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PubMed1986

Characterization of the thioredoxin system in the facultative phototroph Rhodobacter sphaeroides Y.

This paper reports the purification and characterization of a thioredoxin system (thioredoxin, thioredoxin reductase, NADPH) from the facultative phototroph Rhodobacter sphaeroides Y. Rhodobacter sph. Y thioredoxin was purified to homogeneity with an assay based on the reduction of 5,5'-dithiobis(2-nitrobenzoic acid) by NADPH and Escherichia coli thioredoxin reductase. Rhodobacter sph. Y thioredoxin reductase was purified with the same assay using NADPH and E. coli thioredoxin. Rhodobacter sph. Y thioredoxin contained 102 amino acid residues and had a single intrachain disulfide bond. The two half-cystine residues are part of the active site made up of the sequence -Ala-Glu-Trp-Cys-Gly-Pro-Cys-Arg- which is identical to that of E. coli thioredoxin except for the presence of an Arg instead of a Lys. Rhodobacter sph. Y thioredoxin contains two tryptophan residues. The fluorescence intensity of the tryptophan residues is quenched in oxidized thioredoxin; on reduction, a much smaller increase is observed with Rhodobacter sph. Y thioredoxin than with the E. coli protein. However, the presence of 5 M guanidine X HCl results in the complete exposure of the two tryptophan residues. Rhodobacter sph. Y thioredoxin reductase has structural and functional similarities to E. coli thioredoxin reductase: it has a molecular mass of 68 kDa, and consists of two, probably identical, subunits. Each subunit has one bound FAD molecule. The enzyme is highly specific for NADPH; it is also highly specific for Rhodobacter sph. Y thioredoxin with a Km value of 3.3 +/- 0.6 microM. A kinetic study of the two thioredoxin systems shows that they have a high degree of cross-reactivity.

Amino Acids↗
PubMed2003

Role of Rhodobacter sp. strain PS9, a purple non-sulfur photosynthetic bacterium isolated from an anaerobic swine waste lagoon, in odor remediation.

Temporal pigmentation changes resulting from the development of a purple color in anaerobic swine waste lagoons were investigated during a 4-year period. The major purple photosynthetic bacterium responsible for these color changes and the corresponding reductions in odor was isolated from nine photosynthetic lagoons. By using morphological, physiological, and phylogenetic characterization methods we identified the predominant photosynthetic bacterium as a new strain of Rhodobacter, designated Rhodobacter sp. strain PS9. Rhodobacter sp. strain PS9 is capable of photoorganotrophic growth on a variety of organic compounds, including all of the characteristic volatile organic compounds (VOC) responsible for the odor associated with swine production facilities (J. A. Zahn, A. A. DiSpirito, Y. S. Do, B. E. Brooks, E. E. Copper, and J. L. Hatfield, J. Environ. Qual. 30:624-634, 2001). The seasonal variations in airborne VOC emitted from waste lagoons showed that there was a 80 to 93% decrease in the concentration of VOC during a photosynthetic bloom. During the height of a bloom, the Rhodobacter sp. strain PS9 population accounted for 10% of the total community and up to 27% of the eubacterial community based on 16S ribosomal DNA signals. Additional observations based on seasonal variations in meteorological, biological, and chemical parameters suggested that the photosynthetic blooms of Rhodobacter sp. strain PS9 were correlated with lagoon water temperature and with the concentrations of sulfate and phosphate. In addition, the photosynthetic blooms of Rhodobacter sp. strain PS9 were inversely correlated with the concentrations of protein and fluoride.

Anaerobiosis↗
PubMed2002

Proposal of Pseudorhodobacter ferrugineus gen nov, comb nov, for a non-photosynthetic marine bacterium, Agrobacterium ferrugineum, related to the genus Rhodobacter.

