Two β-Galactosidases from the Human Isolate Bifidobacterium breve DSM 20213: Molecular Cloning and Expression,Biochemical Characterization and Synthesis of Galacto-Oligosaccharides

Two β-galactosidases, β-gal I and β-gal II, from Bifidobacterium breve DSM 20213, which was isolated from the intestine of an infant, were overexpressed in Escherichia coli with co-expression of the chaperones GroEL/GroES, purified to electrophoretic homogeneity and biochemically characterized. Both β-gal I and β-gal II belong to glycoside hydrolase family 2 and are homodimers with native molecular masses of 220 and 211 kDa, respectively. The optimum pH and temperature for hydrolysis of the two substrates o-nitrophenyl-β-D-galactopyranoside (oNPG) and lactose were determined at pH 7.0 and 50°C for β-gal I, and at pH 6.5 and 55°C for β-gal II, respectively. The k _cat/K _m values for oNPG and lactose hydrolysis are 722 and 7.4 mM⁻¹s⁻¹ for β-gal I, and 543 and 25 mM⁻¹s⁻¹ for β-gal II. Both β-gal I and β-gal II are only moderately inhibited by their reaction products D-galactose and D-glucose. Both enzymes were found to be very well suited for the production of galacto-oligosaccharides with total GOS yields of 33% and 44% of total sugars obtained with β-gal I and β-gal II, respectively. The predominant transgalactosylation products are β-D-Galp-(1→6)-D-Glc (allolactose) and β-D-Galp-(1→3)-D-Lac, accounting together for more than 75% and 65% of the GOS formed by transgalactosylation by β-gal I and β-gal II, respectively, indicating that both enzymes have a propensity to synthesize β-(1→6) and β-(1→3)-linked GOS. The resulting GOS mixtures contained relatively high fractions of allolactose, which results from the fact that glucose is a far better acceptor for galactosyl transfer than galactose and lactose, and intramolecular transgalactosylation contributes significantly to the formation of this disaccharide.     

A New Extracellular β-Galactosidase Producing Kluyveromyces sp. PCH397 from Yak Milk and Its Applications for Lactose Hydrolysis and Prebiotics Synthesis

β-Galactosidase is a crucial glycoside hydrolase enzyme with potential applications in the dairy, food, and pharmaceutical industries. The enzyme is produced in the intracellular environment by bacteria and yeast. The present study reports yeast Kluyveromyces sp. PCH397 isolated from yak milk, which has displayed extracellular β-galactosidase activity in cell-free supernatant through the growth phase. To investigate further, cell counting and methylene blue staining of culture collected at different growth stages were performed and suggested for possible autolysis or cell lysis, thereby releasing enzymes into the extracellular medium. The maximum enzyme production (9.94 ± 2.53U/ml) was achieved at 37 °C in a modified deMan, Rogosa, and Sharpe (MRS) medium supplemented with lactose (1.5%) as a carbon source. The enzyme showed activity at a wide temperature range (4–50 °C), maximum at 50 °C in neutral pH (7.0). In addition to the hydrolysis of lactose (5.0%), crude β-galactosidase also synthesized vital prebiotics (i.e., lactulose and galacto-oligosaccharides (GOS)). Additionally, β-fructofuranosidase (FFase) activity in the culture supernatant ensued the synthesis of a significant prebiotic, fructo-oligosaccharides (FOS). Hence, the unique features such as extracellular enzymes production, efficient lactose hydrolysis, and broad temperature functionality by yeast isolate PCH397 are of industrial relevance. In conclusion, the present study unrevealed for the first time, extracellular production of β-galactosidase from a new yeast source and its applications in milk lactose hydrolysis and synthesis of valuable prebiotics of industrial importance.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12088-021-00955-1.Keyword: β-Galactosidase, Lactulose, Galacto-oligosaccharides, Fructo-oligosaccharides, Milk-microbes

