A phylogenetic analysis of the purple photosynthetic bacteria

Molecular Physiology of Anaerobic Phototrophic Purple and Green Sulfur Bacteria

There are two main types of bacterial photosynthesis: oxygenic (cyanobacteria) and anoxygenic (sulfur and non-sulfur phototrophs). Molecular mechanisms of photosynthesis in the phototrophic microorganisms can differ and depend on their location and pigments in the cells. This paper describes bacteria capable of molecular oxidizing hydrogen sulfide, specifically the families Chromatiaceae and Chlorobiaceae, also known as purple and green sulfur bacteria in the process of anoxygenic photosynthesis. Further, it analyzes certain important physiological processes, especially those which are characteristic for these bacterial families. Primarily, the molecular metabolism of sulfur, which oxidizes hydrogen sulfide to elementary molecular sulfur, as well as photosynthetic processes taking place inside of cells are presented. Particular attention is paid to the description of the molecular structure of the photosynthetic apparatus in these two families of phototrophs. Moreover, some of their molecular biotechnological perspectives are discussed.

Analysis of hydrothermal vent-associated symbionts by ribosomal RNA sequences

Ribosomal RNA (rRNA) sequences were used to establish the phylogenetic affiliations of symbioses in which prokaryotes appear to confer sulfur-based chemoautotrophy on their invertebrate hosts. Two submarine hydrothermal vent animals, the vestimentiferan tube worm Riftia pachyptila and the clam Calyptogena magnifica, and a tidal-flat bivalve, Solemya velum, were inspected. 5S rRNA's were extracted from symbiont-bearing tissues, separated into unique forms, and their nucleotide sequences determined and related to other 5S rRNA's in a phylogenetic tree analysis. The prokaryotic symbionts are related to one another and affiliated with the same narrow phylogenetic grouping as Escherichia coli and Pseudomonas aeruginosa. The sequence comparisons suggest that Riftia is more closely related to the bivalves than their current taxonomic status would suggest.

Rhodospirillum rubrum has a family I pyrophosphatase: purification, cloning, and sequencing

The cytoplasmic pyrophosphatase of the photosynthetic bacterium Rhodospirillum rubrum was purified to electrophoretic homogeneity. The enzyme is a homohexamer of 20-kDa monomers. The gene was cloned and sequenced. Alignment of the deduced 179-amino-acid protein with known bacterial pyrophosphatases revealed conservation of all residues in the active site. Attempts to obtain an insertion mutant of the cytoplasmic pyrophosphatase gene did not yield any cell completely devoid of cytoplasmic pyrophosphatase activity. The mutants obtained showed 50% of the enzymatic activity and grew in twice the generation time of wild-type cells. This suggests that the membrane-bound pyrophosphatase of Rsp. rubrum is not sufficient for a normal growth rate, whereas the cytoplasmic enzyme is essential for growth. The characteristics of the gene and the encoded protein fit those of prokaryotic family I pyrophosphatases.

The phylogeny of purple bacteria: the alpha subdivision

The technique of oligonucleotide cataloging shows the purple photosynthetic eubacteria to comprise three major subdivisions, temporarily called alpha, beta, and gamma--previously designated groups I-III by Gibson et al. (1979). Each subdivision contains a number of non-photosynthetic genera in addition to the photosynthetic ones. The alpha subdivision, the subject of the present report, contains most but not all of the species that fall into the classically defined genera Rhodospirillum, Rhodopseudomonas and Rhodomicrobium. Intermingled with these are a variety of non-photosynthetic species from genera such as Agrobacterium, Rhizobium, Azospirillum, Nitrobacter, Erythrobacter, Phenylobacterium, Aquaspirillum, and Paracoccus. The phylogenetic substructure of the alpha subdivision is presented and the evolutionary significance of the admixture of biochemical phenotypes is discussed.

