Characterization of alkaline phosphatase PhoK from Sphingomonas sp. BSAR-1 for phosphate monoester synthesis and hydrolysis

The biocatalytic activity of a so far underexploited alkaline phosphatase, PhoK from Sphingomonas sp. BSAR-1, was extensively studied in transphosphorylation and hydrolysis reactions. The use of high-energy phosphate donors and oligophosphates as suitable phosphate donors was evaluated, as well as the hydrolytic activity on a variety of phosphate monoesters. While substrates bearing free hydroxy group displayed only moderate reactivity as acceptors for transphosphorylation by PhoK, strong hydrolytic activity on a broad variety of phosphate monoesters under alkaline conditions was observed. Site-directed mutagenesis of selected amino acid residues in the active site provided valuable insights on their involvement in enzyme catalysis. The key residue Thr89 so far postulated to engage in enzyme phosphorylation was confirmed to be crucial for catalysis and could be replaced by serine, albeit with much lower catalytic efficiency.     

Differentiating Intracellular from Extracellular Alkaline Phosphatase Activity in Soil by Sonication

Differentiating intracellular from extracellular enzyme activity is important in soil enzymology, but not easy. Here, we report on an adjusted sonication method for the separation of intracellular from extracellular phosphatase activity in soil. Under optimal sonication conditions [soil:water ratio  =  1/8 (w/v) and power density  =  15 watt ml^-1], the activity of alkaline phosphomonoesterase (phosphatase) in a Haplic Cambisol soil increased with sonication time in two distinct steps. A first plateau of enzyme activity was reached between 60 and 100 s, and a second higher plateau after 300 s. We also found that sonication for 100 s under optimal conditions activated most (about 80%) of the alkaline phosphatase that was added to an autoclaved soil, while total bacteria number was not affected. Sonication for 300 s reduced the total bacteria number by three orders of magnitude but had no further effects on enzyme activity. Our results indicate that the first plateau of alkaline phosphatase activity was derived from extracellular enzymes attached to soil particles, and the second plateau to the combination of extracellular and intracellular enzymes after cell lysis. We conclude that our adjusted sonication method may be an alternative to the currently used physiological and chloroform-fumigation methods for differentiating intracellular from extracellular phosphatase activity in soil. Further testing is needed to find out whether this holds for other soil types.     

Phytochemical profile and antioxidant activity of extracts of the peruvian peppertree Schinus areira L. from Chile

The Andean tree Schinus areira L. has multiple traditional uses, from the treatment of bronchitis and rheumatic diseases to menstrual cycle regulation and wound healing. With reported hypotensive, analgesic, antitumoral and anti-inflammatory properties, it acts predominantly against diseases related to oxidative stress. This study focuses on the antioxidant activity and phytochemical profile of the extracts of Schinus areira L.Serial extraction of the fruits was performed both by maceration and by Soxhlet. Total phenols and flavonoids were measured using the Folin-Ciocalteu method and AlCl₃, respectively. In vitro antioxidant activity was determined by FRAP and DPPH.Results were similar for both extraction methods. Primary metabolites detected included carbohydrates, proteins and amino acids; secondary metabolites included tannins, flavonoids, saponins, steroids and triterpenes. Antioxidant activity was confirmed for ethyl acetate, methanolic and aqueous extracts. The methanolic extract had both the highest polyphenol content (>195 mg GAE/ g dry weight) and the highest antioxidant activity [EC₅₀ > 476 μg/mL; >273 mg AA/g dry weight (DPPH); >301 mg AA/ g dry weight (FRAP)]. The extract does not produce macrophage cytotoxicity in RAW 264.7, which is indicated by an average cytotoxicity of 2% over 24 h.Our study serves as a starting point for future research on the pharmacological properties of Schinus areira L.     

