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

Widespread dissolved inorganic carbon-modifying toolkits in genomes of autotrophic Bacteria and Archaea and how they are likely to bridge supply from the environment to demand by autotrophic pathways

Using dissolved inorganic carbon (DIC) as a major carbon source, as autotrophs do, is complicated by the bedeviling nature of this substance. Autotrophs using the Calvin-Benson-Bassham cycle (CBB) are known to make use of a toolkit comprised of DIC transporters and carbonic anhydrase enzymes (CA) to facilitate DIC fixation. This minireview provides a brief overview of the current understanding of how toolkit function facilitates DIC fixation in Cyanobacteria and some Proteobacteria using the CBB and continues with a survey of the DIC toolkit gene presence in organisms using different versions of the CBB and other autotrophic pathways (reductive citric acid cycle, Wood-Ljungdahl pathway, hydroxypropionate bicycle, hydroxypropionate-hydroxybutyrate cycle, and dicarboxylate-hydroxybutyrate cycle). The potential function of toolkit gene products in these organisms is discussed in terms of CO2 and HCO3- supply from the environment and demand by the autotrophic pathway. The presence of DIC toolkit genes in autotrophic organisms beyond those using the CBB suggests the relevance of DIC metabolism to these organisms and provides a basis for better engineering of these organisms for industrial and agricultural purposes.

Microbial methane emissions from the non-methanogenesis processes: A critical review

Methane, a potent greenhouse gas of global importance, has traditionally been considered as an end product of microbial methanogenesis of organic matter. Paradoxically, growing evidence has shown that some microbes, such as cyanobacteria, algae, fungi, purple non-sulfur bacteria, and cryptogamic covers, produce methane in oxygen-saturated aquatic and terrestrial ecosystems. The non-methanogenesis process could be an important potential contributor to methane emissions. This systematic review summarizes the knowledge of microorganisms involved in the non-methanogenesis process and the possible mechanisms of methane formation. Cyanobacteria-derived methane production may be attributed to either demethylation of methyl phosphonates or linked to light-driven primary productivity, while algae produce methane by utilizing methylated sulfur compounds as possible carbon precursors. In addition, fungi produce methane by utilizing methionine as a possible carbon precursor, and purple non-sulfur bacteria reduce carbon dioxide to methane by nitrogenase. The microbial methane distribution from the non-methanogenesis processes in aquatic and terrestrial environments and its environmental significance to global methane emissions, possible mechanisms of methane production in each open water, water-to-air methane fluxes, and the impact of climate change on microorganisms are also discussed. Finally, future perspectives are highlighted, such as establishing more in-situ experiments, quantifying methane flux through optimizing empirical models, distinguishing individual methane sources, and investigating nitrogenase-like enzyme systems to improve our understanding of microbial methane emission from the non-methanogenesis process.

The MpsAB Bicarbonate Transporter Is Superior to Carbonic Anhydrase in Biofilm-Forming Bacteria with Limited CO2 Diffusion

CO2 and bicarbonate are required for carboxylation reactions, which are essential in most bacteria. To provide the cells with sufficient CO2, there exist two dissolved inorganic carbon supply (DICS) systems: the membrane potential-generating system (MpsAB) and the carbonic anhydrase (CA). Recently, it has been shown that MpsAB is a bicarbonate transporter that is present not only in photo- and autotrophic bacteria, but also in a diverse range of nonautotrophic microorganisms. Since the two systems rarely coexist in a species but are interchangeable, we investigated what advantages the one system might have over the other. Using the genus Staphylococcus as a model, we deleted the CA gene can in Staphylococcus carnosus and mpsABC genes in Staphylococcus aureus. Deletion of the respective gene in one or the other species led to growth inhibition that could only be reversed by CO2 supplementation. While the S. carnosus Δcan mutant could be fully complemented with mpsABC, the S. aureus ΔmpsABC mutant was only partially complemented by can, suggesting that MpsAB outperforms CA. Interestingly, we provide evidence that mucus biofilm formation such as that involving polysaccharide intercellular adhesin (PIA) impedes the diffusion of CO2 into cells, making MpsAB more advantageous in biofilm-producing strains or species. Coexpression of MpsAB and CA does not confer any growth benefits, even under stress conditions. In conclusion, the distribution of MpsAB or CA in bacteria does not appear to be random as expression of bicarbonate transporters provides an advantage where diffusion of CO2 is impeded. IMPORTANCE CO2 and bicarbonate are required for carboxylation reactions in central metabolism and biosynthesis of small molecules in all bacteria. This is achieved by two different systems for dissolved inorganic carbon supply (DICS): these are the membrane potential-generating system (MpsAB) and the carbonic anhydrase (CA), but both rarely coexist in a given species. Here, we compared both systems and demonstrate that the distribution of MpsAB and/or CA within the phylum Firmicutes is apparently not random. The bicarbonate transporter MpsAB has an advantage in species where CO2 diffusion is hampered-for instance, in mucus- and biofilm-forming bacteria. However, coexpression of MpsAB and CA does not confer any growth benefits, even under stress conditions. Given the clinical relevance of Staphylococcus in the medical environment, such findings contribute to the understanding of bacterial metabolism and thus are crucial for exploration of potential targets for antimicrobials. The knowledge gained here as exemplified by staphylococcal species could be extended to other pathogenic bacteria.