A phylogenetic definition of the major eubacterial taxa

Through oligonucleotide signature analysis of 16S ribosomal RNAs, it is possible to define ten major groups of eubacteria. These are: (1) the Gram positive bacteria, (2) the purple photosynthetic bacteria and their relatives, (3) the spirochetes and their relatives, (4) the sulfur-dependent eubacteria and their relatives, (5) the bacteroides, flavobacteria and cytophagas and their relatives, (6) the cyanobacteria, (7) the green sulfur bacteria, (8) the green non-sulfur bacteria and their relatives, (9) the radio-resistant micrococci, and (10) the planctomyces and their relatives. Although no consensus exists as regards the taconomic terminology, these ten groupings are appropriately termed eubacterial Phyla or Divisions. The major subdivisions of those Phyla or Divisions that have been extensively characterized can also be defined by characteristic oligonucleotide signatures.

Gram-positive bacteria: possible photosynthetic ancestry

A 16S ribosomal RNA gene has been sequenced from Heliobacterium chlorum, the recently discovered photosynthetic bacterium that contains a novel form of chlorophyll. Comparisons with other 16S ribosomal RNA sequences show that the organism belongs to the Gram-positive bacteria (one of ten eubacterial "phyla")--more precisely to the so-called low G + C (G, guanine; C, cytosine) subdivision thereof. This brings to five the number of such phyla that contain photosynthetic species, the other four being the purple bacteria and relatives, the green sulfur bacteria, the green nonsulfur bacteria, and the cyanobacteria. The finding suggests that Gram-positive bacteria may be of photosynthetic ancestry, and it strengthens the case for a common photosynthetic ancestry for all eubacteria.

Phylogenetic origins of the plant mitochondrion based on a comparative analysis of 5S ribosomal RNA sequences

The complete nucleotide sequences of 5S ribosomal RNAs from Rhodocyclus gelatinosa, Rhodobacter sphaeroides, and Pseudomonas cepacia were determined. Comparisons of these 5S RNA sequences show that rather than being phylogenetically related to one another, the two photosynthetic bacterial 5S RNA sequences show that rather than being phylogenetically related to one another, the two photosynthetic bacterial 5S RNAs share more sequence and signature homology with the RNAs of two nonphotosynthetic strains. Rhodobacter sphaeroides is specifically related to Paracoccus denitrificans and Rc. gelatinosa is related to Ps. cepacia. These results support earlier 16S ribosomal RNA studies and add two important groups to the 5S RNA data base. Unique 5S RNA structural features previously found in P. denitrificans are present also in the 5S RNA of Rb. sphaeroides; these provide the basis for subdivisional signatures. The immediate consequence of our obtaining these new sequences is that we are able to clarify the phylogenetic origins of the plant mitochondrion. In particular, we find a close phylogenetic relationship between the plant mitochondria and members of the alpha subdivision of the purple photosynthetic bacteria, namely, Rb. sphaeroides, P. denitrificans, and Rhodospirillum rubrum.

A 3-hydroxy-3-methylglutaryl-CoA lyase gene in the photosynthetic bacterium Rhodospirillum rubrum

A 1.2 kb long DNA segment from Rhodospirillum rubrum has been sequenced (EMBL/GenBank accession number: U41280). This DNA segment includes the first sequenced gene for a putative 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase, termed hmgL, from a photosynthetic organism. The sequenced segment also contains a ribosome-binding site and two clusters of possible-35 and -10 promotor sequences preceding the hmgL gene. Translation of the gene would yield a 303 amino-acid-long protein with a calculated molecular weight of 31.1 kDa. This protein shows 55-60% identity and approx. 75% similarity, including conservative substitutions, with the three eukaryotic and the single prokaryotic HMG-CoA lyases which previously have been sequenced. The R. rubrum enzyme showed stronger homology to the chicken HMG-CoA lyase than to the other bacterial protein. Significant sequence similarity was also found with homocitrate synthases from nitrogen-fixing prokaryotes. In contrast to the other sequenced prokaryotic HMG-CoA lyase (from Pseudomonas mevalonii), the R. rubrum hmgL does not seem to appear in an operon together with a HMG-CoA reductase. The hmgL gene was transcribed in photosynthetically grown cells as judged by amplification of cDNAs synthesised from DNA-free total RNA. In addition, HMG-CoA lyase activity was found in R. rubrum cells grown anaerobically in the light with leucine as the carbon source.

