Structural gene regions of Rhodobacter sphaeroides involved in CO2 fixation

Photosynthetic electron transport and anaerobic metabolism in purple non-sulfur phototrophic bacteria

Purple non-sulfur phototrophic bacteria, exemplified by Rhodobacter capsulatus and Rhodobacter sphaeroides, exhibit a remarkable versatility in their anaerobic metabolism. In these bacteria the photosynthetic apparatus, enzymes involved in CO2 fixation and pathways of anaerobic respiration are all induced upon a reduction in oxygen tension. Recently, there have been significant advances in the understanding of molecular properties of the photosynthetic apparatus and the control of the expression of genes involved in photosynthesis and CO2 fixation. In addition, anaerobic respiratory pathways have been characterised and their interaction with photosynthetic electron transport has been described. This review will survey these advances and will discuss the ways in which photosynthetic electron transport and oxidation-reduction processes are integrated during photoautotrophic and photoheterotrophic growth.

Multiple copies of genes coding for electron transport proteins in the bacterium Nitrosomonas europaea

The genome of Nitrosomonas europaea contains at least three copies each of the genes coding for hydroxylamine oxidoreductase (HAO) and cytochrome c554. A copy of an HAO gene is always located within 2.7 kb of a copy of a cytochrome c554 gene. Cytochrome P-460, a protein that shares very unusual spectral features with HAO, was found to be encoded by a gene separate from the HAO genes.

Cloning and expression of the D-ribulose-1,5-bisphosphate carboxylase/oxygenase form II gene from Thiobacillus intermedius in Escherichia coli

Both form I and II ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) genes were detected in Thiobacillus intermedius by heterologous hybridization using specific probes from Anacystis nidulans and Rhodobacter sphaeroides, respectively. However, only the previously reported form I enzyme could be demonstrated in cells grown under a number of different conditions. The reason(s) why the form II gene is not expressed in T. intermedius is/are not clear at this time. The form II gene was isolated from a lambda library by screening with the Rb. sphaeroides probe. A SalI fragment from this clone was ligated into pUC8 and transformed into Escherichia coli DH5 alpha. Subclones pTi20IIA and pTi20IIB representing both orientations relative to the lac promoter were isolated. Low levels of RuBisCO activity were detected in both induced and non-induced pTi20IIA indicating the probable expression from a T. intermedius promoter. Induced pTi20IIB produced much higher levels of enzyme activity. Analysis of cell-free extracts using sucrose density gradients confirmed the expression of a form II RuBisCO similar in size to that found in Rhodobacter capsulatus. Other Calvin cycle genes were not clustered with either the form I or form II genes.

rbcR [correction of rcbR], a gene coding for a member of the LysR family of transcriptional regulators, is located upstream of the expressed set of ribulose 1,5-bisphosphate carboxylase/oxygenase genes in the photosynthetic bacterium Chromatium vinosum

An open reading frame, rbcR, was identified 226 bp upstream of rbcAB, i.e., the ribulose 1,5-bisphosphate carboxylase genes expressed in the phototrophic purple bacterium Chromatium vinosum. Several features reveal that rbcR encodes a member of the LysR family of transcriptional regulators, in which an anomalous content of lysine and arginine residues (Lys/Arg anomaly) was found. The expression of rbcR in Escherichia coli as a protein fused to the N-terminal region of beta-galactosidase led to reduced expression of rbcAB. Thus, rbcR is likely to encode a trans-acting transcriptional regulator of rbcAB expression in C. vinosum.