Characterization and High-Level Periplasmic Expression of Thermostable α-Carbonic Anhydrase from Thermosulfurimonas Dismutans in Escherichia Coli for CO2 Capture and Utilization

Carbonic anhydrase (CA) is a diffusion-controlled enzyme that rapidly catalyzes carbon dioxide (CO₂) hydration. CA has been considered as a powerful and green catalyst for bioinspired CO₂ capture and utilization (CCU). For successful industrial applications, it is necessary to expand the pool of thermostable CAs to meet the stability requirement under various operational conditions. In addition, high-level expression of thermostable CA is desirable for the economical production of the enzyme. In this study, a thermostable CA (tdCA) of Thermosulfurimonas dismutans isolated from a deep-sea hydrothermal vent was expressed in Escherichia coli and characterized in terms of expression level, solubility, activity and stability. tdCA showed higher solubility, activity, and stability compared to those of CA from Thermovibrio ammonificans, one of the most thermostable CAs, under low-salt aqueous conditions. tdCA was engineered for high-level expression by the introduction of a point mutation and periplasmic expression via the Sec-dependent pathway. The combined strategy resulted in a variant showing at least an 8.3-fold higher expression level compared to that of wild-type tdCA. The E. coli cells with the periplasmic tdCA variant were also investigated as an ultra-efficient whole-cell biocatalyst. The engineered bacterium displayed an 11.9-fold higher activity compared to that of the recently reported system with a halophilic CA. Collectively these results demonstrate that the highly expressed periplasmic tdCA variant, either in an isolated form or within a whole-cell platform, is a promising biocatalyst with high activity and stability for CCU applications.

Regulatory components of carbon concentrating mechanisms in aquatic unicellular photosynthetic organisms

This review provides an insight into the regulation of the carbon concentrating mechanisms (CCMs) in lower organisms like cyanobacteria, proteobacteria, and algae. CCMs evolved as a mechanism to concentrate CO₂ at the site of primary carboxylating enzyme Ribulose-1, 5-bisphosphate carboxylase oxygenase (Rubisco), so that the enzyme could overcome its affinity towards O₂ which leads to wasteful processes like photorespiration. A diverse set of CCMs exist in nature, i.e., carboxysomes in cyanobacteria and proteobacteria; pyrenoids in algae and diatoms, the C₄ system, and Crassulacean acid metabolism in higher plants. Prime regulators of CCM in most of the photosynthetic autotrophs belong to the LysR family of transcriptional regulators, which regulate the activity of the components of CCM depending upon the ambient CO₂ concentrations. Major targets of these regulators are carbonic anhydrase and inorganic carbon uptake systems (CO₂ and HCO₃^- transporters) whose activities are modulated either at transcriptional level or by changes in the levels of their co-regulatory metabolites. The article provides information on the localization of the CCM components as well as their function and participation in the development of an efficient CCM. Signal transduction cascades leading to activation/inactivation of inducible CCM components on perception of low/high CO₂ stimuli have also been brought into picture. A detailed study of the regulatory components can aid in identifying the unraveled aspects of these mechanisms and hence provide information on key molecules that need to be explored to further provide a clear understanding of the mechanism under study.