Classification and identification of bacteria: current approaches to an old problem. Overview of methods used in bacterial systematics

Most of the bacterial species are still unknown. Consequently, our knowledge about bacterial ecology is poor and expectations about specialized species with novel enzymatic functions or new products are high. Thus, bacterial identification is a growing field of interest within microbiology. In this review, suitability of developments for identification based on miniaturized biochemical and physiological investigations of bacteria are evaluated. Special emphasis is given to chemotaxonomic methods such as analysis of quinone system, fatty acid profiles, polar lipid patterns, polyamine patterns, whole cell sugars, peptidoglycan diaminoacids, as well as analytical fingerprinting methods and cellular protein patterning. 16S rDNA sequencing introduced to investigate the phylogenetic relationships of bacteria, nucleic acids hybridization techniques and G + C content determination are discussed as well as restriction fragment length polymorphism (RFLP), macrorestriction analysis and random amplified polymorphic DNA (RAPD). The importance of the different approaches in classification and identification of bacteria according to phylogenetic relationships are demonstrated on selected examples.

Posttranslational regulation of nitrogenase in Rhodospirillum rubrum strains overexpressing the regulatory enzymes dinitrogenase reductase ADP-ribosyltransferase and dinitrogenase reductase activating glycohydrolase

Rhodospirillum rubrum strains that overexpress the enzymes involved in posttranslational nitrogenase regulation, dinitrogenase reductase ADP-ribosyltransferase (DRAT) and dinitrogenase reductase activating glycohydrolase (DRAG), were constructed, and the effect of this overexpression on in vivo DRAT and DRAG regulation was investigated. Broad-host-range plasmid constructs containing a fusion of the R. rubrum nifH promoter and translation initiation sequences to the second codon of draT, the first gene of the dra operon, were constructed. Overexpression plasmid constructs which overexpressed (i) only functional DRAT, (ii) only functional DRAG and presumably the putative downstream open reading frame (ORF)-encoded protein, or (iii) all three proteins were generated and introduced into wild-type R. rubrum. Overexpression of DRAT still allowed proper regulation of nitrogenase activity, with ADP-ribosylation of dinitrogenase reductase by DRAT occurring only upon dark or ammonium stimuli, suggesting that DRAT is still regulated upon overexpression. However, overexpression of DRAG and the downstream ORF altered nitrogenase regulation such that dinitrogenase reductase did not accumulate in the ADP-ribosylated form under inactivation conditions, suggesting that DRAG was constitutively active and that therefore DRAG regulation is altered upon overexpression. Proper DRAG regulation was observed in a strain overexpressing DRAT, DRAG, and the downstream ORF, suggesting that a proper balance of DRAT and DRAG levels is required for proper DRAG regulation.

Characterization of the pet operon of Rhodospirillum rubrum

The three genes of the pet operon, coding, respectively, for the Rieske iron-sulfur protein, cytochrome b and cytochrome c 1 components of the cytochrome bc 1 complex in the photosynthetic bacterium Rhodospirillum rubrum have been sequenced. The amino acid sequences deduced for these three peptides from the nucleotide sequences of the genes have been confirmed, in part, by direct sequencing of portions of the three peptides separated from a sample of the purified, detergent-solubilized complex. These sequences show considerable homology with those previously obtained for the pet operons of other photosynthetic bacteria. Northern blots of R. rubrum mRNA have established that the operon is transcribed as a single polycistronic message, the start site of which has been determined by both primer extension and nuclease protection. Photosynthetic growth of R. rubrum was shown to be inhibited by antimycin A, a specific inhibitor of cytochrome bc 1 complexes, and antimycin A-resistant mutants of R. rubrum have been isolated. Preliminary results suggest that it may be possible to express the R. rubrum pet operon in a strain of the photosynthetic bacterium Rhodobacter capsulatus from which the native pet operon has been deleted.

