 open reading frames that encode enzymes involved in bacteriochlorophyll/porphyrin biosynthesis, carotenoid biosynthesis, and photosynthetic electron transfer were identified in about 100 kilobase pairs of genomic sequence. Phylogenetic analyses of multiple magnesium-tetrapyrrole biosynthesis genes using a combination of distance, maximum parsimony, and maximum likelihood methods indicate that heliobacteria are closest to the last common ancestor of all oxygenic photosynthetic lineages and that green sulfur bacteria and green nonsulfur bacteria are each other's closest relatives. Parsimony and distance analyses further identify purple bacteria as the earliest emerging photosynthetic lineage. These results challenge previous conclusions based on 16S ribosomal RNA and Hsp60/Hsp70 analyses that green nonsulfur bacteria or heliobacteria are the earliest phototrophs. The overall consensus of our phylogenetic analysis, that bacteriochlorophyll biosynthesis evolved before chlorophyll biosynthesis, also argues against the long-held Granick hypothesis.

Reconstitution of light-independent protochlorophyllide reductase from purified bchl and BchN-BchB subunits. In vitro confirmation of nitrogenase-like features of a bacteriochlorophyll biosynthesis enzyme

Protochlorophyllide reductase catalyzes the reductive formation of chlorophyllide from protochlorophyllide during biosynthesis of chlorophylls and bacteriochlorophylls. The light-independent (dark) form of protochlorophyllide reductase plays a key role in the ability of gymnosperms, algae, and photosynthetic bacteria to green (form chlorophyll) in the dark. Genetic and sequence analyses have indicated that dark protochlorophyllide reductase consists of three protein subunits that exhibit significant sequence similarity to the three subunits of nitrogenase, which catalyzes the reductive formation of ammonia from dinitrogen. However, unlike the well characterized features of nitrogenase, there has been no previous biochemical characterization of dark protochlorophyllide reductase. In this study, we report the first reproducible demonstration of dark protochlorophyllide reductase activity from purified protein subunits that were isolated from the purple nonsulfur photosynthetic bacterium Rhodobacter capsulatus. Two of the three subunits (Bchl and BchN) were expressed in R. capsulatus as S tag fusion proteins that facilitated affinity purification. The third subunit (BchB) was co-purified with the BchN protein indicating that BchN and BchB proteins form a tight complex. Dark protochlorophyllide reductase activity was shown to be dependent on the presence of all three subunits, ATP, and the reductant dithionite. The similarity of dark protochlorophyllide reductase to nitrogenase is discussed.

Interacting regulatory circuits involved in orderly control of photosynthesis gene expression in Rhodobacter sphaeroides 2.4.1

FnrL, the homolog of the global anaerobic regulator Fnr, is required for the induction of the photosynthetic apparatus in Rhodobacter sphaeroides 2.4.1. Thus, the precise role of FnrL in photosynthesis (PS) gene expression and its interaction(s) with other regulators of PS gene expression are of considerable importance to our understanding of the regulatory circuitry governing spectral complex formation. Using a CcoP and FnrL double mutant strain, we obtained results which suggested that FnrL is not involved in the transduction of the inhibitory signal, by which PS gene expression is "silenced," emanating from the cbb(3) oxidase encoded by the ccoNOQP operon under aerobic conditions. The dominant effect of the ccoP mutation in the FnrL mutant strain with respect to spectral complex formation under aerobic conditions and restoration of a PS-positive phenotype suggested that inactivation of the cbb(3) oxidase to some extent bypasses the requirement for FnrL in the formation of spectral complexes. Additional analyses revealed that anaerobic induction of the bchE, hemN, and hemZ genes, which are involved in the tetrapyrrole biosynthetic pathways, requires FnrL. Thus, FnrL appears to be involved at multiple loci involved in the regulation of PS gene expression. Additionally, bchE was also shown to be regulated by the PrrBA two-component system, in conjunction with hemN and hemZ. These and other results to be discussed permit us to more accurately describe the role of FnrL as well as the interactions between the FnrL, PrrBA, and other regulatory circuits in the regulation of PS gene expression.