Expression and characterization of a codon-optimized alkaline-stable carbonic anhydrase from Aliivibrio salmonicida for CO2 sequestration applications

The CO₂ mineralization process, accelerated by carbonic anhydrase (CA) was proposed for the efficient capture and storage of CO₂, the accumulation of which in the atmosphere is the main cause of global warming. Here, we characterize a highly stable form of the cloned CA from the Gram-negative marine bacterium Aliivibrio salmonicida, named ASCA that can promote CO₂ absorption in an alkaline solvent required for efficient carbon capture. We designed a mature form of ASCA (mASCA) using a codon optimization of ASCA gene and removal of ASCA signal peptide. mASCA was highly expressed (255 mg/L) with a molecular weight of approximately 26 kDa. The mASCA enzyme exhibited stable esterase activity within a temperature range of 10-60 °C and a pH range of 6-11. mASCA activity remained stable for 48 h at pH 10. We also investigated its inhibition profiles using inorganic anions, such as acetazolamide, sulfanilamide, iodide, nitrate, and azide. We also demonstrate that mASCA is capable of catalyzing the conversion of CO₂ to CaCO₃ (calcite form) in the presence of Ca²⁺. It should be noted that mASCA enzyme exhibits high production yield and sufficient stabilities against relatively high temperature and alkaline pH, which are required conditions for the development of more efficient enzymatic CCS systems.

Light-driven carbon dioxide reduction to methane by nitrogenase in a photosynthetic bacterium

Nitrogenase is an ATP-requiring enzyme capable of carrying out multielectron reductions of inert molecules. A purified remodeled nitrogenase containing two amino acid substitutions near the site of its FeMo cofactor was recently described as having the capacity to reduce carbon dioxide (CO2) to methane (CH4). Here, we developed the anoxygenic phototroph, Rhodopseudomonas palustris, as a biocatalyst capable of light-driven CO2 reduction to CH4 in vivo using this remodeled nitrogenase. Conversion of CO2 to CH4 by R. palustris required constitutive expression of nitrogenase, which was achieved by using a variant of the transcription factor NifA that is able to activate expression of nitrogenase under all growth conditions. Also, light was required for generation of ATP by cyclic photophosphorylation. CH4 production by R. palustris could be controlled by manipulating the distribution of electrons and energy available to nitrogenase. This work shows the feasibility of using microbes to generate hydrocarbons from CO2 in one enzymatic step using light energy.

Novel alkalistable α-carbonic anhydrase from the polyextremophilic bacterium Bacillus halodurans: characteristics and applicability in flue gas CO2 sequestration

The emissions of CO2 into the atmosphere have been constantly rising due to anthropogenic activities, which have led to global warming and climate change. Among various methods proposed for mitigating CO2 levels in the atmosphere, carbonic anhydrase (CA)-mediated carbon sequestration represents a greener and safer approach to capture and convert it into stable mineral carbonates. Despite the fact that CA is an extremely efficient metalloenzyme that catalyzes the hydration of CO2 (CO2 + H2O ↔ HCO3 (-) + H(+)) with a kcat of ∼10(6) s(-1), a thermostable, and alkalistable CA is desirable for the process to take place efficiently. The purified CA from alkaliphilic, moderately thermophilic, and halotolerant Bacillus halodurans TSLV1 (BhCA) is a homodimeric enzyme with a subunit molecular mass of ~37 kDa with stability in a broad pH range between 6.0 and 11.0. It has a moderate thermostability with a T1/2 of 24.0 ± 1.0 min at 60 °C. Based on the sensitivity of CA to specific inhibitors, BhCA is an α-CA; this has been confirmed by nucleotide/amino acid sequence analysis. This has a unique property of stimulation by SO4 (2-), and it remains unaffected by SO3 (2-), NOx, and most other components present in the flue gas. BhCA is highly efficient in accelerating the mineralization of CO2 as compared to commercial bovine carbonic anhydrase (BCA) and is also efficient in the sequestration of CO2 from the exhaust of petrol driven car, thus, a useful biocatalyst for sequestering CO2 from flue gas.

The selective expression of carbonic anhydrase genes of Aspergillus nidulans in response to changes in mineral nutrition and CO2 concentration

Carbonic anhydrase (CA) plays an important role in the formation and evolution of life. However, to our knowledge, there has been no report on CA isoenzyme function differentiation in fungi. Two different CA gene sequences in Aspergillus nidulans with clear genetic background provide us a favorable basis for studying function differentiation of CA isoenzymes. Heterologously expressed CA1 was used to test its weathering ability on silicate minerals and real-time quantitative PCR was used to detect expression of the CA1 and CA2 genes at different CO2 concentrations and in the presence of different potassium sources. The northern blot method was applied to confirm the result of CA1 gene expression. Heterologously expressed CA1 significantly promoted dissolution of biotite and wollastonite, and CA1 gene expression increased significantly in response to soluble K-deficiency. The northern blot test further showed that CA1 participated in K-feldspar weathering. In addition, the results showed that CA2 was primary involved in adapting to CO2 concentration change. Taken together, A. nidulans can choose different CA to meet their survival needs, which imply that some environmental microbes have evolved different CAs to adapt to changes in CO2 concentration and acquire mineral nutrition so that they can better adapt to environmental changes. Inversely, their adaption may impact mineral weathering and/or CO2 concentration, and even global change.