The pet genes of Rhodospirillum rubrum: cloning and sequencing of the genes for the cytochrome bc1-complex

A cytochrome bc1-complex of Rs. rubrum was isolated and the three subunits were purified to homogeneity. The N-terminal amino acid sequence of the purified subunits was determined by automatic Edman degradation. The pet genes of Rhodospirillum rubrum coding for the three subunits of the cytochrome bc1-complex were isolated from a genomic library of Rs. rubrum using oligonucleotides specific for conserved regions of the subunits from other organisms and a heterologous probe derived from the genes for the complex of Rb. capsulatus. The complete nucleotide sequence of a 5500 bp SalI/SphI fragment is described which includes the pet genes and three additional unidentified open reading frames. The N-terminal amino acid sequence of the isolated subunits was used for the identification of the three genes. The genes encoding the subunits are organized as follows: Rieske protein, cytochrome b, cytochrome c1. Comparison of the N-terminal protein sequences with the protein sequences deduced from the nucleotide sequence showed that only cytochrome c1 is processed during transport and assembly of the three subunits of the complex. Only the N-terminal methionine of the Rieske protein is cleaved off. The similarity of the deduced amino acid sequence of the three subunits to the corresponding subunits of other organisms is described and implications for structural features of the subunits are discussed.

Distribution of delta-aminolevulinic acid biosynthetic pathways among phototrophic bacterial groups

Two biosynthetic pathways are known for the universal tetrapyrrole precursor, delta-aminolevulinic acid (ALA). In the ALA synthase pathway which was first described in animal and some bacterial cells, the pyridoxal phosphate-dependent enzyme ALA synthase catalyzes condensation of glycine and succinyl-CoA to form ALA with the loss of C-1 of glycine as CO2. In the five-carbon pathway which was first described in plant and algal cells, the carbon skeleton of glutamate is converted intact to ALA in a proposed reaction sequence that requires three enzymes, tRNA(Glu), ATP, Mg2+, NADPH, and pyridoxal phosphate. We have examined the distribution of the two ALA biosynthetic pathways among various genera, using cell-free extracts obtained from representative organisms. Evidence for the operation of the five-carbon pathway was obtained by the measurement of RNase-sensitive label incorporation from glutamate into ALA, using 3,4-[3H]glutamate or 1-[14C]glutamate as substrate. ALA synthase activity was indicated by RNase-insensitive incorporation of label from 2-[14C]glycine into ALA. The distribution of the two pathways among the bacteria tested was in general agreement with their previously established phylogenetic relationships and clearly indicates that the five-carbon pathway is the more ancient process, whereas the pathway utilizing ALA synthase probably evolved much later. The five-carbon pathway is apparently the more widely utilized one among bacteria, while the ALA synthase pathway seems to be limited to the alpha subgroup of purple bacteria.

Lipopolysaccharides of Thiocystis violacea, Thiocapsa pfennigii, and Chromatium tepidum, species of the family Chromatiaceae

The lipopolysaccharides (LPS) of three species of purple sulfur bacteria (Chromatiaceae), Thiocystis violacea, Thiocapsa pfennigii, and the moderately thermophilic bacterium Chromatium tepidum, were isolated. The LPS of Thiocystis violacea and Chromatium tepidum contained typical O-specific sugars, indicating O-chains. Long O-chains were confirmed for these species by sodium deoxycholate gel electrophoresis of their LPS. Thiocapsa pfennigii, however, had short or no O-chains. The core region of the LPS of all three species comprised D-glycero-D-mannoheptose as the only heptose and 2-keto-3-deoxyoctonate. The lipid A, obtained from the LPS by mild acid hydrolysis, contained glucosamine as the main amino sugar. Amide-bound 3-hydroxymyristic acid was the only hydroxy fatty acid. The main ester-bound fatty acid in all lipid A fractions was 12:0. Mannose and small amounts of 2,3-diamino-2,3-dideoxy-D-glucose were common constituents of the lipid A of the three Chromatiaceae species investigated. All lipid A fractions were essentially free of phosphate.