Roseovarius tolerans gen. nov., sp. nov., a budding bacterium with variable bacteriochlorophyll a production from hypersaline Ekho Lake

Eight Gram-negative, aerobic, pointed and budding bacteria were isolated from various depths of the hypersaline, heliothermal and meromictic Ekho Lake (Vestfold Hills, East Antarctica). The cells contained storage granules and daughter cells could be motile. Bacteriochlorophyll a was sometimes produced, but production was repressed by constant dim light. The strains tolerated a wide range of temperature, pH, concentrations of artificial seawater and NaCl, but had an absolute requirement for sodium ions. Glutamate was metabolized with and without an additional source of combined nitrogen. The dominant fatty acid was C18:1; other characteristic fatty acids were C18:2, C12:0 2-OH, C12:1 3-OH, C16:1, C16:0 and C18:0. The main polar lipids were diphosphatidylglycerol, phosphatidylethanolamine, phosphatidylglycerol and phosphatidylcholine. The DNA G+C base composition was 62-64 mol%. 16S rRNA gene sequence comparisons showed that the isolates were phylogenetically close to the genera Antarctobacter, 'Marinosulfonomonas', Octadecabacter, Sagittula, Sulfitobacter and Roseobacter. Morphological, physiological and genotypic differences to these previously described and distinct genera support the description of a new genus and a new species, Roseovarius tolerans gen. nov., sp. nov. The type strain is EL-172T (= DSM 11457T).

Biosynthesis of the 3-acetyl and 13(1)-oxo groups of bacteriochlorophyll a in the facultative aerobic bacterium, Rhodovulum sulfidophilum--the presence of both oxygenase and hydratase pathways for isocyclic ring formation

Using (18)O-labelling and mass spectrometry, we have examined bacteriochlorophyll a formation in Rhodovulum sulfidophilum, formerly known as Rhodobacter sulfidophilus, which forms large amounts of BCh1 a both aerobically in the dark and anaerobically in the light. R. sulfidophilum, growing under strict anaerobiosis in the light, possesses hydratases which incorporate (18)O label from H2(18)O into both the 13(1)-oxo and 3-acetyl oxygens; in addition, the four carboxyl oxygens at C13(3) and C17(3) were labelled by H2(18)O. Under aerobic conditions in the dark, the labelling of the 13(1)-oxo group by H2(18)O was reduced indicating that (16)O was being incorporated into this group from air. R. sulfidophilum, grown in the dark under an atmosphere initially containing 50% (18)O2 in Ar, possessed an oxygenase which incorporated (18)O label from (18)O2 specifically into the 13(1)-oxo group; under these conditions the acetyl and carboxyl groups remained unlabelled. Thus, both an oxygenase and hydratase operate in R. sulfidophilum to form the 13(1)-oxo group of ring E of BCh1 a; the 3-acetyl group oxygen, however, arises only from water via a hydratase.

Characterization of the Rhodobacter capsulatus housekeeping RNA polymerase. In vitro transcription of photosynthesis and other genes

To begin to characterize biochemically the transcriptional activation systems in photosynthetic bacteria, the Rhodobacter capsulatus RNA polymerase (RNAP) that contains the sigma70 factor (R. capsulatus RNAP/sigma70) was purified and characterized using two classical sigma70 type promoters, the bacteriophage T7A1 and the RNA I promoters. Transcription from these promoters was sensitive to rifampicin, RNase, and monoclonal antibody 2G10 (directed against the Escherichia coli sigma70 subunit). Specific transcripts were detected in vitro for R. capsulatus cytochrome c2 (cycA) and fructose-inducible (fruB) promoters and genes induced in photosynthesis (puf and puc) and bacteriochlorophyll biosynthesis (bchC). Alignment of these natural promoters activated by R. capsulatus RNAP/sigma70 indicated a preference for the sequence TTGAC at the -35 region for strong in vitro transcription. To test the -35 recognition pattern, the R. capsulatus nifA1 promoter, which exhibits only three of the five consensus nucleotides at the -35 region, was mutated to four and five of the consensus nucleotides. Although the nifA1 wild type promoter showed no transcription, the double mutated promoter exhibited high levels of in vitro transcription by the purified R. capsulatus RNAP/sigma70 enzyme. Similarities and differences between the RNAPs and the promoters of R. capsulatus and E. coli are discussed.

Autofluorescence of live purple bacteria in the near infrared

We have developed a novel microscope with which to study the fluorescence of cells in the near-infrared region (lambda = 750-2500 nm). For one of its first applications we report on the autofluorescence of live purple bacteria, Rhodospirillum rubrum, and suggest that the autofluorescent component is bacteriochlorophyll. The rapid fading of the autofluorescence of fixed bacteria and of purified bacteriochlorophyll suggests that the live bacteria are able to regenerate their pigment with a time constant of approximately 20 s.

