Molecular and functional characterization of the Rhodopseudomonas palustris no. 7 phosphoenolpyruvate carboxykinase gene

Microplastics enhanced the allelopathy of pyrogallol on toxic Microcystis with additional risks: Microcystins release and greenhouse gases emissions

Cyanobacteria blooms (CBs) caused by eutrophication pose a global concern, especially Microcystis aeruginosa (M. aeruginosa), which could release harmful microcystins (MCs). The impact of microplastics (MPs) on allelopathy in freshwater environments is not well understood. This study examined the joint effect of adding polystyrene (PS-MPs) as representative MPs and two concentrations (2 and 8 mg/L) of pyrogallol (PYR) on the allelopathy of M. aeruginosa. The results showed that the addition of PS-MPs intensified the inhibitory effect of 8 mg/L PYR on the growth and photosynthesis of M. aeruginosa. After a 7-day incubation period, the cell density decreased to 69.7 %, and the chl-a content decreased to 48 % compared to the condition without PS-MPs (p < 0.05). Although the growth and photosynthesis of toxic Microcystis decreased with the addition of PS-MPs, the addition of PS-MPs significantly resulted in a 3.49-fold increase in intracellular MCs and a 1.10-fold increase in extracellular MCs (p < 0.05). Additionally, the emission rates of greenhouse gases (GHGs) (carbon dioxide, nitrous oxide and methane) increased by 2.66, 2.23 and 2.17-fold, respectively (p < 0.05). In addition, transcriptomic analysis showed that the addition of PS-MPs led to the dysregulation of gene expression related to DNA synthesis, membrane function, enzyme activity, stimulus detection, MCs release and GHGs emissions in M. aeruginosa. PYR and PS-MPs triggered ROS-induced membrane damage and disrupted photosynthesis in algae, leading to increased MCs and GHG emissions. PS-MPs accumulation exacerbated this issue by impeding light absorption and membrane function, further heightening the release of MCs and GHGs emissions. Therefore, PS-MPs exhibited a synergistic effect with PYR in inhibiting the growth and photosynthesis of M. aeruginosa, resulting in additional risks such as MCs release and GHGs emissions. These results provide valuable insights for the ecological risk assessment and control of algae bloom in freshwater ecosystems.

Genetic analysis of electroactive biofilms

Geobacter biofilms synthesize an electroactive exopolysaccharide matrix with conductive pili and c-cytochromes that spatially organizes cells optimally for growth and electron transport to iron oxide substrates, soluble metal contaminants, and current-harvesting electrodes. Despite its relevance to bioremediation and bioenergy applications, little is known about the developmental stages leading to the formation of mature (>20 μm thick) electroactive biofilms. Thus, we developed a transposon mutagenesis method and a high-throughput screening assay and identified mutants of Geobacter sulfurreducens PCA interrupted in the initial stages of surface colonization (attachment and monolayer formation) and the vertical growth and maturation of multilayered biofilms. The molecular dissection of biofilm formation demonstrated that cells undergo a regulated developmental program to first colonize the surface to saturation and then synthesize an electroactive matrix to support optimal cell growth within structured communities. Transitioning from a monolayer to a multilayered, mature biofilm required the expression of conductive pili, consistent with the essential role of these extracellular protein appendages as electronic conduits across all layers of the biofilms. The genetic screening also identified cell envelope processes, regulatory pathways, and electron transport components not previously linked to biofilm formation. These genes provide much-needed understanding of the cellular reprogramming needed to build electroactive biofilms. Importantly, they serve as predictive markers of the physiology and reductive capacity of Geobacter biofilms during the bioremediation of toxic metals and radionuclides and current harvesting in bioelectrochemical systems.