Application of a Novel Alkali-Tolerant Thermostable DyP-Type Peroxidase from Saccharomonospora viridis DSM 43017 in Biobleaching of Eucalyptus Kraft Pulp

Saccharomonospora viridis is a thermophilic actinomycete that may have biotechnological applications because of its dye decolorizing activity, though the enzymatic oxidative system responsible for this activity remains elusive. Bioinformatic analysis revealed a DyP-type peroxidase gene in the genome of S. viridis DSM 43017 with sequence similarity to peroxidase from dye-decolorizing microbes. This gene, svidyp, consists of 1,215 bp encoding a polypeptide of 404 amino acids. The gene encoding SviDyP was cloned, heterologously expressed in Escherichia coli, and then purified. The recombinant protein could efficiently decolorize several triarylmethane dyes, anthraquinonic and azo dyes under neutral to alkaline conditions. The optimum pH and temperature for SviDyP was pH 7.0 and 70°C, respectively. Compared with other DyP-type peroxidases, SviDyP was more active at high temperatures, retaining>63% of its maximum activity at 50–80°C. It also showed broad pH adaptability (>35% activity at pH 4.0–9.0) and alkali-tolerance (>80% activity after incubation at pH 5–10 for 1 h at 37°C), and was highly thermostable (>60% activity after incubation at 70°C for 2 h at pH 7.0). SviDyP had an accelerated action during the biobleaching of eucalyptus kraft pulp, resulting in a 21.8% reduction in kappa number and an increase of 2.98% (ISO) in brightness. These favorable properties make SviDyP peroxidase a promising enzyme for use in the pulp and paper industries.     

Characterization of a Highly Thermostable Alkaline Phosphatase from the Euryarchaeon Pyrococcus abyssi

This work reports the first isolation and characterization of an alkaline phosphatase (AP) from a hyperthermophilic archaeon. An AP gene from Pyrococcus abyssi, a euryarchaeon isolated from a deep-sea hydrothermal vent, was cloned and the enzyme expressed in Escherichia coli. Analysis of the sequence showed conservation of the active site and structural elements of the E. coli AP. The recombinant AP was purified and characterized. Monomeric and homodimeric active forms were detected, with a monomer molecular mass of 54 kDa. Apparent optimum pH and temperature were estimated at 11.0 and 70°C, respectively. Thus far, P. abyssi AP has been demonstrated to be the most thermostable AP, with half-lives at 100 and 105°C of 18 and 5 h, respectively. Enzyme activity was found to be dependent on divalent cations: metal ion chelators inhibited activity, whereas the addition of exogenous Mg(II), Zn(II), and Co(II) increased activity. The enzyme was inhibited by inorganic phosphate, but not by molybdate and vanadate. Strong inhibitory effects were observed in the presence of thiol-reducing agents, although cysteine residues of the P. abyssi AP were not found to be incorporated within intra- or interchain disulfide bonds. In addition, P. abyssi AP was demonstrated to dephosphorylate linear DNA fragments with dephosphorylation efficiencies of 93.8 and 84.1% with regard to cohesive and blunt ends, respectively.     

Phosphatase synthesis in Klebsiella (Aerobacter) aerogenes growing in continuous culture

1. Phosphatase synthesis was studied in Klebsiella aerogenes grown in a wide range of continuous-culture systems. 2. Maximum acid phosphatase synthesis was associated with nutrient-limited, particularly carbohydrate-limited, growth at a relatively low rate, glucose-limited cells exhibiting the highest activity. Compared with glucose as the carbon-limiting growth material, other sugars not only altered the activity but also changed the pH–activity profile of the enzyme(s). 3. The affinity of the acid phosphatase in glucose-limited cells towards p-nitrophenyl phosphate (K_m 0.25–0.43mm) was similar to that of staphylococcal acid phosphatase but was ten times greater than that of the Escherichia coli enzyme. 4. PO₄³⁻-limitation derepressed alkaline phosphatase synthesis but the amounts of activity were largely independent of the carbon source used for growth. 5. The enzymes were further differentiated by the effect of adding inhibitors (F⁻, PO₄³⁻) and sugars to the reaction mixture during the assays. In particular, it was shown that adding glucose, but not other sugars, stimulated the rate of hydrolysis of p-nitrophenyl phosphate by the acid phosphatase in carbohydrate-limited cells at low pH values (<4.6) but inhibited it at high pH values (>4.6). Alkaline phosphatase activity was unaffected. 6. The function of phosphatases in general is discussed and possible mechanisms for the glucose effect are outlined.     