β-Galactosidase (EC 3.2.1.23) hydrolyzes the glycosidic bond in β-galactosides and finds applications in the food industry [1, 2]. The trans-glycosylation property of β-galactosidase (β-gal) is widely used to produce various galactosylated products and prebiotics such as GOS and lactulose [3–7]. The β-gal enzyme is produced intracellularly by many bacteria and yeast, a major constraint for industrial production [1, 8]. Therefore, extracellular β-gal producing bacteria/yeast are of huge relevance. Hence, the present work revealed an efficient extracellular β-gal producing microbe from dairy products of the Indian Himalaya and evaluated its applications in lactose hydrolysis and prebiotics’ synthesis.In this study, twenty milk and four curd samples were collected from the Lahaul and Pangi valleys of Himachal Pradesh, India. The samples were plated on MRS and Elliker agar medium (Himedia, India) for 2–7 days at 28 °C and 37 °C until visible microbial growth. Morphologically distinct isolates were screened for β-gal activity using X-Gal and IPTG plate assay [6, 9]. The positive isolates were screened for β-gal production in liquid MRS medium. The β-gal activity was expressed as U/mg dcw (dry cell weight) for whole cells and U/ml for cell-free supernatant [10, 11]. Yeast isolate PCH397 showing the highest and extracellular enzymatic activity was selected. The culture and reaction conditions for maximum β-gal activity were optimized. FFase activity of whole cells and cell-free supernatant was estimated as described by Lincoln and More [12].The cell-free supernatant (β-gal) was employed for applications in lactose hydrolysis and prebiotic synthesis. The enzyme was incubated with lactose solution (5%, w/v) at 37 °C for lactose hydrolysis followed by thin layer chromatography (TLC) [13] analysis and quantification using the ImageJ program (http://rsbweb.nih.gov/ij/). Further, the cell-free supernatant was incubated with milk at 4 °C for milk lactose hydrolysis. Samples were withdrawn at different time intervals and analyzed for residual lactose concentration using ultra-high performance liquid chromatography-quadrupole-time of flight-ion mobility mass spectrometry (UHPLC-Q-TOF-IMS) [14]. Prebiotic production was carried out by mixing an equal volume of the enzyme with a sugar solution i.e., lactose (40%, w/v) for GOS, and lactose (20%, w/v) + fructose (20%, w/v) for lactulose and FOS production, respectively at 50 °C for 24 h [6]. Samples were analyzed by TLC for GOS, UHPLC-Q-TOF-IMS for FOS and lactulose synthesis.The study resulted in the isolation of 203 morphologically distinct microbes, 62 of which were tested positive for β-gal. Based on quantitative screening, eight isolates showing maximum β-gal activity were selected and examined for the intracellular and extracellular enzymatic activities (Table S1). Yeast isolate PCH397 exhibited maximum extracellular β-gal activity (9.94 ± 2.53 U/ml) along with FFase activity (0.59 ± 0.155) after 48 h of incubation. Isolate PCH397 was identified as Kluyveromyces marxianus by its morphological and molecular characterization (Fig. S1). Phylogenetic tree based on ITS DNA sequence showed similarity (99.63%) with Kluyveromyces marxianus CBS712. To the best of our knowledge, the genus Kluyveromyces has not been reported earlier for extracellular β-gal production. In the past, efforts were made to produce β-gal extracellularly through permeabilization or incorporation of signal peptide to β-gal gene in a fusion construct [15, 16]. The isolate PCH397 was selected due to its generally regarded as safe (GRAS) status and the novel feature of extracellular enzyme production.Highest β-gal activity in the extracellular environment was observed when PCH397 was grown in MRS medium supplemented with 1.5% (w/v) lactose as a substrate and incubated at 37 °C for 48 h (Fig. S2). PCH397 produced extracellular β-gal at lower lactose concentration (1.5%) as compared to various Kluyveromyces spp. [15] where 3% lactose has been used in the growth medium for intracellular β-gal production. Further, whether the extracellular enzyme activity is due to the secretion or cell lysis, the CFU count and cell viability were checked by the methylene blue test. The decreased cell count in the late stationary phase for live cells (Fig. S3) and increased number of methylene blue stained cells indicated cell death (Fig S4). These results suggested that cell lysis in the late stationary phase leads to the secretion of enzymes in extracellular medium. The extracellular production of enzyme would lead to a lower production costs of the enzyme.Cell-free supernatant showed the highest β-gal activity at pH 7.0 in 10 mM sodium phosphate buffer at 50 °C in 5 min (Fig S2). The β-gal enzyme from the current finding holds promise in the sweet whey and milk lactose hydrolysis [1] due to its neutral pH optima. Also, β-gal, which is functional at high temperatures, is used in the synthesis of oligosaccharides [1, 3]. High temperature increases the reaction rate as well as lactose solubility, thus, facilitating transgalactosylation reactions [17]. The β-gal activity (9 U/ml) in cell-free supernatant of PCH397 completely hydrolyzed 5.0% of lactose within 8 h at 37 °C (Fig. 1a, S5a). In a recent study, 5.0% lactose was also hydrolyzed by purified β-gal (5 U/ml) of Paenibacillus barengoltzii CAU904 within 8 h at 40 °C [13]. Under refrigerated conditions (4 °C), the cell free supernatant hydrolyzed ~ 50% milk lactose within 36 h and ~ 80% in 72 h (Fig. 1b, S5b). Since β-gal of PCH397 is active at 4 °C, the enzyme could be utilized to hydrolyze lactose in dairy products under refrigerated conditions. Lactose-free milk products or low-lactose milk products are important dietary constituents for lactose intolerant individuals and deliver essential nutrients to combat nutritional deficiencies [18]. Even with commercially purified enzymes, 100% milk-lactose hydrolysis could not be achieved at a low temperature [19]. However, the crude enzyme from the present investigation can efficiently hydrolyze milk lactose at ambient and refrigerated conditions, reducing the cost associated with enzyme purification. Additionally, the source of enzyme is Kluyveromyces sp. which has GRAS status, therefore, can be used in food applications.Open in a separate windowFig. 1Lactose hydrolysis by crude β-gal of PCH397. a Relative quantification of the hydrolysed products from lactose (5%, w/v) at 37 °C for 24 h. b Relative decrease in lactose concentration (%) at refrigerated conditions obtained by UHPLC-QTOF-IMSFurther, the enzyme was evaluated for its ability to catalyze transgalactosylation reactions at 50 °C. The crude enzyme was incubated with different substrate mixture viz. lactose and fructose. After 8 h of incubation, 50% of lactose was hydrolyzed into glucose, galactose, and GOS (Fig. S6a). Maximum GOS production was achieved after 12 h (Fig. 2a). The purified β-gal from Paenibacillus barengoltzii synthesized GOS from 350 g/L of lactose within 4 h [13]. Though GOS synthesis was faster in comparison to the current study, it is to be noted that we used a crude enzyme mixture instead of a purified enzyme. The crude enzyme has also shown FFase activity (Table S1), and was used for the synthesis of FOS from lactose and fructose mixture. UHPLC-Q-TOF-IMS analysis confirmed the formation of FOS (Fig. 2b). Multiple peaks were observed in the sample containing lactulose, one of which was identical with the peak of lactulose standard (Fig. 2c) as confirmed by HPAEC-PAD (Fig. S6b). The lactulose formation was maximum at 20 h of incubation (Fig. S6c).Open in a separate windowFig. 2Hydrolysis and transgalactosylation of lactose by crude enzyme from PCH397 having β-gal and FFase activity. a Relative quantification of the hydrolyzed and transgalactosylated products. UHPLC-QTOF-IMS detection of prebiotics b FOS and c lactulose with their respective standardIt is the first report of simultaneous co-synthesis of multiple prebiotics i.e., GOS, FOS, and lactulose using a yeast strain. Similar reports for GOS and FOS synthesis have been attempted by enzymatic means from fungal sources in the past [6]. The synthesis of multiple prebiotics is very advantageous. Numerous studies have shown that blended consumption of multiple prebiotics including GOS and FOS has many health benefits [20–24]. The combination of GOS, FOS, and lactulose can be of considerable importance for their prebiotic applications. In conclusion, our findings revealed a yeast source for the cost-effective production of β-galactosidase and a strategy for co-synthesis of valuable prebiotics, which is not reported in the past. The utilization of a yeast source with GRAS status for lactose hydrolysis and co-synthesis of prebiotics promises various health benefits and commercial relevance.     