Comparative proteomic and genomic analyses of Brucella abortus biofilm and planktonic cells

The present study aimed to explore the differences in protein and gene expression of Brucella abortus cultured under biofilm and planktonic conditions. The proteins unique to biofilms and planktonic B. abortus were separated by two-dimensional (2-D) electrophoresis and then identified by matrix-assisted laser desorption/ionization-tandem time of flight-mass spectrometry (MALDI-TOF/TOF-MS). High-throughput sequencing and bioinformatic analyses were performed to identify differentially expressed genes between B. abortus cultured under biofilm and planktonic conditions. The proteins and genes identified by proteomic and genomic analyses were further evaluated via western blot and reverse transcription-quantitative polymerase chain reaction (RT-qPCR) analyses. 2-D electrophoresis identified 20 differentially expressed protein spots between biofilms and planktonic cells, which corresponded to 18 individual proteins (12 downregulated and 6 upregulated) after MALDI-TOF/TOF-MS analysis, including elongation factor Tu and enolase. RT-qPCR analysis revealed that all of the 18 genes were downregulated in biofilms compared with planktonic cells. Western blot analysis identified 9 downregulated and 3 upregulated proteins. High-throughput sequencing and bioinformatic analyses identified 14 function and pathway-associated genes (e.g., BAbS19_I14970). RT-qPCR analysis of the 14 genes showed that they were upregulated in biofilm compared with in planktonic state. In conclusion, these differentially expressed genes may play important roles in bacterial defense, colonization, invasion, and virulence.

Unusual Phosphoenolpyruvate (PEP) Synthetase-Like Protein Crucial to Enhancement of Polyhydroxyalkanoate Accumulation in Haloferax mediterranei Revealed by Dissection of PEP-Pyruvate Interconversion Mechanism

Phosphoenolpyruvate (PEP)/pyruvate interconversion is a major metabolic point in glycolysis and gluconeogenesis and is catalyzed by various sets of enzymes in different Archaea groups. In this study, we report the key enzymes that catalyze the anabolic and catabolic directions of the PEP/pyruvate interconversion in Haloferax mediterranei The in silico analysis showed the presence of a potassium-dependent pyruvate kinase (PYK_Hm [HFX_0773]) and two phosphoenol pyruvate synthetase (PPS) candidates (PPS_Hm [HFX_0782] and a PPS homolog protein named PPS-like [HFX_2676]) in this strain. Expression of the pyk _Hm gene and pps _Hm was induced by glycerol and pyruvate, respectively; whereas the pps-like gene was not induced at all. Similarly, genetic analysis and enzyme activities of purified proteins showed that PYK_Hm catalyzed the conversion from PEP to pyruvate and that PPS_Hm catalyzed the reverse reaction, while PPS-like protein displayed no function in PEP/pyruvate interconversion. Interestingly, knockout of the pps-like gene led to a 70.46% increase in poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) production. The transcriptome sequencing (RNA-Seq) and quantitative reverse transcription-PCR (qRT-PCR) results showed that many genes responsible for PHBV monomer supply and for PHBV synthesis were upregulated in a pps-like gene deletion strain and thereby improved PHBV accumulation. Additionally, our phylogenetic evidence suggested that PPS-like protein diverged from PPS enzyme and evolved as a distinct protein with novel function in haloarchaea. Our findings attempt to fill the gaps in central metabolism of Archaea by providing comprehensive information about key enzymes involved in the haloarchaeal PEP/pyruvate interconversion, and we also report a high-yielding PHBV strain with great future potentials.IMPORTANCE Archaea, the third domain of life, have evolved diversified metabolic pathways to cope with their extreme habitats. Phosphoenol pyruvate (PEP)/pyruvate interconversion during carbohydrate metabolism is one such important metabolic process that is highly differentiated among Archaea However, this process is still uncharacterized in the haloarchaeal group. Haloferax mediterranei is a well-studied haloarchaeon that has the ability to produce polyhydroxyalkanoates (PHAs) under unbalanced nutritional conditions. In this study, we identified the key enzymes involved in this interconversion and discussed their differences with their counterparts from other members of the Archaea and Bacteria domains. Notably, we found a novel protein, phosphoenolpyruvate synthetase-like (PPS-like), which exhibited high homology to PPS enzyme. However, PPS-like protein has evolved some distinct sequence features and functions, and strikingly the corresponding gene deletion helped to enhance poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) synthesis significantly. Overall, we have filled the gap in knowledge about PEP/pyruvate interconversion in haloarchaea and reported an efficient strategy for improving PHBV production in H. mediterranei.