Induction of Rat Liver Alkaline Phosphatase by Bile Duct Ligation

Bile duct ligation causes a five- to sevenfold increase in the activity of rat liver alkaline phosphatase within 12 hours after ligation and a similar rise in the activity of alkaline phosphatase in serum. The increased serum activity is due entirely to the appearance of a new isoenzyme that has the properties of rat liver alkaline phosphatase. The increase in both serum and liver alkaline phosphatase is prevented by the prior administration of cycloheximide in a dose that inhibits protein synthesis by 70%. Rat liver alkaline phosphatase was then purified to homogeneity. Antibody was raised to purified rat liver alkaline phosphatase in rabbits. The antibody was coupled to sepharose 4B and affinity columns made. ³-H-leucine was then injected into the portal veins of sham operated rats and rats with bile duct ligation four hours after ligation. One hour after injection and five hours after ligation, animals were sacrificed. Liver alkaline phosphatase was purified by means of affinity chromatography and double immunoprecipitation with rabbit antibody to rat liver alkaline phosphatase and goat anti-rabbit gamma globulin. Bile duct ligation increased the incorporation of ³-H-leucine into liver alkaline phosphatase more than threefold compared with sham operated rats, 164 CPM/mg protein vs. 49 CPM/mg protein (p < .001). The data indicate that the increased activity of rat liver alkaline phosphatase after bile duct ligation is due to enzyme induction rather than to activation of a pre-existing, relatively inactive enzyme.     

Lyophilized,non-viable,recombinant E. coli cells for cadmium bioprecipitation and recovery

The phoN gene, encoding a non-specific acid phosphatase from Salmonella enterica sv. Typhi, was cloned and overexpressed into Escherichia coli. The E. coli cells bearing phoN showed high acid phosphatase activity and removed 83% of cadmium from a 1 mM solution in 3 h, when provided with 5 mM β-glycerophosphate as a source of phosphate. Such cells, when lyophilized without any lyo/cryoprotectant, were rendered non-viable but fully retained cadmium precipitation ability. Lyophilized recombinant cells could be stored at room temperature, without significant loss of activity for up to 6 months, and removed upto 21 g cadmium/g dry weight. The precipitated cadmium could be easily recovered from the cells by dilute acid wash, following which the cells retained their cadmium precipitation ability, facilitating their reuse. The use of genetically engineered, non-viable E. coli cells offers an environmentally safe biotechnology for bioremediation of cadmium from contaminated sites.     

Francisella DnaK Inhibits Tissue-nonspecific Alkaline Phosphatase

Following pulmonary infection with Francisella tularensis, we observed an unexpected but significant reduction of alkaline phosphatase, an enzyme normally up-regulated following inflammation. However, no reduction was observed in mice infected with a closely related Gram-negative pneumonic organism (Klebsiella pneumoniae) suggesting the inhibition may be Francisella-specific. In similar fashion to in vivo observations, addition of Francisella lysate to exogenous alkaline phosphatase (tissue-nonspecific isozyme) was inhibitory. Partial purification and subsequent proteomic analysis indicated the inhibitory factor to be the heat shock protein DnaK. Incubation with increasing amounts of anti-DnaK antibody reduced the inhibitory effect in a dose-dependent manner. Furthermore, DnaK contains an adenosine triphosphate binding domain at its N terminus, and addition of adenosine triphosphate enhances dissociation of DnaK with its target protein, e.g. alkaline phosphatase. Addition of adenosine triphosphate resulted in decreased DnaK co-immunoprecipitated with alkaline phosphatase as well as reduction of Francisella-mediated alkaline phosphatase inhibition further supporting the binding of Francisella DnaK to alkaline phosphatase. Release of DnaK via secretion and/or bacterial cell lysis into the extracellular milieu and inhibition of plasma alkaline phosphatase could promote an orchestrated, inflammatory response advantageous to Francisella.     