Expression and characterization of codon-optimized carbonic anhydrase from Dunaliella species for CO(2) sequestration application

Carbonic anhydrases (CAs) have been given much attention as biocatalysts for CO(2) sequestration process because of their ability to convert CO(2) to bicarbonate. Here, we expressed codon-optimized sequence of α-type CA cloned from Dunaliella species (Dsp-aCAopt) and characterized its catalyzing properties to apply for CO(2) to calcite formation. The expressed amount of Dsp-aCAopt in Escherichia coli is about 50 mg/L via induction of 1.0 mM isopropyl-β-D-thiogalactopyranoside at 20 °C (for the case of intact Dsp-aCA, negligible). Dsp-aCAopt enzyme shows 47 °C of half-denaturation temperature and show wide pH stability (optimum pH 7.6/10.0). Apparent values of K (m) and V (max) for p-nitrophenylacetate substrate are 0.91 mM and 3.303 × 10(-5) μM min(-1). The effects of metal ions and anions were investigated to find out which factors enhance or inhibit Dsp-aCAopt activity. Finally, we demonstrated that Dsp-aCAopt enzyme can catalyze well the conversion of CO(2) to CaCO(3), as the calcite form, in the Ca(2+) solution [8.9 mg/100 μg (172 U/mg enzyme) with 10 mM of Ca(2+)]. The obtained expression and characterization results of Dsp-aCAopt would be usefully employed for the development of efficient CA-based system for CO(2)-converting/capturing processes.

Crystallographic study of wild-type carbonic anhydrase alpha CA1 from Chlamydomonas reinhardtii

Carbonic anhydrases (CAs) are ubiquitously distributed and are grouped into three structurally independent classes (alphaCA, betaCA and gammaCA). Most alphaCA enzymes are monomeric, but alphaCA1 from Chlamydomonas reinhardtii is a dimer that is uniquely stabilized by disulfide bonds. In addition, during maturation an internal peptide of 35 residues is removed and three asparagine residues are glycosylated. In order to obtain insight into the effects of these structural features on CA function, wild-type C. reinhardtii alphaCA1 has been crystallized in space group P6(5), with unit-cell parameters a=b=134.3, c=120.2 A. The crystal diffracted to 1.88 A resolution and a preliminary solution of its crystal structure has been obtained by the MAD method.

Expression and function of four carbonic anhydrase homologs in the deep-sea chemolithoautotroph Thiomicrospira crunogena

The hydrothermal vent chemolithoautotroph Thiomicrospira crunogena grows rapidly in the presence of low concentrations of dissolved inorganic carbon (DIC) (= CO(2) + HCO(3)(-) + CO(3)(-2)). Its genome encodes alpha-carbonic anhydrase (alpha-CA), beta-CA, carboxysomal beta-like CA (CsoSCA), and a protein distantly related to gamma-CA. The purposes of this work were to characterize the gene products, determine whether they were differentially expressed, and identify those that are necessary for DIC uptake and fixation. When expressed in Escherichia coli, CA activity was detectable for alpha-CA, beta-CA, and CsoSCA but not for the gamma-CA-like protein. alpha-CA and CsoSCA but not beta-CA were inhibited by sulfonamide inhibitors. CsoSCA was also inhibited by dithiothreitol. When grown under DIC limitation in chemostats, T. crunogena transcribed csoSCA more frequently than when ammonia limited, while genes encoding alpha-CA and beta-CA were not differentially transcribed under these conditions. Cell extracts from T. crunogena grown under both DIC- and ammonia-limited conditions had CA activity that was strongly inhibited by sulfonamides, though extracts from nitrogen-limited cells had some CA activity that was resistant, perhaps due to a higher level of beta-CA activity. Based on predictions from the SignalP software program, subcellular location when expressed in E. coli, and carbonic anhydrase assays conducted on intact T. crunogena cells, alpha-CA is located in the periplasm. However, inhibition of alpha-CA by acetazolamide had only a minor impact on rates of DIC uptake or fixation. Conversely, inhibition of CsoSCA with ethoxyzolamide inhibited carbon fixation but not DIC uptake, consistent with this enzyme functioning to facilitate DIC interconversion and fixation within carboxysomes.