A genetic tool for production of GFP-expressing Rhodopseudomonas palustris for visualization of bacterial colonization

Development of a genetic tool for visualization of photosynthetic bacteria (PSB) is essential for understanding microbial function during their interaction with plant and microflora. In this study, Rhodopseudomonas palustris GJ-22-gfp harboring the vector pBBR1-pckA_PT-gfp was constructed using an electroporation transformation method and was used for dynamic tracing of bacteria in plants. The results showed that strain GJ-22-gfp was stable and did not affect the biocontrol function, and the Confocal Laser Scanning Microscopy (CLSM) results indicated it could successfully colonised on the surface of leaf and root of tobacco and rice. In tobacco leaves, cells formed aggregates on the mesophyll epidermal cells. While in rice, no aggregate was found. Instead, the fluorescent cells colonise the longitudinal intercellular spaces between epidermal cells. In addition, the results of strain GJ-22 on the growth promotion and disease resistance of tobacco and rice indicated that the different colonization patterns might be related to the bacteria could induce systemic resistance in tobacco.

Metabolic engineering of Rhodopseudomonas palustris for the obligate reduction of n-butyrate to n-butanol

BACKGROUND

Rhodopseudomonas palustris is a versatile microbe that encounters an innate redox imbalance while growing photoheterotrophically with reduced substrates. The resulting excess in reducing equivalents, together with ATP from photosynthesis, could be utilized to drive a wide range of bioconversions. The objective of this study was to genetically modify R. palustris to provide a pathway to reduce n-butyrate into n-butanol for maintaining redox balance.

RESULTS

Here, we constructed and expressed a plasmid-based pathway for n-butanol production from Clostridium acetobutylicum ATCC 824 in R. palustris. We maintained the environmental conditions in such a way that this pathway functioned as the obligate route to re-oxidize excess reducing equivalents, resulting in an innate selection pressure. The engineered strain of R. palustris grew under otherwise restrictive redox conditions and achieved concentrations of 1.5 mM n-butanol at a production rate of 0.03 g L^-1 day^-1 and a selectivity (i.e., products compared to the consumed substrate) of close to 40%. Since the theoretical maximum selectivity is 45%, the engineered strain converted close to its maximum selectivity.

CONCLUSIONS

The innate redox imbalance of R. palustris can be used to drive the reduction of n-butyrate into n-butanol after expression of a plasmid-based enzyme from a butanol-producing Clostridium strain.

Domestic wastewater treatment with purple phototrophic bacteria using a novel continuous photo anaerobic membrane bioreactor

A key future challenge of domestic wastewater treatment is nutrient recovery while still achieving acceptable discharge limits. Nutrient partitioning using purple phototrophic bacteria (PPB) has the potential to biologically concentrate nutrients through growth. This study evaluates the use of PPB in a continuous photo-anaerobic membrane bioreactor (PAnMBR) for simultaneous organics and nutrient removal from domestic wastewater. This process could continuously treat domestic wastewater to discharge limits (<50 mgCOD L(-1), 5 mgN L(-1), 1.0 mgP L(-1)). Approximately 6.4 ± 1.3 gNH4-N and 1.1 ± 0.2 gPO4-P for every 100 gSCOD were removed at a hydraulic retention time of 8-24 h and volumetric loading rates of 0.8-2.5 COD kg m(3) d(-1). Thus, a minimum of 200 mg L(-1) of ethanol (to provide soluble COD) was required to achieve these discharge limits. Microbial community through sequencing indicated dominance of >60% of PPB, though the PPB community was highly variable. The outcomes from the current work demonstrate the potential of PPB for continuous domestic (and possibly industrial) wastewater treatment and nutrient recovery. Technical challenges include the in situ COD supply in a continuous reactor system, as well as efficient light delivery. Addition of external (agricultural or fossil) derived organics is not financially nor environmentally justified, and carbon needs to be sourced internally from the biomass itself to enable this technology. Reduced energy consumption for lighting is technically feasible, and needs to be addressed as a key objective in scaleup.