The marine gram-negative non-photosynthetic bacterium, Agrobacterium ferrugineum IAM 12616(T) forms one cluster with the species of the photosynthetic genus Rhodobacter in phylogenetic trees based on molecules of 16S rRNA, 23S rRNA and DNA gyrases. Agrobacterium ferrugineum and Rhodobacter species are similar in that growth occurs without NaCl in the culture medium (optimal NaCl concentration for growth of P. ferrugineus is 1%) and their major hydroxy fatty acid compositions are 3-hydroxy decanoic acids (3-OH 10:0) and 3-hydroxy tetradecanoic acids (3-OH 14:1). However, A. ferrugineum differs from Rhodobacter species in G+C content (58 mol% in A. ferrugineum versus 64-73 mol% in Rhodobacter species), in having an insertion in its 16S rRNA gene sequence, and in lacking photosynthetic abilities, bacteriochlorophyll a and intracytoplasmic membrane systems. Furthermore, experiments using PCR and Southern hybridization show that A. ferrugineum does not have puhA gene and puf genes localized near the opposite ends of the photosynthesis gene cluster of Rhodobacter capsulatus. It suggests that A. ferrugineum may not have any genes for photosynthesis. We propose the transfer of A. ferrugineum IAM 12616(T) to the genus Pseudorhodobacter gen. nov. as Pseudorhodobacter ferrugineus comb. nov. Although Pseudorhodobacter ferrugineus disturbs the phylogenetic monophyly of the genus Rhodobacter, this taxonomic proposal seems adequate until it has been clarified whether P. ferrugineus possesses an incomplete photosynthetic apparatus.

Base Composition↗
PubMed1995

Refinement of the high-resolution physical and genetic map of Rhodobacter capsulatus and genome surveys using blots of the cosmid encyclopedia.

Cosmids from a library containing Rhodobacter capsulatus DNA fragments were previously ordered in two contigs: one corresponding to the chromosome and one to a 134 kb plasmid. This map contained 40 regions connected only by colony hybridization. To confirm the linkage and correct the map, the actual sizes of the overlaps were determined by blot-hybridization with Rhodobacter chromosomal DNA and by mapping of additional cosmids. Several revisions of the earlier map include single cosmid shifts and inversions. One additional gap in a cosmid contig was also found, raising the possibility that the chromosome is not a contiguous circle. About 2500 additional EcoRI,BamHI and HindIII restriction sites were added to the 560 EcoRV sites previously mapped onto the Rhodobacter chromosome, increasing the resolution of the physical map to the size of individual genes. Twenty-five new markers were located on the genetic map. The 48 markers now mapped represent nearly 300 genes and ORFs cloned from different species of Rhodobacter. The orientation of transcription of the four rrn operons was established using 16S rRNA- and 23S rRNA-specific probes and digestion with the rare-cutting enzyme, CeuI. Gel blots of 192 cosmids of the miniset of R.capsulatus digested with EcoRV were prepared. Such a hybridization template represents the whole genome cut into 560 DNA fragments varying in size from 0.4 to 25 kb. This template was used for high-resolution mapping of single genes, analysis of total genomic DNAs from related Rhodobacter strains and differentially expressed RNAs.

Chromosome Mapping↗
PubMed1997

Magnesium chelatase: association with ribosomes and mutant complementation studies identify barley subunit Xantha-G as a functional counterpart of Rhodobacter subunit BchD.

Magnesium chelatase catalyses the insertion of Mg2+ into protoporphyrin and is found exclusively in organisms which synthesise chlorophyll or bacteriochlorophyll. Soluble protein preparations containing >10 mg protein/ml, obtained by gentle lysis of barley plastids and Rhodobacter sphaeroplasts, inserted Mg2+ into deuteroporphyrin IX in the presence of ATP at rates of 40 and 8 pmoles/mg protein per min, respectively. With barley extracts optimal activity was observed with 40 mM Mg2+. The activity was inhibited by micromolar concentrations of chloramphenicol. Mutations in each of three genetic loci, Xantha-f, -g and -h, in barley destroyed the activity. However, Mg-chelatase activity was reconstituted in vitro by combining pairwise the plastid stroma protein preparations from non-leaky xantha-f -g and -h mutants. This establishes that, as in Rhodobacter, three proteins are required for the insertion of magnesium into protoporphyrin IX in barley. These three proteins, Xantha-F, -G and -H, are referred to as Mg-chelatase subunits and they appear to exist separate from each other in vivo. Active preparations from barley and Rhodobacter yielded pellet and supernatant fractions upon centrifugation for 90 min at 272,000 x g. The pellet and the supernatant were inactive when assayed separately, but when they were combined activity was restored. Differential distribution of the Mg-chelatase subunits in the fractions was established by in vitro complementation assays using stroma protein from the xantha-f, -g, and -h mutants. Xantha-G protein was confined to the pellet fraction, while Xantha-H was confined to the supernatant. Reconstitution assays using purified recombinant BchH, BchI and partially purified BchD revealed that the pellet fraction from Rhodobacter contained the BchD subunit. The pellet fractions from both barley and Rhodobacter contained ribosomes and had an A260:A280 ratio of 1.8. On sucrose density gradients both Xantha-G and BchD subunits migrated with the plastid and bacterial ribosomal RNA, respectively.