DNA binding characteristics of CrtJ. A redox-responding repressor of bacteriochlorophyll, carotenoid, and light harvesting-II gene expression in Rhodobacter capsulatus

Previous genetic analysis indicated that the photosynthesis gene cluster from Rhodobacter capsulatus coded for the transcription factor, CrtJ, that is responsible for aerobic repression of bacteriochlorophyll, carotenoid, and light harvesting-II gene expression. In this study, we have heterologously overexpressed and purified CrtJ to homogeneity and shown by gel mobility shift assays that CrtJ is biologically active. DNase I footprint analysis confirms molecular genetic studies by showing that CrtJ binds to conserved palindromic sequences that overlap the -10 and -35 promoter regions of the bchC operon. Graphs of the percentage of DNA bound versus protein concentration show sigmoidal curves, which is highly indicative of cooperative binding of CrtJ to the two palindromic sites. A binding constant for interaction of CrtJ with the palindrome that spans the -10 region was calculated to be 4.8 x 10(-9) M, whereas affinity for the palindrome that spans the -35 region was found to be 2.9 x 10(-9) M. Binding of CrtJ to the bchC promoter region was also found to be redox-sensitive, with CrtJ exhibiting a 4.5-fold higher binding affinity under oxidizing versus reducing conditions.

Characterization of chlorophyll a and bacteriochlorophyll a synthases by heterologous expression in Escherichia coli

Genes coding for putative chlorophyll a synthase (chlG) from Synechocystis sp. PCC 6803 and bacteriochlorophyll a synthase (bchG) from Rhodobacter capsulatus were amplified by the polymerase chain reaction and cloned into T7 RNA polymerase-based expression plasmids. In vitro enzymatic assays indicated that heterologous expression of the chlG and bchG gene products in Escherichia coli conferred chlorophyll a and bacteriochlorophyll a synthase activity, respectively. Chlorophyll a synthase utilized chlorophyllide a, but not bacteriochlorophyllide a, as a substrate, whereas bacteriochlorophyll a synthase utilized bacteriochlorophyllide a, but not chlorophyllide a. Both enzymes were also observed to exhibit a marked preference for phytyl diphosphate over geranylgeranyl diphosphate.

Cloning, sequencing and functional assignment of the chlorophyll biosynthesis gene, chlP, of Synechocystis sp. PCC 6803

A gene from the cyanobacterium Synechocystis sp. PCC 6803 has been cloned and sequenced, and subsequently used to partially complement a bchP mutant of the purple photosynthetic bacterium Rhodobacter sphaeroides. This mutant is blocked in the terminal hydrogenation steps of bchla biosynthesis and possesses only bchl esterified with geranylgeraniol. It also has a reduced cellular level of the light-harvesting LH2 complex, and the 850 nm absorbance maximum of LH2 is red-shifted by approximately 6 nm. Upon heterologous expression of the Synechocystis bchP homologue, not only are hydrogenated forms of bchlaGG detectable, but the level of LH2 is increased and the red-shift reversed by several nm. We conclude that this gene, which we term chlP, encodes the enzyme catalysing the stepwise hydrogenation of geranylgeraniol to phytol during chla biosynthesis.

Cloning and characterization of the chlorophyll biosynthesis gene chlM from Synechocystis PCC 6803 by complementation of a bacteriochlorophyll biosynthesis mutant of Rhodobacter capsulatus

A bacteriochlorophyll a biosynthesis mutant of the purple photosynthetic bacterium Rhodobacter capsulatus was functionally complemented with a cosmid genomic library from Synechocystis sp. PCC 6803. The complemented R. capsulatus strain contains a defined mutation in the bchM gene that codes for Mg-protoporphyrin IX methyltransferase, the enzyme which converts Mg-protoporphyrin IX to Mg-protoporphyrin IX methylester using S-adenosyl-L-methionine as a cofactor. Since chlorophyll biosynthesis also requires the same methylation reaction, the Synechocystis genome should similarly code for a Mg-protoporphyrin IX methyltransferase. Sequence analysis of the complementing Synechocystis cosmid indicates that it contains an open reading frame exhibiting 29% sequence identity to BchM. In addition, expression of the Synechocystis gene in the R. capsulatus bchM mutant via the strong R. capsulatus puc promoter was shown to support nearly wild-type levels of bacteriochlorophyll a synthesis. To our knowledge, the Synechocystis sequence thus represents the first chlorophyll biosynthesis gene homolog of bchM. The complementing Synechocystis cosmid was also shown to code for a gene product that is a member of a highly conserved family of RNA binding proteins, the function of which in cyanobacteria remains undetermined.

Cloning and overexpression of the Rhodobacter capsulatus hemH gene

In photosynthetically grown Rhodobacter capsulatus, heme is a qualitatively minor end product of the common tetrapyrrole pathway, but it may play a significant regulatory role. Heme is synthesized from protoporphyrin by the product of the hemH gene, ferrochelatase. We have cloned the R. capsulatus hemH gene by complementation of an Escherichia coli hemH mutant. When a plasmid carrying the hemH gene is returned to R. capsulatus, ferrochelatase activity increases, aminolevulinate synthase activity decreases, and bacteriochlorophyll levels are dramatically lowered. This is the first in vivo evidence to suggest that heme feedback inhibits aminolevulinate synthase in R. capsulatus, thereby reducing porphyrin synthesis.