Dynamics of metabolic activities and gene expression in the Roseobacter clade bacterium Phaeobacter sp. strain MED193 during growth with thiosulfate

Metagenomic analyses of surface seawater reveal that genes for sulfur oxidation are widespread in bacterioplankton communities. However, little is known about the metabolic processes used to exploit the energy potentially gained from inorganic sulfur oxidation in oxic seawater. We therefore studied the sox gene system containing Roseobacter clade isolate Phaeobacter sp. strain MED193 in acetate minimal medium with and without thiosulfate. The addition of thiosulfate enhanced the bacterial growth yields up to 40% in this strain. Concomitantly, soxB and soxY gene expression increased about 8-fold with thiosulfate and remained 11-fold higher than that in controls through stationary phase. At stationary phase, thiosulfate stimulated protein synthesis and anaplerotic CO2 fixation rates up to 5- and 35-fold, respectively. Several genes involved in anaplerotic CO2 fixation (i.e., pyruvate carboxylase, propionyl coenzyme A [CoA], and crotonyl-CoA carboxylase) were highly expressed during active growth, coinciding with high CO2 fixation rates. The high expression of key genes in the ethylmalonyl-CoA pathway suggests that this is an important pathway for the utilization of two-carbon compounds in Phaeobacter sp. MED193. Overall, our findings imply that Roseobacter clade bacteria carrying sox genes can use their lithotrophic potential to gain additional energy from sulfur oxidation for both increasing their growth capacity and improving their long-term survival.

Enhancement of photoheterotrophic biohydrogen production at elevated temperatures by the expression of a thermophilic clostridial hydrogenase

The working temperature of a photobioreactor under sunlight can be elevated above the optimal growth temperature of a microorganism. To improve the biohydrogen productivity of photosynthetic bacteria at higher temperatures, a [FeFe]-hydrogenase gene from the thermophile Clostridium thermocellum was expressed in the mesophile Rhodopseudomonas palustris CGA009 (strain CGA-CThydA) using a log-phase expression promoter P( pckA ) to drive the expression of heterogeneous hydrogenase gene. In contrast, a mesophilic Clostridium acetobutylicum [FeFe]-hydrogenase gene was also constructed and expressed in R. palustris (strain CGA-CAhydA). Both transgenic strains were tested for cell growth, in vivo hydrogen production rate, and in vitro hydrogenase activity at elevated temperatures. Although both CGA-CThydA and CGA-CAhydA strains demonstrated enhanced growth over the vector control at temperatures above 38 °C, CGA-CThydA produced more hydrogen than the other strains. The in vitro hydrogenase activity assay, measured at 40 °C, confirmed that the activity of the CGA-CThydA hydrogenase was higher than the CGA-CAhydA hydrogenase. These results showed that the expression of a thermophilic [FeFe]-hydrogenase in R. palustris increased the growth rate and biohydrogen production at elevated temperatures. This transgenic strategy can be applied to a broad range of purple photosynthetic bacteria used to produce biohydrogen under sunlight.

Phototrophic growth of a Rubisco-deficient mesophilic purple nonsulfur bacterium harboring a Type III Rubisco from a hyperthermophilic archaeon