Effects of Dried Distillers’ Grain on Fecal Prevalence and Growth of Escherichia coli O157 in Batch Culture Fermentations from Cattle

Distillers’ grains (DG), a by-product of ethanol production, are fed to cattle. Associations between Escherichia coli O157 prevalence and feeding of DG were investigated in feedlot cattle (n = 379) given one of three diets: steam-flaked corn (SFC) and 15% corn silage with 0 or 25% dried distillers’ grains (DDG) or SFC with 5% corn silage and 25% DDG. Ten fecal samples were collected from each pen weekly for 12 weeks to isolate E. coli O157. Cattle fed 25% DDG with 5 or 15% silage had a higher (P = 0.01) prevalence of E. coli O157 than cattle fed a diet without DDG. Batch culture ruminal or fecal microbial fermentations were conducted to evaluate the effect of DDG on E. coli O157 growth. The first study utilized microbial inocula from steers fed SFC or dry-rolled corn with 0 or 25% DDG and included their diet as the substrate. Ruminal microbial fermentations from steers fed DDG had higher E. coli O157 contents than ruminal microbial fermentations from steers fed no DDG (P < 0.05) when no substrate was included. Fecal fermentations showed no DDG effect on E. coli O157 growth. In the second study with DDG as a substrate, ruminal fermentations with 0.5 g DDG had higher (P < 0.01) E. coli O157 concentrations at 24 h than ruminal fermentations with 0, 1, or 2 g DDG. In fecal fermentations, 2 g DDG resulted in a higher concentration (P < 0.05) at 24 h than 0, 0.5, or 1 g DDG. The results indicate that there is a positive association between DDG and E. coli O157 in cattle, and the findings should have important ramifications for food safety.     

Biochemical Properties and Crystal Structure of a β-Phenylalanine Aminotransferase from Variovorax paradoxus

By selective enrichment, we isolated a bacterium that can use β-phenylalanine as a sole nitrogen source. It was identified by 16S rRNA gene sequencing as a strain of Variovorax paradoxus. Enzyme assays revealed an aminotransferase activity. Partial genome sequencing and screening of a cosmid DNA library resulted in the identification of a 1,302-bp aminotransferase gene, which encodes a 46,416-Da protein. The gene was cloned and overexpressed in Escherichia coli. The recombinant enzyme was purified and showed a specific activity of 17.5 U mg⁻¹ for (S)-β-phenylalanine at 30°C and 33 U mg⁻¹ at the optimum temperature of 55°C. The β-specific aminotransferase exhibits a broad substrate range, accepting ortho-, meta-, and para-substituted β-phenylalanine derivatives as amino donors and 2-oxoglutarate and pyruvate as amino acceptors. The enzyme is highly enantioselective toward (S)-β-phenylalanine (enantioselectivity [E], >100) and derivatives thereof with different substituents on the phenyl ring, allowing the kinetic resolution of various racemic β-amino acids to yield (R)-β-amino acids with >95% enantiomeric excess (ee). The crystal structures of the holoenzyme and of the enzyme in complex with the inhibitor 2-aminooxyacetate revealed structural similarity to the β-phenylalanine aminotransferase from Mesorhizobium sp. strain LUK. The crystal structure was used to rationalize the stereo- and regioselectivity of V. paradoxus aminotransferase and to define a sequence motif with which new aromatic β-amino acid-converting aminotransferases may be identified.     

Sensitivity of Escherichia coli O157:H7 to Commercially Available Alkaline Cleaners and Subsequent Resistance to Heat and Sanitizers