Heterologous Expression of Lactose- and Galactose-Utilizing Pathways from Lactic Acid Bacteria in Corynebacterium glutamicum for Production of Lysine in Whey

The genetic determinants for lactose utilization from Lactobacillus delbrueckii subsp. bulgaricus ATCC 11842 and galactose utilization from Lactococcus lactis subsp. cremoris MG 1363 were heterologously expressed in the lysine-overproducing strain Corynebacterium glutamicum ATCC 21253. The C. glutamicum strains expressing the lactose permease and β-galactosidase genes of L. delbrueckii subsp. bulgaricus exhibited β-galactosidase activity in excess of 1,000 Miller units/ml of cells and were able to grow in medium in which lactose was the sole carbon source. Similarly, C. glutamicum strains containing the lactococcal aldose-1-epimerase, galactokinase, UDP-glucose-1-P-uridylyltransferase, and UDP-galactose-4-epimerase genes in association with the lactose permease and β-galactosidase genes exhibited β-galactosidase levels in excess of 730 Miller units/ml of cells and were able to grow in medium in which galactose was the sole carbon source. When grown in whey-based medium, the engineered C. glutamicum strain produced lysine at concentrations of up to 2 mg/ml, which represented a 10-fold increase over the results obtained with the lactose- and galactose-negative control, C. glutamicum 21253. Despite their increased catabolic flexibility, however, the modified corynebacteria exhibited slower growth rates and plasmid instability.     