The hyperthermophilic archaeon, Thermococcus kodakaraensis KOD1 harbors a structurally novel, Type III Rubisco (Rbc(Tk)). In terms of protein engineering of Rubiscos, the enzyme may provide an alternative target to the conventional Type I and Type II enzymes. With a future aim to improve the catalytic properties of Rbc(Tk), here we examined whether or not the enzyme could support growth of a mesophilic organism dependent on CO2 fixation. Via double-crossover homologous recombination, we first deleted three Rubisco genes present on the chromosome of the photosynthetic mesophile Rhodopseudomonas palustris No. 7. The mutant strain (delta3) could neither grow under photoautotrophic nor photoheterotrophic conditions. We introduced the rbc(Tk) gene into strain delta3 either on a plasmid, or by integrating the gene onto the chromosome. The two transformant strains harboring rbc(Tk) displayed growth under photoautotrophic and photoheterotrophic conditions, both dependent on CO2 fixation. Specific growth rates and Rubisco activity levels were compared under photoheterotrophic conditions among the two transformants and the wild-type strain. We observed that the levels of Rubisco activity in the respective cell-free extracts correlated well with the specific growth rates. Immunoprecipitation experiments revealed that Rubisco activity detected in the transformants was derived solely from Rbc(Tk). These results demonstrated that the Type III Rbc(Tk) from a hyperthermophile could support CO2 fixation in a mesophilic organism, and that the specific growth rate of the transformant can be used as a convenient parameter for selection of engineered proteins with improved Rubisco activity.

Crystal structure of Anaerobiospirillum succiniciproducens PEP carboxykinase reveals an important active site loop

The 2.2 Angstroms resolution crystal structure of the enzyme phosphoenolpyruvate carboxykinase (PCK) from the bacterium Anaerobiospirillum succiniciproducens complexed with ATP, Mg(2+), Mn(2+) and the transition state analogue oxalate has been solved. The 2.4 Angstroms resolution native structure of A. succiniciproducens PCK has also been determined. It has been found that upon binding of substrate, PCK undergoes a conformational change. Two domains of the molecule fold towards each other, with the substrates and metal ions held in a cleft formed between the two domains. This domain movement is believed to accelerate the reaction PCK catalyzes by forcing bulk solvent molecules out of the active site. Although the crystal structure of A. succiniciproducens PCK with bound substrate and metal ions is related to the structures of PCK from Escherichia coli and Trypanosoma cruzi, it is the first crystal structure from this class of enzymes that clearly shows an important surface loop (residues 383-397) from the C-terminal domain, hydrogen bonding with the peptide backbone of the active site residue Arg60. The interaction between the surface loop and the active site backbone, which is a parallel beta-sheet, seems to be a feature unique of A. succiniciproducens PCK. The association between the loop and the active site is the third type of interaction found in PCK that is thought to play a part in the domain closure. This loop also appears to help accelerate catalysis by functioning as a 'lid' that shields water molecules from the active site.

Phosphoenolpyruvate carboxykinase is an acid-induced, chromosomally encoded virulence factor in Agrobacterium tumefaciens

The pckA gene, encoding phosphoenolpyruvate carboxykinase, catalyzes the reversible decarboxylation and phosphorylation of oxaloacetate to form phosphoenolpyruvate. Located on the circular chromosome of Agrobacterium, this locus is adjacent to the loci chvG and chvI, encoding a two-component regulatory system that has been shown to be important in virulence. Using a reporter gene fusion, studies showed that the pckA gene is induced by acidic pH but not by acetosyringone. This acid induction is regulated by the chvG-chvI regulatory system, which controls acid-inducible genes. A pckA mutant had no demonstrable PckA enzyme activity and grew on AB minimal medium with glucose but did not grow on the same medium with succinate as the sole carbon source and was more inhibited in its growth than the wild-type strain by an acidic environment. A pckA mutant was highly attenuated in tumor-inducing ability on tobacco leaf disks and was severely attenuated in vir gene expression. Although vir gene induction was completely restored when a constitutive virG gene was introduced into the mutant strain, virulence was only partially restored. These results suggest that avirulence may be due to a combination of the inhibition of this mutant in the acidic plant wound environment and the poor induction of the vir genes.

Xanthomonas campestris pv. campestris possesses a single gluconeogenic pathway that is required for virulence

Disruption of ppsA, a key gene in gluconeogenesis, of Xanthomonas campestris pv. campestris resulted in the failure of the pathogen to grow in medium with pyruvate or C4-dicarboxylates as the sole carbon source and a significant reduction in virulence, indicating that X. campestris pv. campestris possesses only the malic enzyme-PpsA route in gluconeogenesis, which is required for virulence.