The effects of seven commercially available alkaline cleaners used in the food processing industry, 0.025 M NaOH, and 0.025 M KOH on viability of wild-type (EDL 933) and rpoS-deficient (FRIK 816-3) strains of Escherichia coli O157:H7 in logarithmic and stationary phases of growth were determined. Cells were treated at 4 or 23°C for 2, 10, or 30 min. Cleaners 2, 4, 6, and 7, which contained hypochlorite and <11% NaOH and/or KOH (pH 11.2 to 11.7), killed significantly higher numbers of cells than treatment with cleaner 3, containing sodium metasilicate (pH 11.4) and <10% KOH, and cleaner 5, containing ethylene glycol monobutyl ether (pH 10.4). There were no differences in the sensitivities of logarithmic and stationary-phase cells to the alkaline cleaners. Treatment with KOH or NaOH (pH 12.2) was not as effective as four out of seven commercial cleaners in killing E. coli O157:H7, indicating that chlorine and other cleaner components have bactericidal activity at high pH. Stationary-phase cells of strain EDL 933 that had been exposed to cleaner 7 at 4 or 23°C and strain FRIK 816-3 exposed to cleaner 7 at 23°C had significantly higher D_55°C (decimal reduction time, minutes at 55°C) values than control cells or cells exposed to cleaner 5, indicating that exposure to cleaner 7 confers cross-protection to heat. Cells of EDL 933 treated with cleaner 7 at 12°C showed significantly higher D_55°C values than cells of FRIK 816-3, indicating that rpoS may play a role in cross-protection. Stationary-phase cells treated with cleaner 5 or cleaner 7 at 4 or 12°C were not cross-protected against subsequent exposure to sanitizers containing quaternary ammonium compounds or sodium hypochlorite, or to cetylpyridinium chloride and benzalkonium chloride.     

Engineering of Deinococcus radiodurans R1 for Bioprecipitation of Uranium from Dilute Nuclear Waste

Genetic engineering of radiation-resistant organisms to recover radionuclides/heavy metals from radioactive wastes is an attractive proposition. We have constructed a Deinococcus radiodurans strain harboring phoN, a gene encoding a nonspecific acid phosphatase, obtained from a local isolate of Salmonella enterica serovar Typhi. The recombinant strain expressed an ~27-kDa active PhoN protein and efficiently precipitated over 90% of the uranium from a 0.8 mM uranyl nitrate solution in 6 h. The engineered strain retained uranium bioprecipitation ability even after exposure to 6 kGy of ⁶⁰Co gamma rays. The PhoN-expressing D. radiodurans offers an effective and eco-friendly in situ approach to biorecovery of uranium from dilute nuclear waste.     

Production of Alkaline Phosphatase from Escherichia coli in Continuous Culture

A 2-liter continuous culture for the production of Escherichia coli rich in alkaline phosphatase is described. The maximal output is greater than 250 mg of enzyme per day. The enzyme yield per unit of bacterial dry weight is only one-fifth of that obtained in earlier investigations. However, because of the increased bacterial density, the output per unit volume of culture is more than four times as great.     

Role of Calf-Adapted Escherichia coli in Maintenance of Antimicrobial Drug Resistance in Dairy Calves

The prevalence of antimicrobial drug-resistant bacteria is typically highest in younger animals, and prevalence is not necessarily related to recent use of antimicrobial drugs. In dairy cattle, we hypothesize that antimicrobial drug-resistant, neonate-adapted bacteria are responsible for the observed high frequencies of resistant Escherichia coli in calves. To explore this issue, we examined the age distribution of antimicrobial drug-resistant E. coli from Holstein cattle at a local dairy and conducted an experiment to determine if low doses of oxytetracycline affected the prevalence of antimicrobial drug-resistant E. coli. Isolates resistant to tetracycline (>4 μg/ml) were more prevalent in <3-month-old calves (79%) compared with lactating cows (14%). In an experimental trial where calves received diets supplemented with or without oxytetracycline, the prevalence of tetracycline-resistant E. coli was slightly higher for the latter group (P = 0.039), indicating that drug use was not required to maintain a high prevalence of resistant E. coli. The most common resistance pattern among calf E. coli isolates included resistance to streptomycin (>12 μg/ml), sulfadiazine (>512 μg/ml), and tetracycline (>4 μg/ml) (SSuT), and this resistance pattern was most prevalent during the period when calves were on milk diets. To determine if prevalence was a function of differential fitness, we orally inoculated animals with nalidixic acid-resistant strains of SSuT E. coli and susceptible E. coli. Shedding of SSuT E. coli was significantly greater than that of susceptible strains in neonatal calves (P < 0.001), whereas there was no difference in older animals (P = 0.5). These data support the hypothesis that active selection for traits linked to the SSuT phenotype are responsible for maintaining drug-resistant E. coli in this population of dairy calves.     