A phylogenetic survey of budding, and/or prosthecate, non-phototrophic eubacteria: membership of Hyphomicrobium, Hyphomonas, Pedomicrobium, Filomicrobium, Caulobacter and "dichotomicrobium" to the alpha-subdivision of purple non-sulfur bacteria

The phylogenetic position of various budding and/or or prosthecate Gram-negative eubacteria was determined by different methods. Members of the genera Hyphomicrobium, Filomicrobium, Pedomicrobium were investigated by 16S rRNA cataloguing, a 1373 nucleotide long portion of the 16S rRNA was sequenced from Hyphomicrobium vulgare and the 5S rRNAs were analyzed from two Hyphomicrobium strains, Hyphomonas polymorpha and Caulobacter crescentus. Comparison with published sequences indicated a membership of all of these organisms to the alpha subdivision of purple bacteria. While C. crescentus and Hyphomonas polymorpha constitute separate individual lines of descent, the position of a coherent cluster embracing Hyphomicrobium, Pedomicrobium and Filomicrobium is not yet settled. 16S rRNA cataloguing indicate the presence of a distinct line equivalent to other subgroups in its phylogenetic depth. 5S rRNA analysis, on the other hand, groups Hyphomicrobium vulgare and strain IFAM 1761 with members of subgroup alpha-2 (Rhodopseudomonas palustris, Nitrobacter winogradskyi and relatives). In contrast to the present classification, Pedomicrobium ferrugineum and Filimicrobium fusiforme are more closely related to certain Hyphomicrobium strains than these are related among each other. Budding mode of reproduction and prosthecate morphology are dominating morphological features of members of the alpha subdivision. These characteristics may gain diagnostic significance in a future formal description of this subdivision and its subgroups as a higher rank.

Organization and structure of the genes for the cytochrome b/c1 complex in purple photosynthetic bacteria. A phylogenetic study describing the homology of the b/c1 subunits between prokaryotes, mitochondria, and chloroplasts

The cytochrome b/c1 complex is an ubiquitous energy transducing enzyme, part of the electron transport chain of prokaryotes, mitochondria, and chloroplasts (b6/f). In the ancient purple photosynthetic bacteria, the b/c1 complex occupies a central metabolic role, being part of their photosynthetic and respiratory electron transport chain. In Rhodobacter the three subunits of the b/c1 complex are FeS protein, cytochrome b, and cytochrome c1, and they are encoded by a constitutively expressed operon named fbc. The organization of the genes for the cytochrome b/c1 complex, the modality of transcription, and the biogenesis of the encoded polypeptides will be described. The Rhodobacter species used to isolate the fbc genes, previously reported as R. sphaeroides was identified as R. capsulatus. Further biochemical characterization of the prokaryotic b/c1 complex indicated that the three polypeptides encoded by the fbc operon comprise the entire catalytic structure: ubiquinol-cytochrome-c reductase. The amino acid sequences of the three b/c1 subunits from the photosynthetic bacterium Rhodobacter capsulatus were compared with the corresponding sequences from yeast mitochondria and spinach chloroplasts. The high homology found between the sequences of all three redox polypeptides from R. capsulatus and yeast mitochondria (cytochrome b 41%, FeS protein 46%, cytochrome c1 31%) provided further evidence that mitochondria arose from the phylogenetic line of purple bacteria. The structure of cytochrome b also exhibited considerable homology to chloroplast cytochrome b6 plus subunit IV (26%). The amino acid sequence of the Rieske FeS protein from R. capsulatus and chloroplasts were found to be conserved only in the C-terminal part (14% total identity), whereas the homology between cytochrome c1 and cytochrome f is very weak (12%), despite similar topology of the two polypeptides. Analysis of the homology suggested that the catalytic sites quinol oxidase (Q0) and quinone reductase (Qi) arose monophonetically, whereas cytochrome c and plastocyanin reductase sites are not homologous and could derive from diverse ancestral genes by convergent evolution.