The PEP-pyruvate-oxaloacetate node as the switch point for carbon flux distribution in bacteria

In many organisms, metabolite interconversion at the phosphoenolpyruvate (PEP)-pyruvate-oxaloacetate node involves a structurally entangled set of reactions that interconnects the major pathways of carbon metabolism and thus, is responsible for the distribution of the carbon flux among catabolism, anabolism and energy supply of the cell. While sugar catabolism proceeds mainly via oxidative or non-oxidative decarboxylation of pyruvate to acetyl-CoA, anaplerosis and the initial steps of gluconeogenesis are accomplished by C3- (PEP- and/or pyruvate-) carboxylation and C4- (oxaloacetate- and/or malate-) decarboxylation, respectively. In contrast to the relatively uniform central metabolic pathways in bacteria, the set of enzymes at the PEP-pyruvate-oxaloacetate node represents a surprising diversity of reactions. Variable combinations are used in different bacteria and the question of the significance of all these reactions for growth and for biotechnological fermentation processes arises. This review summarizes what is known about the enzymes and the metabolic fluxes at the PEP-pyruvate-oxaloacetate node in bacteria, with a particular focus on the C3-carboxylation and C4-decarboxylation reactions in Escherichia coli, Bacillus subtilis and Corynebacterium glutamicum. We discuss the activities of the enzymes, their regulation and their specific contribution to growth under a given condition or to biotechnological metabolite production. The present knowledge unequivocally reveals the PEP-pyruvate-oxaloacetate nodes of bacteria to be a fascinating target of metabolic engineering in order to achieve optimized metabolite production.

Metabolic analysis of Corynebacterium glutamicum during lactate and succinate productions under oxygen deprivation conditions

Lactate and succinate were produced from glucose by Corynebacterium glutamicum under oxygen deprivation conditions without growth. Addition of bicarbonate to the reaction mixture led not only to a 3.6-fold increase in succinate production rate, but also to a 2.3- and 2.5-fold increase, respectively, of the rates of lactate production and glucose consumption, compared to the control. Furthermore, when small amounts of pyruvate were added to the reaction mixture, acid production rates and the glucose consumption rate were multiplied by a factor ranging from 2 to 3. These phenomena were paralleled by an increase in the NAD(+)/NADH ratio, thus corroborating the view that the efficient regeneration of NAD(+) could be triggered by the addition of either bicarbonate or pyruvate. To investigate the global metabolism of corynebacteria under oxygen deprivation conditions, we engineered several strains where the genes coding for key metabolic enzymes had been inactivated by gene disruption and replacement. A lactate dehydrogenase (LDH)-deficient mutant was not able to produce lactate, suggesting this enzyme has no other isozyme. Although a pyruvate carboxylase (pyc) mutant exhibited similar behavior to that of the wild type, phosphoenolpyruvate carboxylase (ppc) mutants were characterized by a dramatic decrease in succinate production, which was concomitant to decreased lactate production and glucose consumption rates. This set of observations corroborates the view that in coryneform bacteria under oxygen deprivation conditions the major anaplerotic reaction is driven by the ppc gene product rather than by the pyc gene product. Moreover, intracellular NADH concentrations in C. glutamicum were observed to correlate to oxygen-deprived metabolic flows.