Sequence analysis and phylogenetic reconstruction of the genes encoding the large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase from the chlorophyll b-containing prokaryote Prochlorothrix hollandica

Prochlorophytes similar to Prochloron sp. and Prochlorothrix hollandica have been suggested as possible progenitors of the plastids of green algae and land plants because they are prokaryotic organisms that possess chlorophyll b (chl b). We have sequenced the Prochlorothrix genes encoding the large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase(rubisco), rbcL and rbcS, for comparison with those of other taxa to assess the phylogenetic relationship of this species. Length differences in the large subunit polypeptide among all sequences compared occur primarily at the amino terminus, where numerous short gaps are present, and at the carboxy terminus, where sequences of Alcaligenes eutrophus and non-chlorophyll b algae are several amino acids longer. Some domains in the small subunit polypeptide are conserved among all sequences analyzed, yet in other domains the sequences of different phylogenetic groups exhibit specific structural characteristics. Phylogenetic analyses of rbcL and rbcS using Wagner parsimony analysis of deduced amino acid sequences indicate that Prochlorothrix is more closely related to cyanobacteria than to the green plastid lineage. The molecular phylogenies suggest that plastids originated by at least three separate primary endosymbiotic events, i.e., once each leading to green algae and land plants, to red algae, and to Cyanophora paradoxa. The Prochlorothrix rubisco genes show a strong GC bias, with 68% of the third codon positions being G or C. Factors that may affect the GC content of different genomes are discussed.

Phosphoribulokinase activity and regulation of CO2 fixation critical for photosynthetic growth of Rhodobacter sphaeroides

The Rhodobacter sphaeroides genome contains two unlinked genetic regions each encoding numerous proteins involved in CO2 fixation which include phosphoribulokinases (prkA and prkB), ribulose 1,5-bisphosphate carboxylase/oxygenase (rbcLS and rbcR) (P. L. Hallenbeck and S. Kaplan, Photosynth. Res. 19:63-71, 1988; F. R. Tabita, Microbiol. Rev. 52:155-189, 1988), and two open reading frames linked to rbcLS and rbcR, namely, cfxA and cfxB, respectively (P. L. Hallenbeck, R. Lerchen, P. Hessler, and S. Kaplan, J. Bacteriol. 172:1736-1748). In this study, we examined the unique role(s) of each phosphoribulokinase activity in the regulation of CO2 fixation. Strains were constructed which contain null mutations in prkA and/or prkB. Studies utilizing these strains suggested that CO2 fixation plays an essential role in attaining the cellular redox balance necessary for photoheterotrophic growth. The presence of an external electron acceptor can negate the requirement for CO2 for photoheterotrophic growth. Each form of phosphoribulokinase and ribulose 1,5-bisphosphate carboxylase/oxygenase was shown to have distinct roles in CO2 metabolism when cells were exposed to extremes in CO2 levels. Evidence is also presented which unequivocally demonstrated that regulation of the expression of the enzymes involved in CO2 metabolism is effective at the transcriptional level. Although the two regions of the DNA involved in CO2 fixation are physically unlinked, each region of the DNA can have a profound effect on the expression of the other region of the DNA.

Roles of CfxA, CfxB, and external electron acceptors in regulation of ribulose 1,5-bisphosphate carboxylase/oxygenase expression in Rhodobacter sphaeroides

The Rhodobacter sphaeroides genome contains two unlinked genetic regions each encoding a series of proteins involved in CO2 fixation which include phosphoribulokinase (prkA and prkB) and ribulose 1,5-bisphosphate carboxylase/oxygenase (rbcLS and rbcR) (P. L. Hallenbeck and S. Kaplan, Photosynth. Res. 19:63-71, 1988; F. R. Tabita, Microbiol. Rev. 52:155-189, 1988). We examined the effect of CO2 in the presence and absence of an alternate electron acceptor, dimethyl sulfoxide, on the expression of rbcR and rbcLS in photoheterotrophically grown R. sphaeroides. The expression of both rbcR and rbcLS was shown to depend on the CO2 concentration when succinate was used as the carbon source. It was also demonstrated that CO2 fixation is critical for photoheterotrophic growth but could be replaced by the alternative reduction of dimethyl sulfoxide to dimethyl sulfide. Dimethyl sulfoxide severely depressed both rbcR and rbcLS expression in cells grown photoheterotrophically at CO2 concentrations of 0.05% or greater. However, cells grown photoheterotrophically in the absence of exogenous CO2 but in the presence of dimethyl sulfoxide had intermediate levels of expression of rbcL and rbcR, suggesting partially independent control by limiting CO2 tension. We also present evidence for the existence of two gene products, namely, CfxA and CfxB, which are encoded by genes immediately upstream of rbcLS and rbcR, respectively. Strains were constructed which contained null mutations in cfxA and/or cfxB. Each mutation eliminated expression of the linked downstream rbc operon. Further, studies utilizing these strains demonstrated that each form of ribulose 1,5-bisphosphate carboxylase/oxygenase plays an essential role in maintaining the cellular redox balance during photoheterotrophic growth at differing CO2 concentrations.