Chlorophyll↗
PubMedDate not supplied

[Cloning and identification of the gene controlling nitrogen metabolism in the photosynthetic purple bacteria Rhodobacter sphaeroides].

A recombinant plasmid has been selected from the genomic library of Rhodobacter sphaeroides that restores the properties of the wild type strain in the mutant Drn121. The latter possesses the derepressed synthesis of nitrogenase when grown in the light, inability of nitrogen fixation in the dark and growth on potassium nitrate as a single source of nitrogen, disruption of ammonium ions and methylamine transportation, decreased activity of glutamine synthetase. The gene complementing the drn121 mutation is localized within the EcoRI-HindIII fragment of Rhodobacter sphaeroides chromosome 2.25 kb in size. Analysis of the fragment nucleotide sequence has revealed the fragments with a high level of homology to regulatory genes ntrB (the 3'-end) and ntrC of Rhodobacter capsulatus. The plasmid pRCN102, containing the nifR3-ntrB-ntrC operon of Rhodobacter capsulatus, is able to complement the drn121 mutation while its derivatives having inactivated ntrN or ntrC genes are not. Hence, in Rhodobacter sphaeroides mutant Drn121 the mutation is localized in ntrC gene the product of which is involved not only in nitrogen fixation but also in nitrogen metabolism on the whole.

Chromosome Mapping↗
PubMed1990

Chloramphenicol acetyltransferase expression in marine Rhodobacter sp. NKPB 0021 by use of shuttle vectors containing the minimal replicon of an endogenous plasmid.

A vector, pUK318, was constructed to allow the expression of foreign genes in the marine photosynthetic bacterium Rhodobacter sp. NKPB 0021. This strain has been cured of its two endogenous plasmids. pUK318 consists of a 2.3-kb PstI-BamHI restriction fragment, containing a marine Rhodobacter plasmid replication region, cloned into pUC18. This fragment was derived from plasmid pRD31, a 3.1-kb endogenous plasmid purified from the marine strain Rhodobacter sp. NKPB 043402. A kanamycin resistance gene from Tn903 was cloned into the PstI restriction site to provide antibiotic selection. pUK318 was transferred to Rhodobacter sp. NKPB 0021 by transformation, and efficiencies of 7.2 x 10(-5) were obtained. Furthermore, pUK318 was stably maintained when transformants were grown either heterotrophically or photosynthetically in the absence of antibiotics. pUK318 was used to express the Escherichia coli chloramphenicol acetyl transferase (CAT) gene in Rb. NKPB 0021. Transformants expressed a maximum CAT activity of 1.12 mmol/min/g dry cells. In addition, the DNA region essential for pUK318 replication in Rb. NKPB 0021 was localized to a 1.36-kb HincII-PstI fragment. This is the first report of a plasmid vector containing a marine Rhodobacter-specific replicon that allows stable expression of foreign genes in the absence of antibiotic selection.

Chloramphenicol O-Acetyltransferase↗
PubMed1997

Identification, sequence analysis, and expression of the lepB gene for a leader peptidase in Rhodobacter capsulatus.

The leader peptidase (signal peptidase I) gene, lepB, of Rhodobacter capsulatus has been cloned and sequenced. The amino acid sequence of the predicted protein exhibits similarity to other known bacterial leader peptidases. R. capsulatus belongs to the alpha-subdivision of purple bacteria and thus is a relative of mitochondria in eukaryotes. Like the yeast mitochondrial inner membrane proteases IMP1 and IMP2, the leader peptidase from Rhodobacter has only one membrane-spanning segment. Sequence comparison of the Rhodobacter Lep protein with IMP1 and IMP2 did not reveal a higher overall similarity than between other prokaryotic signal peptidases and the mitochondrial enzymes. Expression studies using lacZ fusions in combination with primer extension analysis provide evidence for a weak promoter located a short distance from the transcription start of the lepB gene. Failure to establish a Rhodobacter strain with a disrupted lepB gene indicates that this gene is essential.

Amino Acid Sequence↗
PubMed1996

Identification and analysis of the rnc gene for RNase III in Rhodobacter capsulatus.