Transport and Metabolism of Lactose, Glucose, and Galactose in Homofermentative Lactobacilli

A number of species of lactobacilli were examined for their ability to ferment both the glucose and galactose moieties of lactose. Lactobacillus helveticus strains metabolized both the glucose and galactose moieties, whereas L. bulgaricus, L. lactis, and L. acidophilus strains metabolized only the glucose moiety and released galactose into the growth medium. All four species tested contained β-galactosidase activity, and no significant phospho-β-galactosidase activity was observed. L. bulgaricus and L. helveticus had a phosphoenolpyruvate (PEP):glucose phosphotransferase system for the uptake of glucose, but no evidence for a PEP:lactose phosphotransferase or PEP:galactose phosphotransferase system was obtained.     

Characterization of Thermoanaerobacter Glucose Isomerase in Relation to Saccharidase Synthesis and Development of Single-Step Processes for Sweetener Production

Regulation of glucose isomerase synthesis was studied in Thermoanaerobacter strain B6A, which fermented a wide variety of carbohydrates including glucose, xylose, lactose, starch, and xylan. Glucogenic amylase activities and β-galactosidase were produced constitutively, whereas the synthesis of glucose isomerase was induced by either xylose or xylan. Production of these saccharidase activities was not significantly repressed by the presence of glucose or 2-deoxyglucose in the growth media. Glucose isomerase production was optimized by controlling the culture pH at 5.5 during xylose fermentation. The apparent temperature and pH optima for these cell-bound saccharidase activities were as follows: glucose isomerase, 80°C, pH 7.0 to 7.5; glucogenic amylase, 70°C, pH 5.0 to 5.5; and β-galactosidase, 60°C, pH 6.0 to 6.5 Glucose isomerase, glucogenic amylase, and β-galactosidase were produced in xylose-grown cells that were active and stable at 60 to 70°C and pH 6.0 to 6.5. Under single-step process conditions, these saccharidase activities in whole cells or cell extracts converted starch or lactose directly into fructose mixtures. A total of 96% of initial liquefied starch was converted into a 49:51 mixture of glucose and fructose, whereas 85% of initial lactose was converted into a 40:31:29 mixture of galactose, glucose, and fructose.     

Structure of Acidic Polysaccharide Produced by Strain No. 626 of Aeromonas hydrophila

Lactobacillus acidophilus strain TK8912 was found to carry six plasmids having molecular masses of 40, 17, 10.5, 5, 2, and 1.8 megadaltons (Mdal). An acriflavin-induced, Lac^- mutant had lost a 17-Mdal plasmid (pLA102) and phospho-β-galactosidase (P-β-gal) activity. Since strain TK1-2TL (Lac⁺ transformant) restored P-β-gal activity, pLA102 is likely to encode a P-β-gal gene required for lactose metabolism in strain TK8912. It is also suggested that the gene for the tagatose 6-phosphate pathway, which is responsible for galactose metabolism, is encoded by the 40-Mdal plasmid pLA101.     

Characterization of a Novel β-Galactosidase from Bifidobacterium adolescentis DSM 20083 Active towards Transgalactooligosaccharides

This paper reports on the effects of both reducing and nonreducing transgalactooligosaccharides (TOS) comprising 2 to 8 residues on the growth of Bifidobacterium adolescentis DSM 20083 and on the production of a novel β-galactosidase (β-Gal II). In cells grown on TOS, in addition to the lactose-degrading β-Gal (β-Gal I), another β-Gal (β-Gal II) was detected and it showed activity towards TOS but not towards lactose. β-Gal II activity was at least 20-fold higher when cells were grown on TOS than when cells were grown on galactose, glucose, and lactose. Subsequently, the enzyme was purified from the cell extract of TOS-grown B. adolescentis by anion-exchange chromatography, adsorption chromatography, and size-exclusion chromatography. β-Gal II has apparent molecular masses of 350 and 89 kDa as judged by size-exclusion chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis, respectively, indicating that the enzyme is active in vivo as a tetramer. β-Gal II had an optimal activity at pH 6 and was not active below pH 5. Its optimum temperature was 35°C. The enzyme showed highest V_max values towards galactooligosaccharides with a low degree of polymerization. This result is in agreement with the observation that during fermentation of TOS, the di- and trisaccharides were fermented first. β-Gal II was active towards β-galactosyl residues that were 1→4, 1→6, 1→3, and 1↔1 linked, signifying its role in the metabolism of galactooligosaccharides by B. adolescentis.     