PhoN-expressing, lyophilized, recombinant Deinococcus radiodurans cells for uranium bioprecipitation

Employment of genetically engineered radiation resistant organisms to recover radionuclides/heavy metals from radioactive wastes is an attractive proposition. Cells of recombinant Deinococcus radiodurans strain expressing, a non-specific acid phosphatase encoding phoN gene, were lyophilized. Lyophilized recombinant Deinococcus cells retained viability and PhoN activity and could efficiently precipitate uranium from aqueous solutions for up to six months of storage at room temperature. Batch process for uranium removal using lyophilized cells was more efficient compared to a flow through system, in terms of percent uranium removed, substrate conservation and time taken. Lyophilized recombinant Deinococcus cells exhibited high loading of up to 5.7 g uranium/g dry weight of cells in a batch process at 20 mM input uranium concentration. Lyophilization deflated the cells but did not alter gross cell morphology or surface nucleation capability of cells for uranium precipitation. The precipitated uranyl phosphate remained tightly associated with the cell surface, thus facilitating easy recovery.     

Fate of Escherichia coli during Ensiling of Wheat and Corn

A recombinant Escherichia coli strain carrying a plasmid with an antibiotic resistance marker and expressing the green fluorescent protein was inoculated at a concentration of 3.8 × 10⁸ CFU/g into direct-cut wheat (348 g of dry matter kg⁻¹), wilted wheat (450 g of dry matter kg⁻¹), and corn (375 g of dry matter kg⁻¹). The forages were ensiled in mini-silos. The treatments included control (no E. coli added), application of tagged E. coli, and delayed sealing of the inoculated wheat. Three silos per treatment were sampled on predetermined dates, and the numbers of E. coli were determined on Chromocult TBX medium with or without kanamycin. Colonies presumptively identified as E. coli were also tested for fluorescence activity. Addition of E. coli at the time of ensiling resulted in a more rapid decrease in the pH but had almost no effect on the chemical composition of the final silages or their aerobic stability. E. coli disappeared from the silages when the pH decreased below 5.0. It persisted longer in silages of wilted wheat, in which the pH declined more slowly. Control silages of all crops also contained bacteria, presumptively identified as E. coli, that were resistant to the antibiotic, which suggests that some epiphytic strains are naturally resistant to antibiotics.     

Microbial Community Dynamics and Activity Link to Indigo Production from Indole in Bioaugmented Activated Sludge Systems

Biosynthesis of the popular dyestuff indigo from indole has been comprehensively studied using pure cultures, but less has been done to characterize the indigo production by microbial communities. In our previous studies, a wild strain Comamonas sp. MQ was isolated from activated sludge and the recombinant Escherichia coli _nagAc carrying the naphthalene dioxygenase gene (nag) from strain MQ was constructed, both of which were capable of producing indigo from indole. Herein, three activated sludge systems, G1 (non-augmented control), G2 (augmented with Comamonas sp. MQ), and G3 (augmented with recombinant E. coli _nagAc), were constructed to investigate indigo production. After 132-day operation, G3 produced the highest yields of indigo (99.5 ± 3.0 mg/l), followed by G2 (27.3 ± 1.3 mg/l) and G1 (19.2 ± 1.2 mg/l). The microbial community dynamics and activities associated with indigo production were analyzed by Illumina Miseq sequencing of 16S rRNA gene amplicons. The inoculated strain MQ survived for at least 30 days, whereas E. coli _nagAc was undetectable shortly after inoculation. Quantitative real-time PCR analysis suggested the abundance of naphthalene dioxygenase gene (nagAc) from both inoculated strains was strongly correlated with indigo yields in early stages (0–30 days) (P < 0.001) but not in later stages (30–132 days) (P > 0.10) of operation. Based on detrended correspondence analysis (DCA) and dissimilarity test results, the communities underwent a noticeable shift during the operation. Among the four major genera (> 1% on average), the commonly reported indigo-producing populations Comamonas and Pseudomonas showed no positive relationship with indigo yields (P > 0.05) based on Pearson correlation test, while Alcaligenes and Aquamicrobium, rarely reported for indigo production, were positively correlated with indigo yields (P < 0.05). This study should provide new insights into our understanding of indigo bio-production by microbial communities.     