Physical and genetic mapping of the Rhodobacter sphaeroides 2.4.1 genome: genome size, fragment identification, and gene localization

Four restriction endonucleases, AseI (5'-ATTAAT), SpeI (5'-ACTAGT), DraI (5'-TTTAAA), and SnaBI (5'-TACGTA), generated DNA fragments of suitable size distributions for mapping the genome of Rhodobacter sphaeroides by transverse alternating field electrophoresis. AseI produced 17 fragments, ranging in size from 3 to 1,105 kilobases (kb), SpeI yielded 16 fragments (12 to 1,645 kb), DraI yielded at least 25 fragments (6 to 800 kb), and SnaBI generated 10 fragments (12 to 1,225 kb). A total genome size of approximately 4,400 +/- 112 kb was determined by summing the fragment lengths in each of the digests generated by using the different restriction endonucleases. The total genomic DNA consisted of chromosomal DNA (3,960 +/- 112 kb) and the five endogenous plasmids (approximately 450 kb total) whose cognate DNA fragments have been unambiguously identified. A number of genes have been physically mapped to the AseI-generated restriction endonuclease fragments of total genomic DNA by Southern hybridization analysis with either homologous or heterologous specific gene probes or, in the case of several auxotrophic and pigment-biosynthetic mutants apparently generated by Tn5, a Tn5-specific probe. Other genes have been mapped by a comparison with wild-type patterns of the electrophoretic banding patterns of the AseI-digested genomic DNA derived from mutants generated by the insertion of either kanamycin or spectinomycin-streptomycin resistance cartridges. The relative orientations, distance, and location of the pufBALMX, puhA, cycA, and pucBA operons have also been determined, as have been the relative orientations between prkB and hemT and between prkA and the fbc operon.

Physical and genetic mapping of the Rhodobacter sphaeroides 2.4.1 genome: presence of two unique circular chromosomes

A macrorestriction map representing the complete physical map of the Rhodobacter sphaeroides 2.4.1 chromosomes has been constructed by ordering the chromosomal DNA fragments from total genomic DNA digested with the restriction endonucleases AseI, SpeI, DraI, and SnaBI. Junction fragments and multiple restriction endonuclease digestions of the chromosomal DNAs derived from wild-type and various mutant strains, in conjunction with Southern hybridization analysis, have been used to order all of the chromosomal DNA fragments. Our results indicate that R. sphaeroides 2.4.1 carries two different circular chromosomes of 3,046 +/- 95 and 914 +/- 17 kilobases (kb). Both chromosome I (3,046 kb) and chromosome II (914 kb) contain rRNA cistrons. It appears that only a single copy of the rRNA genes is contained on chromosome I (rrnA) and that two copies are present on chromosome II (rrnB, rrnC). Additionally, genes for glyceraldehyde 3-phosphate dehydrogenase (gapB) and delta-aminolevulinic acid synthase (hemT) are found on chromosome II. In each instance, there appears to be a second copy of each of these genes on chromosome I, but the extent of the DNA homology is very low. Genes giving rise to enzymes involved in CO2 fixation and linked to the gene encoding the form I enzyme (i.e., the form I region) are on chromosome I, whereas those genes representing the form II region are on chromosome II. The complete physical and partial genetic maps for each chromosome are presented.