A Novel Agarolytic β-Galactosidase Acts on Agarooligosaccharides for Complete Hydrolysis of Agarose into Monomers

Marine red macroalgae have emerged to be renewable biomass for the production of chemicals and biofuels, because carbohydrates that form the major component of red macroalgae can be hydrolyzed into fermentable sugars. The main carbohydrate in red algae is agarose, and it is composed of d-galactose and 3,6-anhydro-l-galactose (AHG), which are alternately bonded by β1-4 and α1-3 linkages. In this study, a novel β-galactosidase that can act on agarooligosaccharides (AOSs) to release galactose was discovered in a marine bacterium (Vibrio sp. strain EJY3); the enzyme is annotated as Vibrio sp. EJY3 agarolytic β-galactosidase (VejABG). Unlike the lacZ-encoded β-galactosidase from Escherichia coli, VejABG does not hydrolyze common substrates like lactose and can act only on the galactose moiety at the nonreducing end of AOS. The optimum pH and temperature of VejABG on an agarotriose substrate were 7 and 35°C, respectively. Its catalytic efficiency with agarotriose was also similar to that with agaropentaose or agaroheptaose. Since agarotriose lingers as the unreacted residual oligomer in the currently available saccharification system using β-agarases and acid prehydrolysis, the agarotriose-hydrolyzing capability of this novel β-galactosidase offers an enormous advantage in the saccharification of agarose or agar in red macroalgae for its use as a biomass feedstock for fermentable sugar production.     

IgE Production to α-Gal Is Accompanied by Elevated Levels of Specific IgG1 Antibodies and Low Amounts of IgE to Blood Group B

IgE antibodies to gal-α-1,3-gal-β-1,4-GlcNAc (α-gal) can mediate a novel form of delayed anaphylaxis to red meat. Although IgG antibodies to α-gal (anti-α-gal or anti-Gal) are widely expressed in humans, IgE anti-α-gal is not. We explored the relationship between the IgG and IgE responses to both α-gal and the related blood group B antigen. Contradicting previous reports, antibodies to α-gal were found to be significantly less abundant in individuals with blood group B or AB. Importantly, we established a connection between IgE and IgG responses to α-gal: elevated titers of IgG anti-α-gal were found in IgE-positive subjects. In particular, proportionally more IgG1 anti-α-gal was found in IgE-positive subjects against a background of IgG2 production specific for α-gal. Thus, two types of immune response to α-gal epitopes can be distinguished: a ‘typical’ IgG2 response, presumably in response to gut bacteria, and an ‘atypical’, Th2-like response leading to IgG1 and IgE in addition to IgG2. These results suggest that IgE to a carbohydrate antigen can be formed (probably as part of a glycoprotein or glycolipid) even against a background of bacterial immune stimulation with essentially the same antigen.     

IS1631 Occurrence in Bradyrhizobium japonicum Highly Reiterated Sequence-Possessing Strains with High Copy Numbers of Repeated Sequences RSα and RSβ

From Bradyrhizobium japonicum highly reiterated sequence-possessing (HRS) strains indigenous to Niigata and Tokachi in Japan with high copy numbers of the repeated sequences RSα and RSβ (K. Minamisawa, T. Isawa, Y. Nakatsuka, and N. Ichikawa, Appl. Environ. Microbiol. 64:1845–1851, 1998), several insertion sequence (IS)-like elements were isolated by using the formation of DNA duplexes by denaturation and renaturation of total DNA, followed by treatment with S1 nuclease. Most of these sequences showed structural features of bacterial IS elements, terminal inverted repeats, and homology with known IS elements and transposase genes. HRS and non-HRS strains of B. japonicum differed markedly in the profiles obtained after hybridization with all the elements tested. In particular, HRS strains of B. japonicum contained many copies of IS1631, whereas non-HRS strains completely lacked this element. This association remained true even when many field isolates of B. japonicum were examined. Consequently, IS1631 occurrence was well correlated with B. japonicum HRS strains possessing high copy numbers of the repeated sequence RSα or RSβ. DNA sequence analysis indicated that IS1631 is 2,712 bp long. In addition, IS1631 belongs to the IS21 family, as evidenced by its two open reading frames, which encode putative proteins homologous to IstA and IstB of IS21, and its terminal inverted repeat sequences with multiple short repeats.     