Primary structure of the reaction center from Rhodopseudomonas sphaeroides

The reaction center is a pigment-protein complex that mediates the initial photochemical steps of photosynthesis. The amino-terminal sequences of the L, M, and H subunits and the nucleotide and derived amino acid sequences of the L and M structural genes from Rhodopseudomonas sphaeroides have previously been determined. We report here the sequence of the H subunit, completing the primary structure determination of the reaction center from R. sphaeroides. The nucleotide sequence of the gene encoding the H subunit was determined by the dideoxy method after subcloning fragments into single-stranded M13 phage vectors. This information was used to derive the amino acid sequence of the corresponding polypeptide. The termini of the primary structure of the H subunit were established by means of the amino and carboxy terminal sequences of the polypeptide. The data showed that the H subunit is composed of 260 residues, corresponding to a molecular weight of 28,003. A molecular weight of 100,858 for the reaction center was calculated from the primary structures of the subunits and the cofactors. Examination of the genes encoding the reaction center shows that the codon usage is strongly biased towards codons ending in G and C. Hydropathy analysis of the H subunit sequence reveals one stretch of hydrophobic residues near the amino terminus; the L and M subunits contain five such stretches. From a comparison of the sequences of homologous proteins found in bacterial reaction centers and photosystem II of plants, an evolutionary tree was constructed. The analysis of evolutionary relationships showed that the L and M subunits of reaction centers and the D1 and D2 proteins of photosystem II are descended from a common ancestor, and that the rate of change in these proteins was much higher in the first billion years after the divergence of the reaction center and photosystem II than in the subsequent billion years represented by the divergence of the species containing these proteins.

Nucleotide sequence and transcription of the fbc operon from Rhodopseudomonas sphaeroides. Evaluation of the deduced amino acid sequences of the FeS protein, cytochrome b and cytochrome c1

The fbc operon from Rhodopseudomonas sphaeroides encodes the three redox carriers of the ubiquinol-cytochrome-c reductase (b/c1 complex): FeS protein, cytochrome b and cytochrome c1 [Gabellini, N. et al. (1985) EMBO J.2, 549-553]. The nucleotide sequence of 3874 bp of cloned R. sphaeroides chromosomal DNA, including the three structural genes fbcF, fbcB and fbcC has been determined. The reading frames of the fbc genes could be identified readily since the encoded amino acid sequences are highly homologous with the sequences of the corresponding mitochondrial polypeptides. Initiation and termination points for transcription have been investigated by S1 nuclease protection analysis. The transcription of the fbc operon starts approximately 240 base pairs upstream from the start codon of the fbcF gene and terminates 120 base pairs downstream from the stop codon of the fbcC gene. Nucleotide sequences resembling recognition signals for the binding and release of the RNA polymerase were identified. The N-terminal amino acid sequence of the mature cytochrome c1 was obtained by automated Edman degradation of the isolated subunit, confirming the fbcC reading frame and indicating that the bacterial preapocytochrome c1 has a transient leader sequence including 21 residues. The N-terminal sequence of one hydrophilic peptide of the FeS protein has been also obtained confirming the fbcF reading frame. The deduced amino acid sequences are discussed in relation to the known primary structures of the homologous proteins from mitochondria and chloroplasts. The primary structures of the polypeptides are evaluated with respect to their topology in the membrane, their biogenesis, the structure of the catalytic sites and subunit interactions.