Regulation of expression and biochemical characterization of a beta-class carbonic anhydrase from the plant growth-promoting rhizobacterium, Azospirillum brasilense Sp7

Carbonic anhydrase (CA; [EC 4.2.1.1]) is a ubiquitous enzyme catalysing the reversible hydration of CO(2) to bicarbonate, a reaction that supports various biochemical and physiological functions. Genome analysis of Azospirillum brasilense, a nonphotosynthetic, nitrogen-fixing, rhizobacterium, revealed an ORF with homology to beta-class carbonic anhydrases (CAs). Biochemical characteristics of the beta-class CA of A. brasilense, analysed after cloning the gene (designated as bca), overexpressing in Escherichia coli and purifying the protein by affinity purification, revealed that the native recombinant enzyme is a homotetramer, inhibited by the known CA inhibitors. CA activity in A. brasilense cell extracts, reverse transcriptase (RT)-PCR and Western blot analyses showed that bca was constitutively expressed under aerobic conditions. Lower beta-galactosidase activity in A. brasilense cells harbouring bca promoter: lacZ fusion during the stationary phase or during growth on 3% CO(2) enriched air or at acidic pH indicated that the transcription of bca was downregulated by the stationary phase, elevated CO(2) levels and acidic pH conditions. These observations were also supported by RT-PCR analysis. Thus, beta-CA in A. brasilense seems to be required for scavenging CO(2) from the ambient air and the requirement of CO(2) hydration seems to be higher for the cultures growing exponentially at neutral to alkaline pH.

Characterization of a Mesorhizobium loti alpha-type carbonic anhydrase and its role in symbiotic nitrogen fixation

Carbonic anhydrase (CA) (EC 4.2.1.1) is a widespread enzyme catalyzing the reversible hydration of CO(2) to bicarbonate, a reaction that participates in many biochemical and physiological processes. Mesorhizobium loti, the microsymbiont of the model legume Lotus japonicus, possesses on the symbiosis island a gene (msi040) encoding an alpha-type CA homologue, annotated as CAA1. In the present work, the CAA1 open reading frame from M. loti strain R7A was cloned, expressed, and biochemically characterized, and it was proven to be an active alpha-CA. The biochemical and physiological roles of the CAA1 gene in free-living and symbiotic rhizobia were examined by using an M. loti R7A disruption mutant strain. Our analysis revealed that CAA1 is expressed in both nitrogen-fixing bacteroids and free-living bacteria during growth in batch cultures, where gene expression was induced by increased medium pH. L. japonicus plants inoculated with the CAA1 mutant strain showed no differences in top-plant traits and nutritional status but consistently formed a higher number of nodules exhibiting higher fresh weight, N content, nitrogenase activity, and delta(13)C abundance. Based on these results, we propose that although CAA1 is not essential for nodule development and symbiotic nitrogen fixation, it may participate in an auxiliary mechanism that buffers the bacteroid periplasm, creating an environment favorable for NH(3) protonation, thus facilitating its diffusion and transport to the plant. In addition, changes in the nodule delta(13)C abundance suggest the recycling of at least part of the HCO(3)(-) produced by CAA1.

Characterization of carbonic anhydrase from diversified genus for biomimetic carbon-dioxide sequestration

Diversified group of bacteria were screened for carbonic anhydrase (CA) activity. Significant CA activity was found in crude enzyme extracts of Enterobacter and Aeromonas isolates while minimal or negligible CA activity was observed in case of Shigella and Klebsiella spp. Optimization and characterization study of potent CA producing isolates revealed that the maximum enzyme activity of 3.86 EU/ml was observed in E. taylorae and the optimum pH range for enzyme stability was found to be 7.5-9.0 along with an optimum temperature range of 35-50 °C. The molecular mass of CA was 29-kDa indicating α-type with periplasmic and cytosolic location. Present investigation for the first time reports CA in diversified genus and optimized parameters for enhanced production of CA in Enterobacter sp. & Aeromonas sp. from fresh water bodies that inturn lay down grounds for exploitation of CA from E. taylorae as an efficient catalyst for CO(2) sequestration within a bioreactor.