The large subunit ribosomal RNA of the purple bacterium Rhodobacter capsulatus shows fragmentation into pieces of 14 and 16S, both fragments forming the functional equivalent of intact 23S rRNA. An RNA-processing step removes an extra stem-loop structure from the 23S rRNA [Kordes, E., Jock, S., Fritsch, J., Bosch, F. and Klug, G. (1994) J. Bacteriol., 176, 1121-1127]. Taking advantage of the fragmentation deficient mutant strain Fm65, we used genetic complementation to find the mutated gene responsible for this aberration. It was identified as the Rhodobacter homologue to mc from Escherichia coli encoding endoribonuclease III (RNase III). The predicted protein has 226 amino acids with a molecular weight of 25.5 kDa. It shares high homology with other known RNase III enzymes over the full length. In particular it shows the double-stranded RNA-binding domain (dsRBD) motif essential for binding of dsRNA substrates. The Fm65 mutant has a frame shift mutation resulting in complete loss of the dsRBD rendering the enzyme inactive. The cloned Rhodobacter enzyme can substitute RNase III activity in an RNase III deficient E. coli strain. Contrary to E. coli, the Rhodobacter mc is in one operon together with the lep gene encoding the leader peptidase.

Amino Acid Sequence↗
PubMed1998

Asymmetric reduction of racemic sulfoxides by dimethyl sulfoxide reductases from Rhodobacter capsulatus, Escherichia coli and Proteus species.

The enantioselective reduction of racemic sulfoxides by dimethyl sulfoxide reductases from Rhodobacter capsulatus, Escherichia coli, Proteus mirabilis and Proteus vulgaris was investigated. Purified dimethyl sulfoxide reductase from Rhodobacter capsulatus catalysed the selective removal of (S)-methyl p-tolyl sulfoxide from a racemic mixture of methyl p-tolyl sulfoxide and resulted in an 88% recovery of enantiomerically pure (R)-methyl p-tolyl sulfoxide. Rhodobacter capsulatus was shown to be able to grow photoheterotrophically in the presence of certain chiral sulfoxides under conditions where a sulfoxide is needed as an electron sink. Whole cells of Rhodobacter capsulatus were shown to catalyse the enantioselective reduction of methyl p-tolyl sulfoxide, ethyl 2-pyridyl sulfoxide, methylthiomethyl methyl sulfoxide and methoxymethyl phenyl sulfoxide. Similarly, whole cells of Escherichia coli, Proteus mirabilis and Proteus vulgaris reduced these sulfoxides but with opposite enantioselectivity.

Anaerobiosis↗
PubMed2002

Numerical dominance and phylotype diversity of marine Rhodobacter species during early colonization of submerged surfaces in coastal marine waters as determined by 16S ribosomal DNA sequence analysis and fluorescence in situ hybridization.

Early stages of surface colonization in coastal marine waters appear to be dominated by the marine Rhodobacter group of the alpha subdivision of the division Proteobacteria (alpha-Proteobacteria). However, the quantitative contribution of this group to primary surface colonization has not been determined. In this study, glass microscope slides were incubated in a salt marsh tidal creek for 3 or 6 days. Colonizing bacteria on the slides were examined by fluorescence in situ hybridization by employing DNA probes targeting 16S or 23S rRNA to identify specific phylogenetic groups. Confocal laser scanning microscopy was then used to quantify and track the dynamics of bacterial primary colonists during the early stages of surface colonization and growth. More than 60% of the surface-colonizing bacteria detectable by fluorescence staining (Yo-Pro-1) could also be detected with the Bacteria domain probe EUB338. Archaea were not detected on the surfaces and did not appear to participate in surface colonization. Of the three subdivisions of the Proteobacteria examined, the alpha-Proteobacteria were the most abundant surface-colonizing organisms. More than 28% of the total bacterial cells and more than 40% of the cells detected by EUB338 on the surfaces were affiliated with the marine Rhodobacter group. Bacterial abundance increased significantly on the surfaces during short-term incubation, mainly due to the growth of the marine Rhodobacter group organisms. These results demonstrated the quantitative importance of the marine Rhodobacter group in colonization of surfaces in salt marsh waters and confirmed that at least during the early stages of colonization, this group dominated the surface-colonizing bacterial assemblage.

Colony Count, Microbial↗
PubMed1999

Structural and functional analyses of photosynthetic regulatory genes regA and regB from Rhodovulum sulfidophilum, Roseobacter denitrificans, and Rhodobacter capsulatus.