Effect of the PufQ protein on early steps in the pathway of bacteriochlorophyll biosynthesis in Rhodobacter capsulatus

The addition in trans of the pufQ gene to a strain of Rhodobacter capsulatus from which the entire puf operon had been deleted, increased its ability to synthesize coproporphyrinogen from both delta-aminolevulinic acid and porphobilinogen. Studies at the enzyme level indicated that the conversion of porphobilinogen to uroporphyrinogen III had about a 2-fold higher level of activity in the anaerobically-grown pufQ-containing strain. This increase in activity over the puf-deletion strain appeared to occur during transitions from aerobic to semiaerobic growth conditions. These results indicated that the PufQ protein may exert a stimulatory effect quite early in the pathway of bacteriochlorophyll biosynthesis.

The derivation of the oxygen atoms of the 13(1)-oxo and 3-acetyl groups of bacteriochlorophyll a from water in Rhodobacter sphaeroides cells adapting from respiratory to photosynthetic conditions: evidence for an anaerobic pathway for the formation of isocyclic ring E

Using mass spectrometry, we have demonstrated 18O-labelling of both the 13(1)-oxo and 3-acetyl groups of newly-formed bacteriochlorophyll a synthesized by Rhodobacter sphaeroides cells during adaptation from respiratory to photosynthetic conditions in the presence of H218O. This derivation of the 13(1)-oxo group of bacteriochlorophyll a from water provides a stark contrast with that of chlorophylls in higher plants where ring E formation is an aerobic process in which the 13(1)-oxo group arises from molecular oxygen via an oxygenase activity. The formation of the 3-acetyl group of bacteriochlorophyll a, however, is consistent with the enzymic hydration of the 3-vinyl group of a derivative of chlorophyll a.

appA, a novel gene encoding a trans-acting factor involved in the regulation of photosynthesis gene expression in Rhodobacter sphaeroides 2.4.1

A new gene, the product of which is involved in the regulation of photosynthesis gene expression in the anoxygenic photosynthetic bacterium Rhodobacter sphaeroides 2.4.1, has been identified. The isolation of this gene, designated appA (activation of photopigment and puc expression), was based on its ability, when provided in extra copies, to partially suppress mutations in the two-component PrrB-PrrA regulatory system. The presence of extra copies of the appA gene in either prrB, prrA, or wild-type strains resulted in an activation of puc::lacZ expression under aerobic conditions. Constructed AppA null mutants did not grow photosynthetically and were impaired in the synthesis of both bacteriochlorophyll and carotenoids, as well as the structural proteins of the photosynthetic spectral complexes. When grown anaerobically in the dark, these mutants accumulated bacteriochlorophyll precursors. The expression of lacZ fusions to several photosynthesis genes and operons, including puc, puf, and bchF, was decreased in the AppA mutant strains in comparison with the wild type. To examine the role of AppA involvement in bacteriochlorophyll biosynthesis, we inactivated an early gene, bchE, of the bacteriochlorophyll pathway in both wild-type and AppA- mutant backgrounds. The double mutant, AppA- BchE-, was found to be severely impaired in photosynthesis gene expression, similar to the AppA- BchE+ mutant and in contrast to the AppA+ BchE- mutant. This result indicated that AppA is more likely involved in the regulation of expression of the bch genes than in the biosynthetic pathway per se. The appA gene was sequenced and appears to encode a protein of 450 amino acids with no obvious homology to known proteins.

Identification and sequence analysis of genes involved in late steps in cobalamin (vitamin B12) synthesis in Rhodobacter capsulatus

A 6.4-kb region of a 6.8-kb BamHI fragment carrying Rhodobacter capsulatus genes involved in late steps of cobalamin synthesis has been sequenced. The nucleotide sequence and genetic analysis revealed that this fragment contains eight genes arranged in at least three operons. Five of these eight genes show homology to genes involved in the cobalamin synthesis of Pseudomonas denitrificans and Salmonella typhimurium. The arrangement of these homologous genes differs considerably in the three genera. Upstream of five overlapping genes (named bluFEDCB), a promoter activity could be detected by using lacZ fusions. This promoter shows no regulation by oxygen, vitamin B12 (cobalamin), or cobinamide. Disruption of the bluE gene by a Tn5 insertion (strain AH2) results in reduced expression of the puf and puc operons, which encode pigment-binding proteins of the photosynthetic apparatus. The mutant strain AH2 can be corrected to a wild-type-like phenotype by addition of vitamin B12 or cobinamide dicyanide. Disruption of the bluB gene by an interposon (strain BB1) also disturbs the formation of the photosynthetic apparatus. The mutation of strain BB1 can be corrected by vitamin B12 but not by cobinamide. We propose that a lack of cobalamin results in deregulation and a decreased formation of the photosynthetic apparatus.