Acetate metabolism and its regulation in Corynebacterium glutamicum

The amino acid producing Corynebacterium glutamicum grows aerobically on a variety of carbohydrates and organic acids as single or combined sources of carbon and energy. Among the substrates metabolized are glucose and acetate which both can also serve as substrates for amino acid production. Based on biochemical, genetic and regulatory studies and on quantitative determination of metabolic fluxes during utilization of acetate and/or glucose, this review summarizes the present knowledge on the different steps of the fundamental pathways of acetate utilization in C. glutamicum, namely, on acetate transport, acetate activation, tricarboxylic acid (TCA) cycle, glyoxylate cycle and gluconeogenesis. It becomes evident that, although the pathways of acetate utilization follow the same theme in many bacteria, important biochemical, genetic and regulatory peculiarities exist in C. glutamicum. Recent genome wide and comparative expression analyses in C. glutamicum cells grown on glucose and on acetate substantiated previously identified transcriptional regulation of acetate activating enzymes and of glyoxylate cycle enzymes. Additionally, a variety of genes obviously also under transcriptional control in response to the presence or absence of acetate in the growth medium were uncovered. These genes, thus also belonging to the acetate stimulon of C. glutamicum, include genes coding for TCA cycle enzymes (e.g. aconitase and succinate dehydrogenase), for gluconeogenesis (phosphoenolpyruvate carboxykinase), for glycolysis (pyruvate dehydrogenase E1) and genes coding for proteins with hitherto unknown function. Although the basic mechanism of transcriptional regulation of the enzymes involved in acetate metabolism is not yet understood, some recent findings led to a better understanding of the adaptation of C. glutamicum to acetate at the molecular level.

Construction of a genetically engineered microorganism for CO2 fixation using a Rhodopseudomonas/Escherichia coli shuttle vector

The CO2 fixation ability of Rhodopseudomonas palustris DH was enhanced by introducing the recombinant plasmid pMG-CBBM containing the form II ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) gene (cbbM) isolated from Rps. palustris NO. 7. Sequencing of a 3.0-kb PstI fragment containing the cbbM gene revealed an open reading frame encoding 461 amino acids, homologous to known cbbM genes, with a ribosome binding site upstream of cbbM and a terminator downstream of cbbM, without promoter. pMG-CBBM, a Rhodopseudomonas/Escherichia coli shuttle expression plasmid, was derived from the Rhodopseudomonas/E. coli shuttle cloning vector pMG105, by inserting the promoter of the pckA gene and the cbbM gene into its multiple cloning site. Plasmid pMG-CBBM was transformed into Rps. palustris DH by electroporation, and was stably maintained when transformants were grown either photoheterotrophically or photolithoautotrophically in the absence of antibiotics. This is the first report of an expression plasmid containing a Rps. palustris-specific promoter that allows stable expression of a foreign gene in the absence of antibiotic selection.

Co-expression of five genes in E coli for L-phenylalanine in Brevibacterium flavum

AIM: To study the effect of co-expression of ppsA, pckA, aroG, pheA and tyrB genes on the production of L-phenylalanine, and to construct a genetic engineering strain for L-phenylalanine.

METHODS: ppsA and pckA genes were amplified from genomic DNA of E. coli by polymerase chain reaction, and then introduced into shuttle vectors between E coli and Brevibacterium flavum to generate constructs pJN2 and pJN5. pJN2 was generated by inserting ppsA and pckA genes into vector pCZ; whereas pJN5 was obtained by introducing ppsA and pckA genes into pCZ-GAB, which was originally constructed for co-expression of aroG, pheA and tyrB genes. The recombinant plasmids were then introduced into B. flavum by electroporation and the transformants were used for L-phenylalanine fermentation.

RESULTS: Compared with the original B. flavum cells, all the transformants were showed to have increased five enzyme activities specifically, and have enhanced L-phenylalanine biosynthesis ability variably. pJN5 transformant was observed to have the highest elevation of L-phenylalanine production by a 3.4-fold. Co-expression of ppsA and pckA increased activity of DAHP synthetase significantly.

CONCLUSION: Co-expression of ppsA and pckA genes in B. flavum could remarkably increase the expression of DAHP synthetase; Co-expression of ppsA, pckA, aroG, pheA and tyrB of E. coli in B. flavum was a feasible approach to construct a strain for phenylalanine production.

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.