Genes coding for putative RegA, RegB, and SenC homologues were identified and characterized in the purple nonsulfur photosynthetic bacteria Rhodovulum sulfidophilum and Roseobacter denitrificans, species that demonstrate weak or no oxygen repression of photosystem synthesis. This additional sequence information was then used to perform a comparative analysis with previously sequenced RegA, RegB, and SenC homologues obtained from Rhodobacter capsulatus and Rhodobacter sphaeroides. These are photosynthetic bacteria that exhibit a high level of oxygen repression of photosystem synthesis controlled by the RegA-RegB two-component regulatory system. The response regulator, RegA, exhibits a remarkable 78.7 to 84.2% overall sequence identity, with total conservation within a putative helix-turn-helix DNA-binding motif. The RegB sensor kinase homologues also exhibit a high level of sequence conservation (55.9 to 61.5%) although these additional species give significantly different responses to oxygen. A Rhodovulum sulfidophilum mutant lacking regA or regB was constructed. These mutants produced smaller amounts of photopigments under aerobic and anaerobic conditions, indicating that the RegA-RegB regulon controls photosynthetic gene expression in this bacterium as it does as in Rhodobacter species. Rhodobacter capsulatus regA- or regB-deficient mutants recovered the synthesis of a photosynthetic apparatus that still retained regulation by oxygen tension when complemented with reg genes from Rhodovulum sulfidophilum and Roseobacter denitrificans. These results suggest that differential expression of photosynthetic genes in response to aerobic and anaerobic growth conditions is not the result of altered redox sensing by the sensor kinase protein, RegB.

Aerobiosis↗
PubMedDate not supplied

Differences in codon usage among genes encoding proteins of different function in Rhodobacter capsulatus.

Codon usage in Rhodobacter was evaluated and found to be strikingly different from that in Escherichia coli. While codon usage for genes concerned with nitrogen utilization and carotenoid biosynthesis corresponded to expectation, based on codon usage for Rhodobacter in general, that for the fructose utilization (fru) operon and for the photosynthetic genes encoding the reaction centre and light harvesting proteins exhibited significant deviation from expectation and from each other for specific amino acids. The differences in codon usage for the fru operon versus the photosynthetic genes may reflect different proportions of the various tRNA specific for certain amino acids when cells are grown under heterotrophic versus phototropic conditions. In addition, preferential use of the initiation codon, GTG, was found for the first cistrons of Rhodobacter operons.

Base Sequence↗
PubMed2003

Isolation and molecular characterization of pMG160, a mobilizable cryptic plasmid from Rhodobacter blasticus.

A 3.4-kb cryptic plasmid was obtained from a new isolate of Rhodobacter blasticus. This plasmid, designated pMG160, was mobilizable by the conjugative strain Escherichia coli S17.1 into Rhodobacter sphaeroides, Rhodobacter capsulatus, and Rhodopseudomonas palustris. It replicated in the latter strains but not in Rhodospirillum rubrum, Rhodocyclus gelatinosus, or Bradyrhizobium species. Plasmid pMG160 was stably maintained in R. sphaeroides for more than 100 generations in the absence of selection but showed segregational instability in R. palustris. Instability in R. palustris correlated with a decrease in plasmid copy number compared to the copy number in R. sphaeroides. The complete nucleotide sequence of plasmid pMG160 contained three open reading frames (ORFs). The deduced amino acid sequences encoded by ORF1 and ORF2 showed high degrees of homology to the MobS and MobL proteins that are involved in plasmid mobilization of certain plasmids. Based on homology with the Rep protein of several other plasmids, ORF3 encodes a putative rep gene initiator of plasmid replication. The functions of these sequences were demonstrated by deletion mapping, frameshift analysis, and analysis of point mutations. Two 6.1-kb pMG160-based E. coli-R. sphaeroides shuttle cloning vectors were constructed and designated pMG170 and pMG171. These two novel shuttle vectors were segregationally stable in R. sphaeroides growing under nonselective conditions.

Bacterial Proteins↗
PubMed2001

An alternative to the accepted phylogeny of purple bacteria based on 16S rRNA: analyses of the amino acid sequences of cytochromes C2 and C556 from Rhodobacter (Rhodovulum) sulfidophilus.