Relationship between the Subunits of Leucoplast Pyruvate Kinase from Ricinus communis and a Comparison with the Enzyme from Other Sources

Two cDNA clones, PK_pα and PK_pβ, for the leucoplast isozyme of pyruvate kinase have been isolated and characterized. A Southern blot of castor (Ricinus communis) DNA probed with PK_pα indicates the presence of a single gene for PK_p. Most (1610 base pairs) of the sequence of both cDNAs is identical. These 1610 base pairs begin with an ATG translation initiation codon, and have 248 base pairs of 3′-untranslated and 1362 base pairs of coding sequence. The sequences of the two clones 5′- to the identical regions are different but both encode peptides with a high percentage of hydrophobic amino acids. The derived sequence of PK_pα encodes eight amino acid residues which have been identified as the amino-terminus of one subunit of PK_p from castor seed leucoplasts when the enzyme is purified in the absence of cysteine endopeptidase inhibitors. The sequence upstream of these amino acids is possibly the transit peptide for this protein. When PK_p is extracted under conditions that eliminate its proteolytic degradation, its α-subunit has a relative molecular weight equal to the full-length coding sequence of PK_pα. The data indicate that the transit peptide for the subunit of leucoplast pyruvate kinase encoded by PK_pα is not cleaved until the protein is released from the plastid. The derived amino acid sequences of PK_pα and PK_pβ are most closely related to Escherichia coli pyruvate kinase. Although the residues involved in substrate binding are conserved in leucoplast pyruvate kinase, there is no phosphorylation site and only 5 of 15 amino acids in the E. coli fructose-1,6-bisphosphate binding site are conserved.     

Cloning of Genes Coding for the Three Subunits of Thiocyanate Hydrolase of Thiobacillus thioparus THI 115 and Their Evolutionary Relationships to Nitrile Hydratase

Thiocyanate hydrolase is a newly found enzyme from Thiobacillus thioparus THI 115 that converts thiocyanate to carbonyl sulfide and ammonia (Y. Katayama, Y. Narahara, Y. Inoue, F. Amano, T. Kanagawa, and H. Kuraishi, J. Biol. Chem. 267:9170–9175, 1992). We have cloned and sequenced the scn genes that encode the three subunits of the enzyme. The scnB, scnA, and scnC genes, arrayed in this order, contained open reading frames encoding sequences of 157, 126, and 243 amino acid residues, respectively, for the β, α, and γ subunits, respectively. Each open reading frame was preceded by a typical Shine-Dalgarno sequence. The deduced amino-terminal peptide sequences for the three subunits were in fair agreement with the chemically determined sequences. The protein molecular mass calculated for each subunit was compatible with that determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. From a computer analysis, thiocyanate hydrolase showed significant homologies to bacterial nitrile hydratases known to convert nitrile to the corresponding amide, which is further hydrolyzed by amidase to form acid and ammonia. The two enzymes were homologous over regions corresponding to almost the entire coding regions of the genes: the β and α subunits of thiocyanate hydrolase were homologous to the amino- and carboxyl-terminal halves of the β subunit of nitrile hydratase, and the γ subunit of thiocyanate hydrolase was homologous to the α subunit of nitrile hydratase. Comparisons of the catalytic properties of the two homologous enzymes support the model for the reaction steps of thiocyanate hydrolase that was previously presented on the basis of biochemical analyses.     

Cold-Adapted β-Galactosidase from the Antarctic Psychrophile Pseudoalteromonas haloplanktis

The β-galactosidase from the Antarctic gram-negative bacterium Pseudoalteromonas haloplanktis TAE 79 was purified to homogeneity. The nucleotide sequence and the NH₂-terminal amino acid sequence of the purified enzyme indicate that the β-galactosidase subunit is composed of 1,038 amino acids with a calculated M_r of 118,068. This β-galactosidase shares structural properties with Escherichia coli β-galactosidase (comparable subunit mass, 51% amino sequence identity, conservation of amino acid residues involved in catalysis, similar optimal pH value, and requirement for divalent metal ions) but is characterized by a higher catalytic efficiency on synthetic and natural substrates and by a shift of apparent optimum activity toward low temperatures and lower thermal stability. The enzyme also differs by a higher pI (7.8) and by specific thermodynamic activation parameters. P. haloplanktis β-galactosidase was expressed in E. coli, and the recombinant enzyme displays properties identical to those of the wild-type enzyme. Heat-induced unfolding monitored by intrinsic fluorescence spectroscopy showed lower melting point values for both P. haloplanktis wild-type and recombinant β-galactosidase compared to the mesophilic enzyme. Assays of lactose hydrolysis in milk demonstrate that P. haloplanktis β-galactosidase can outperform the current commercial β-galactosidase from Kluyveromyces marxianus var. lactis, suggesting that the cold-adapted β-galactosidase could be used to hydrolyze lactose in dairy products processed in refrigerated plants.     