Spatial Patterns of Alkaline Phosphatase Expression within Bacterial Colonies and Biofilms in Response to Phosphate Starvation

The expression of alkaline phosphatase in response to phosphate starvation was shown to be spatially and temporally heterogeneous in bacterial biofilms and colonies. A commercial alkaline phosphatase substrate that generates a fluorescent, insoluble product was used in conjunction with frozen sectioning techniques to visualize spatial patterns of enzyme expression in both Klebsiella pneumoniae and Pseudomonas aeruginosa biofilms. Some of the expression patterns observed revealed alkaline phosphatase activity at the boundary of the biofilm opposite the place where the staining substrate was delivered, indicating that the enzyme substrate penetrated the biofilm fully. Alkaline phosphatase accumulated linearly with time in K. pneumoniae colonies transferred from high-phosphate medium to low-phosphate medium up to specific activities of 50 μmol per min per mg of protein after 24 h. In K. pneumoniae biofilms and colonies, alkaline phosphatase was initially expressed in the region of the biofilm immediately adjacent to the carbon and energy source (glucose). In time, the region of alkaline phosphatase expression expanded inward until it spanned most, but not all, of the biofilm or colony depth. In contrast, expression of alkaline phosphatase in P. aeruginosa biofilms occurred in a thin, sharply delineated band at the biofilm-bulk fluid interface. In this case, the band of activity never occupied more than approximately one-sixth of the biofilm. These results are consistent with the working hypothesis that alkaline phosphatase expression patterns are primarily controlled by the local availability of either the carbon and energy source or the electron acceptor.     

Halotolerant Cyanobacterium Aphanothece halophytica Contains NapA-Type Na+/H+ Antiporters with Novel Ion Specificity That Are Involved in Salt Tolerance at Alkaline pH

Aphanothece halophytica is a halotolerant alkaliphilic cyanobacterium which can grow at NaCl concentrations up to 3.0 M and at pH values up to 11. The genome sequence revealed that the cyanobacterium Synechocystis sp. strain PCC 6803 contains five putative Na⁺/H⁺ antiporters, two of which are homologous to NhaP of Pseudomonas aeruginosa and three of which are homologous to NapA of Enterococcus hirae. The physiological and functional properties of NapA-type antiporters are largely unknown. One of NapA-type antiporters in Synechocystis sp. strain PCC 6803 has been proposed to be essential for the survival of this organism. In this study, we examined the isolation and characterization of the homologous gene in Aphanothece halophytica. Two genes encoding polypeptides of the same size, designated Ap-napA1-1 and Ap-napA1-2, were isolated. Ap-NapA1-1 exhibited a higher level of homology to the Synechocystis ortholog (Syn-NapA1) than Ap-NapA1-2 exhibited. Ap-NapA1-1, Ap-NapA1-2, and Syn-NapA1 complemented the salt-sensitive phenotypes of an Escherichia coli mutant and exhibited strongly pH-dependent Na⁺/H⁺ and Li⁺/H⁺ exchange activities (the highest activities were at alkaline pH), although the activities of Ap-NapA1-2 were significantly lower than the activities of the other polypeptides. Only one these polypeptides, Ap-NapA1-2, complemented a K⁺ uptake-deficient E. coli mutant and exhibited K⁺ uptake activity. Mutagenesis experiments suggested the importance of Glu129, Asp225, and Asp226 in the putative transmembrane segment and Glu142 in the loop region for the activity. Overexpression of Ap-NapA1-1 in the freshwater cyanobacterium Synechococcus sp. strain PCC 7942 enhanced the salt tolerance of cells, especially at alkaline pH. These findings indicate that A. halophytica has two NapA1-type antiporters which exhibit different ion specificities and play an important role in salt tolerance at alkaline pH.