It is becoming increasingly apparent from complete genome sequences that 16S rRNA data, as currently interpreted, does not provide an unambiguous picture of bacterial phylogeny. In contrast, we have found that analysis of insertions and deletions in the amino acid sequences of cytochrome c2 has some advantages in establishing relationships and that this approach may have broad utility in acquiring a better understanding of bacterial relationships. The amino acid sequences of cytochromes c2 and c556 have been determined in whole or in part from four strains of Rhodobacter sulfidophilus. The cytochrome c2 contains three- and eight-residue insertions as well as a single-residue deletion in common with the large cytochromes c2 but in contrast to the small cytochromes c2 and mitochondrial cytochromes. In addition, the Rb. sulfidophilus protein shares a rare six- to seven-residue insertion with other Rhodobacter cytochromes c2. The cytochrome c556 is a low-spin class II cytochrome c homologous to the greater family of cytochromes c', which are usually high-spin. The similarity of cytochrome c556 to other species of class II cytochromes is consistent with the relationships deduced from comparisons of cytochromes c2. Thus, our results do not support placement of Rb. sulfidophilus in a separate genus, Rhodovulum, which was proposed primarily on the basis of 16S rRNA sequences. Instead, the Rhodobacter cytochromes c2 are distinct from those of other genera and species of purple bacteria and show a different pattern of relationships among species than reported for 16S rRNA.

Amino Acid Sequence↗
PubMed2006

Investigation of Rhodobacter capsulatus PufX interactions in the core complex of the photosynthetic apparatus.

The photosynthetic apparatus of purple bacteria in the genus Rhodobacter includes a core complex consisting of the reaction centre (RC), light-harvesting complex 1 (LH1), and the PufX protein. PufX modulates LH1 structure and facilitates photosynthetic quinone/quinol exchange. We deleted RC/LH1 genes in pufX+ and pufX++ (merodiploid) strains of Rhodobacter capsulatus, which reduced PufX levels regardless of pufX gene copy number and location. Photosynthetic growth of RC-only strains and independent assembly kinetics of the RC and LH1 were unaffected by pufX merodiploidy, but the absorption spectra of strains expressing the RC plus either LH1 alpha or beta indicated that PufX may influence bacteriochlorophyll binding environments. Significant self-association of the PufX transmembrane segment was detected in a hybrid protein expression system, consistent with a role of PufX in core complex dimerization, as proposed for other Rhodobacter species. Our results indicate that in R. capsulatus PufX has the potential to be a central, homodimeric core complex component, and its cellular level is increased by interactions with the RC and LH1.

Bacterial Proteins↗
PubMed1996

Cloning and sequence analysis of the dimethylsulfoxide reductase structural gene from Rhodobacter capsulatus.

The dimethylsulfoxide reductase structural gene (dorA) of Rhodobacter capsulatus was cloned from a lambda expression library. The nucleotide sequence of the dorA gene was determined and it was found to encode a protein of 825 amino acids. Comparison of the deduced amino-acid sequence of DorA with N-terminal sequence of purified dimethylsulfoxide reductase from Rhodobacter capsulatus showed that the pre-protein possesses a 41-amino-acid N-terminal signal polypeptide. All of the conserved segments which have been described in bacterial enzymes which bind molybdopterin guanine dinucleotide (Berks, B.C., Ferguson, S.J., Moir, J.W.B. and Richardson, D.J. (1995) Biochim, Biophys. Acta 1232, 97-173) were identified in Rhodobacter capsulatus dimethylsulfoxide reductase.

Amino Acid Sequence↗
PubMed2005

The PufX protein of Rhodobacter capsulatus affects the properties of bacteriochlorophyll a and carotenoid pigments of light-harvesting complex 1.

A pufX gene deletion in the purple bacterium Rhodobacter capsulatus causes a severe photosynthetic defect and increases core light-harvesting complex (LH1) protein and bacteriochlorophyll a (BChl) levels. It was suggested that PufX interrupts the LH1 alpha/beta ring around the reaction centre, allowing quinone/quinol exchange. However, naturally PufX(-) purple bacteria grow photosynthetically with an uninterrupted LH1. We discovered that substitutions of the Rhodobacter-specific LH1 alpha seryl-2 decrease carotenoid levels in PufX(-)R. capsulatus. An LH1 alphaS2F mutation improved the photosynthetic growth of a PufX(-) strain lacking the peripheral LH2 antenna, although LH1 BChl absorption remained above wild-type, suggesting that Rhodobacter-specific carotenoid binding is involved in the PufX(-) photosynthetic defect and LH1 expansion is not. Furthermore, PufX overexpression increased LH1-like BChl absorption without inhibiting photosynthetic growth. PufX(+) LH1 alphaS2-substituted mutant strains had wild-type carotenoid levels, indicating that PufX modulates LH1 carotenoid binding, inducing a conformational change that favours quinone/quinol exchange.

Bacterial Proteins↗