Gemini,a Bifunctional Enzymatic and Fluorescent Reporter of Gene Expression

Background

The development of collections of quantitatively characterized standard biological parts should facilitate the engineering of increasingly complex and novel biological systems. The existing enzymatic and fluorescent reporters that are used to characterize biological part functions exhibit strengths and limitations. Combining both enzymatic and fluorescence activities within a single reporter protein would provide a useful tool for biological part characterization.

Methodology/Principal Findings

Here, we describe the construction and quantitative characterization of Gemini, a fusion between the β-galactosidase (β-gal) α-fragment and the N-terminus of full-length green fluorescent protein (GFP). We show that Gemini exhibits functional β-gal activity, which we assay with plates and fluorometry, and functional GFP activity, which we assay with fluorometry and microscopy. We show that the protein fusion increases the sensitivity of β-gal activity and decreases the sensitivity of GFP.

Conclusions/Significance

Gemini is therefore a bifunctional reporter with a wider dynamic range than the β-gal α-fragment or GFP alone. Gemini enables the characterization of gene expression, screening assays via enzymatic activity, and quantitative single-cell microscopy or FACS via fluorescence activity. The analytical flexibility afforded by Gemini will likely increase the efficiency of research, particularly for screening and characterization of libraries of standard biological parts.     

T. cruzi DNA polymerase beta (Tcpolβ) is phosphorylated in vitro by CK1, CK2 and TcAUK1 leading to the potentiation of its DNA synthesis activity

The unicellular protozoan Trypanosoma cruzi is the causing agent of Chagas disease which affects several millions of people around the world. The components of the cell signaling pathways in this parasite have not been well studied yet, although its genome can encode several components able to transduce the signals, such as protein kinases and phosphatases. In a previous work we have found that DNA polymerase β (Tcpolβ) can be phosphorylated in vivo and this modification activates the synthesis activity of the enzyme. Tcpolβ is kinetoplast-located and is a key enzyme in the DNA base excision repair (BER) system. The polypeptide possesses several consensus phosphorylation sites for several protein kinases, however, a direct phosphorylation of those sites by specific kinases has not been reported yet. Tcpolβ has consensus phosphorylation sites for casein kinase 1 (CK1), casein kinase 2 (CK2) and aurora kinase (AUK). Genes encoding orthologues of those kinases exist in T. cruzi and we were able to identify the genes and to express them to investigate whether or no Tcpolβ could be a substrate for in vitro phosphorylation by those kinases. Both CK1 and TcAUK1 have auto-phosphorylation activities and they are able to phosphorylate Tcpolβ. CK2 cannot perform auto-phosphorylation of its subunits, however, it was able to phosphorylate Tcpolβ. Pharmacological inhibitors used to inhibit the homologous mammalian kinases can also inhibit the activity of T. cruzi kinases, although, at higher concentrations. The phosphorylation events carried out by those kinases can potentiate the DNA polymerase activity of Tcpolβ and it is discussed the role of the phosphorylation on the DNA polymerase and lyase activities of Tcpolβ. Taken altogether, indicates that CK1, CK2 and TcAUK1 can play an in vivo role regulating the function of Tcpolβ.     

Protein Engineering of Epoxide Hydrolase from Agrobacterium radiobacter AD1 for Enhanced Activity and Enantioselective Production of (R)-1-Phenylethane-1,2-Diol