Evolution of the regulatory isozymes of 3-deoxy-D-arabino-heptulosonate 7-phosphate synthase present in the Escherichia coli genealogy

The evolutionary history of isozymes for 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthase has been constructed in a phylogenetic cluster of procaryotes (superfamily B) that includes Escherichia coli. Members of superfamily B that have been positioned on a phylogenetic tree by oligonucleotide cataloging possess one or more of four distinct isozymes of DAHP synthase. DAHP synthase-0 is insensitive to feedback inhibition, while DAHP synthase-Tyr, DAHP synthase-Trp, and DAHP synthase-Phe are sensitive to feedback inhibition by L-tyrosine, L-tryptophan, and L-phenylalanine, respectively. The evolutionary history of this isozyme family can be deduced within superfamily B by using a cladistic methodology of maximum parsimony (R. A. Jensen, Mol. Biol. Evol. 2:92-108, 1985). DAHP synthase-0 was found in Acinetobacter species and in Oceanospirillum minutulum, organisms that also possess DAHP synthase-Tyr. These two isozymes were apparently present in a common ancestor that predated the evolutionary divergence of contemporary superfamily B sublineages. DAHP synthase-0 is postulated to have been the evolutionary forerunner of DAHP synthase-Trp. The newly evolved DAHP synthase-Trp is postulated to have possessed sensitivity to feedback inhibition by chorismate as well as by L-tryptophan, chorismate sensitivity having been retained in rRNA group I pseudomonads (minor sensitivity), group V pseudomonads (very sensitive), and Lysobacter enzymogenes (ultrasensitive). Organisms constituting the enteric lineage of the phylogenetic tree (including a cluster of four Oceanospirillum species) have all lost the chorismate sensitivity of DAHP synthase-Trp. The absence of DAHP synthase-Phe in the Oceanospirillum cluster of organisms supports the previous conclusion that DAHP synthase-Phe evolved recently within superfamily B, being present only Escherichia coli and its close relatives.

Nitrous oxide reduction by members of the family Rhodospirillaceae and the nitrous oxide reductase of Rhodopseudomonas capsulata

After growth in the absence of nitrogenous oxides under anaerobic phototrophic conditions, several strains of Rhodopseudomonas capsulata were shown to possess a nitrous oxide reductase activity. The enzyme responsible for this activity had a periplasmic location and resembled a nitrous oxide reductase purified from Pseudomonas perfectomarinus. Electron flow to nitrous oxide reductase was coupled to generation of a membrane potential and inhibited by rotenone but not antimycin. It is suggested that electron flow to nitrous oxide reductase branches at the level of ubiquinone from the previously characterized electron transfer components of R. capsulata. This pathway of electron transport could include cytochrome c', a component hitherto without a recognized function. R. capsulata grew under dark anaerobic conditions in the presence of malate as carbon source and nitrous oxide as electron acceptor. This confirms that nitrous oxide respiration is linked to ATP synthesis. Phototrophically and anaerobically grown cultures of nondenitrifying strains of Rhodopseudomonas sphaeroides, Rhodopseudomonas palustris, and Rhodospirillum rubrum also possessed nitrous oxide reductase activity.

Rhodopseudomonas blastica atp operon. Nucleotide sequence and transcription

The nucleotide sequence has been determined of a 12,368 base-pair region of DNA cloned from the non-sulphur photosynthetic bacterium Rhodopseudomonas blastica. It contains a cluster of six genes of which five encode the subunits of F1-ATPase; the sixth codes for an unknown protein. The genes are arranged in the same order as in the Escherichia coli unc operon, except that the unknown gene is placed between those for gamma and beta subunits. Neither the genes for F0 subunits, nor a homologue of the E. coli uncI gene is associated with this locus. The six genes are transcribed from a single promoter and we have designated this region the R. blastica atp operon. The two distal genes, beta and epsilon, may also be transcribed from a second promoter. Initiation and termination points for transcription have been identified by primer extensions and S1 nuclease mapping experiments. Signals involved in initiation of translation (Shine and Dalgarno sequences) and termination of transcription in the photosynthetic bacterium resemble those in E. coli. However, no common features can be identified in these two bacteria between 5' regions adjacent to sites of initiation of transcription. The sequence also contains a gene that encodes a protein homologous to discoidin, a cell surface lectin of Dictyostelium discoideum thought to be involved in cell--cell aggregation. Seven other reading frames have not been identified.