Cloning of genes participating in aerobic biodegradation of p-cumate from Rhodopseudomonas palustris

Rhodopseudomonas palustris utilizes p-cumate as a carbon source both under anaerobic light and aerobic dark conditions. A gene cluster was isolated whose sequence showed high homology to genes which have been implicated the degradation of p-cumate in Pseudomonas pitida. Seven structural genes coding for dioxygenase-reductase, dihydroxy-dihydro dehydrogenase, and ring cleavage oxygenases were identified. A putative regulator and its possible recognition site was suggested on the basis of homology data. Mutant cells in which a kanamycin cassette was inserted into the dihydroxy-dihydro dehydrogenase gene could not grow aerobically on p-cumate. The mutation had no effect on growth using the para substituted benzoate derivatives 4-hydroxycinnamate, ferulate, protocatechuate, and 2,3,4-trihydroxybenzoate as sole carbon source. Moreover, mutant cells showed a growth pattern similar to wild type cells grown on these compounds under photoheterotrophic anaerobic conditions. These data suggest that genes of this operon are involved specifically in aerobic dissimilation of p-cumate. Intermediate products of p-cumate degradation could be detected from extracts of Escherichia coli heterologously expressing the first 5 genes responsible for the first two steps of p-cumate degradation in R. palustris. Primer extension analysis revealed the transcription regulation of the gene cluster which could be induced with para methyl-, ethyl- and isopropyl (cumate) benzoates. This is the first report on genes involved in aerobic degradation of these compounds in photosynthetic bacteria.

A periplasmic, alpha-type carbonic anhydrase from Rhodopseudomonas palustris is essential for bicarbonate uptake

Intact cells of the purple non-sulfur bacterium Rhodopseudomonas palustris growing anaerobically, but not aerobically, contain carbonic anhydrase (CA) activity. The native enzyme was purified >2000-fold to apparent homogeneity and found to be a dimer with an estimated molecular mass of 54 kDa and a subunit molecular mass of 27 kDa. The CA gene (acaP) was cloned and its sequence revealed that it was homologous to alpha-type CAs. The upstream region of acaP was fused to the lacZ gene and beta-galactosidase activity was measured under different growth conditions. Acetazolamide inhibited purified CA with an IC(50) in the range of 10(-8) M, and in the culture media concentrations as low as 30 microM inhibited phototrophic growth under anaerobic, light conditions when bicarbonate was used. An acaP::KAN:(r) mutant strain was constructed by insertion of a kanamycin-resistance cassette and showed a growth pattern similar to wild-type cells grown in the presence of CA inhibitor. CO(2) gas supplied as an inorganic carbon source reversed the effect of mutation or acetazolamide. CA activity measurements, fusion and Western blot experiments confirmed that CA is expressed under different anaerobic conditions independently of bicarbonate or CO(2) and that there is no expression under aerobic conditions.

Sequence analysis of the cryptic plasmid pMG101 from Rhodopseudomonas palustris and construction of stable cloning vectors

A 15-kb cryptic plasmid was obtained from a natural isolate of Rhodopseudomonas palustris. The plasmid, designated pMG101, was able to replicate in R. palustris and in closely related strains of Bradyrhizobium japonicum and phototrophic Bradyrhizobium species. However, it was unable to replicate in the purple nonsulfur bacterium Rhodobacter sphaeroides and in Rhizobium species. The replication region of pMG101 was localized to a 3.0-kb SalI-XhoI fragment, and this fragment was stably maintained in R. palustris for over 100 generations in the absence of selection. The complete nucleotide sequence of this fragment revealed two open reading frames (ORFs), ORF1 and ORF2. The deduced amino acid sequence of ORF1 is similar to sequences of Par proteins, which mediate plasmid stability from certain plasmids, while ORF2 was identified as a putative rep gene, coding for an initiator of plasmid replication, based on homology with the Rep proteins of several other plasmids. The function of these sequences was studied by deletion mapping and gene disruptions of ORF1 and ORF2. pMG101-based Escherichia coli-R. palustris shuttle cloning vectors pMG103 and pMG105 were constructed and were stably maintained in R. palustris growing under nonselective conditions. The ability of plasmid pMG101 to replicate in R. palustris and its close phylogenetic relatives should enable broad application of these vectors within this group of alpha-proteobacteria.