Whole-genome transcriptional profiling of Bradyrhizobium japonicum during chemoautotrophic growth

Bradyrhizobium japonicum is a facultative chemoautotroph capable of utilizing hydrogen gas as an electron donor in a respiratory chain terminated by oxygen to provide energy for cellular processes and carbon dioxide assimilation via a reductive pentose phosphate pathway. A transcriptomic analysis of B. japonicum cultured chemoautotrophically identified 1,485 transcripts, representing 17.5% of the genome, as differentially expressed when compared to heterotrophic cultures. Genetic determinants required for hydrogen utilization and carbon fixation, including the uptake hydrogenase system and components of the Calvin-Benson-Bassham cycle, were strongly induced in chemoautotrophically cultured cells. A putative isocitrate lyase (aceA; blr2455) was among the most strongly upregulated genes, suggesting a role for the glyoxylate cycle during chemoautotrophic growth. Addition of arabinose to chemoautotrophic cultures of B. japonicum did not significantly alter transcript profiles. Furthermore, a subset of nitrogen fixation genes was moderately induced during chemoautotrophic growth. In order to specifically address the role of isocitrate lyase and nitrogenase in chemoautotrophic growth, we cultured aceA, nifD, and nifH mutants under chemoautotrophic conditions. Growth of each mutant was similar to that of the wild type, indicating that the glyoxylate bypass and nitrogenase activity are not essential components of chemoautotrophy in B. japonicum.

Roles of alpha and beta carbonic anhydrases of Helicobacter pylori in the urease-dependent response to acidity and in colonization of the murine gastric mucosa

Carbon dioxide occupies a central position in the physiology of Helicobacter pylori owing to its capnophilic nature, the large amounts of carbon dioxide produced by urease-mediated urea hydrolysis, and the constant bicarbonate supply in the stomach. Carbonic anhydrases (CA) catalyze the interconversion of carbon dioxide and bicarbonate and are involved in functions such as CO(2) transport or trapping and pH homeostasis. H. pylori encodes a periplasmic alpha-CA (alpha-CA-HP) and a cytoplasmic beta-CA (beta-CA-HP). Single CA inactivation and double CA inactivation were obtained for five genetic backgrounds, indicating that H. pylori CA are not essential for growth in vitro. Bicarbonate-carbon dioxide exchange rates were measured by nuclear magnetic resonance spectroscopy using lysates of parental strains and CA mutants. Only the mutants defective in the alpha-CA-HP enzyme showed strongly reduced exchange rates. In H. pylori, urease activity is essential for acid resistance in the gastric environment. Urease activity measured using crude cell extracts was not modified by the absence of CA. With intact CA mutant cells incubated in acidic conditions (pH 2.2) in the presence of urea there was a delay in the increase in the pH of the incubation medium, a phenotype most pronounced in the absence of H. pylori alpha-CA. This correlated with a delay in acid activation of the urease as measured by slower ammonia production in whole cells. The role of CA in vivo was examined using the mouse model of infection with two mouse-adapted H. pylori strains, SS1 and X47-2AL. Compared to colonization by the wild-type strain, colonization by X47-2AL single and double CA mutants was strongly reduced. Colonization by SS1 CA mutants was not significantly different from colonization by wild-type strain SS1. However, when mice were infected by SS1 Delta(beta-CA-HP) or by a SS1 double CA mutant, the inflammation scores of the mouse gastric mucosa were strongly reduced. In conclusion, CA contribute to the urease-dependent response to acidity of H. pylori and are required for high-grade inflammation and efficient colonization by some strains.

Expression and localization of alpha- and beta-carbonic anhydrase in Helicobacter pylori

Helicobacter pylori, the causative agent of peptic ulcer disease, expresses two different forms of the zinc-containing enzyme carbonic anhydrase (CA) (alpha and beta), catalyzing the reversible hydration of CO(2). Presumably, the high CO(2) requirement of H. pylori implies an important role for this enzyme in the bacterial physiology. In this paper, expression of the CAs has been analyzed in three different strains of the bacterium, 26695, J99 and 17.1, and appears to be independent of CO(2) concentration in the investigated range (0.1-10%). Presence of the potent and highly specific CA inhibitor, acetazolamide, in the medium does not seem to inhibit bacterial growth at the given sulfonamide concentration. Moreover, the localization and distribution of the alpha-CA was analyzed by immunonegative staining, while SDS-digested freeze-fracture immunogold labelling was used for the beta-form of the enzyme. The latter method has the advantage of allowing assessment of protein localization to distinct cell compartments and membrane structures. The resulting electron microscopy images indicate a localization of the beta-CA in the cytosol, on the cytosolic side of the inner membrane and on the outer membrane facing the periplasmic space. The alpha-enzyme was found attached to the surface of the bacterium.