DNA shuffling and saturation mutagenesis of positions F108, L190, I219, D235, and C248 were used to generate variants of the epoxide hydrolase of Agrobacterium radiobacter AD1 (EchA) with enhanced enantioselectivity and activity for styrene oxide and enhanced activity for 1,2-epoxyhexane and epoxypropane. EchA variant I219F has more than fivefold-enhanced enantioselectivity toward racemic styrene oxide, with the enantiomeric ratio value (E value) for the production of (R)-1-phenylethane-1,2-diol increased from 17 for the wild-type enzyme to 91, as well as twofold-improved activity for the production of (R)-1-phenylethane-1,2-diol (1.96 ± 0.09 versus 1.04 ± 0.07 μmol/min/mg for wild-type EchA). Computer modeling indicated that this mutation significantly alters (R)-styrene oxide binding in the active site. Another three variants from EchA active-site engineering, F108L/C248I, I219L/C248I, and F108L/I219L/C248I, also exhibited improved enantioselectivity toward racemic styrene oxide in favor of production of the corresponding diol in the (R) configuration (twofold enhancement in their E values). Variant F108L/I219L/C248I also demonstrated 10-fold- and 2-fold-increased activity on 5 mM epoxypropane (24 ± 2 versus 2.4 ± 0.3 μmol/min/mg for the wild-type enzyme) and 5 mM 1,2-epoxyhexane (5.2 ± 0.5 versus 2.6 ± 0.0 μmol/min/mg for the wild-type enzyme). Both variants L190F (isolated from a DNA shuffling library) and L190Y (created from subsequent saturation mutagenesis) showed significantly enhanced activity for racemic styrene oxide hydrolysis, with 4.8-fold (8.6 ± 0.3 versus 1.8 ± 0.2 μmol/min/mg for the wild-type enzyme) and 2.7-fold (4.8 ± 0.8 versus 1.8 ± 0.2 μmol/min/mg for the wild-type enzyme) improvements, respectively. L190Y also hydrolyzed 1,2-epoxyhexane 2.5 times faster than the wild-type enzyme.     

Pheromone-regulated Genes Required for Yeast Mating Differentiation

Yeast cells mate by an inducible pathway that involves agglutination, mating projection formation, cell fusion, and nuclear fusion. To obtain insight into the mating differentiation of Saccharomyces cerevisiae, we carried out a large-scale transposon tagging screen to identify genes whose expression is regulated by mating pheromone. 91,200 transformants containing random lacZ insertions were screened for β-galactosidase (β-gal) expression in the presence and absence of α factor, and 189 strains containing pheromone-regulated lacZ insertions were identified. Transposon insertion alleles corresponding to 20 genes that are novel or had not previously been known to be pheromone regulated were examined for effects on the mating process. Mutations in four novel genes, FIG1, FIG2, KAR5/ FIG3, and FIG4 were found to cause mating defects. Three of the proteins encoded by these genes, Fig1p, Fig2p, and Fig4p, are dispensible for cell polarization in uniform concentrations of mating pheromone, but are required for normal cell polarization in mating mixtures, conditions that involve cell–cell communication. Fig1p and Fig2p are also important for cell fusion and conjugation bridge shape, respectively. The fourth protein, Kar5p/Fig3p, is required for nuclear fusion. Fig1p and Fig2p are likely to act at the cell surface as Fig1:: β-gal and Fig2::β-gal fusion proteins localize to the periphery of mating cells. Fig4p is a member of a family of eukaryotic proteins that contain a domain homologous to the yeast Sac1p. Our results indicate that a variety of novel genes are expressed specifically during mating differentiation to mediate proper cell morphogenesis, cell fusion, and other steps of the mating process.     

Heterologous Expression of a Bioactive β-Hexosyltransferase,an Enzyme Producer of Prebiotics,from Sporobolomyces singularis

Galacto-oligosaccharides (GOS) are indigestible dietary fibers that are able to reach the lower gastrointestinal tract to be selectively fermented by health-promoting bacteria. In this report, we describe the heterologous expression of an optimized synthetically produced version of the β-hexosyltransferase gene (Bht) from Sporobolomyces singularis. The Bht gene encodes a glycosyl hydrolase (EC 3.2.1.21) that acts as galactosyltransferase, able to catalyze a one-step conversion of lactose to GOS. Expression of the enzyme in Escherichia coli yielded an inactive insoluble protein, while the methylotrophic yeast Pichia pastoris GS115 produced a bioactive β-hexosyltransferase (rBHT). The enzyme exhibited faster kinetics at pHs between 3.5 and 6 and at temperatures between 40 and 50°C. Enzyme stability improved at temperatures lower than 40°C, and glucose was found to be a competitive inhibitor of enzymatic activity. P. pastoris secreted a fraction of the bioactive rBHT into the fermentation broth, while the majority of the enzyme remained associated with the outer membrane. Both the secreted and the membrane-associated forms were able to efficiently convert lactose to GOS. Additionally, resting cells with membrane-bound enzyme converted 90% of the initial lactose into GOS at 68% yield (g/g) (the maximum theoretical is 75%) with no secondary residual (glucose or galactose) products. This is the first report of a bioactive BHT from S. singularis that has been heterologously expressed.