Elucidation of Enzymatic Function Encoded by Fold and Fic Genes from Dimethylglycine Operon

BIOLOGIJA. 2004. Nr. 2. P. 14 © Lietuvos moksløElucidation akademija, of enzymatic 2004 function encoded by folD and fic genes from dimethylglycine operon 1 © Lietuvos mokslø akademijos leidykla, 2004 Elucidation of enzymatic function encoded by folD and fic genes from dimethylglycine operon V. Èasaitë, Analysis of the dimethylglycine oxidase encoding operon from Arthro- R. Meðkienë, bacter globiformis showed the presence of genes encoding putative 5- formiminotetrahydrofolate cyclodeaminase (fic) and 5,10-methylenetetra- R. Meðkys* hydrofolate dehydrogenase/5,10-methenyltetrahydrofolate cyclohydrolase Department of Molecular Microbiology (folD). Both genes were cloned and expressed in Escherichia coli cells. and Biotechnology, Fic and folD genes were found to encode a monofunctional 5,10-methenyl-THF cyclohydrolase and a monofunctional NADP-dependent 5,10- Institute of Biochemistry, methylene-THF dehydrogenase, respectively. Mokslininkø 12, LT-2600 Vilnius, Lithuania. Key words: dimethylglycine oxidase operon, 5,10-methylenetetrahydrofo- late dehydrogenase, 5,10-methenyltetrahydrofolate cyclohydrolase, Art- hrobacter globiformis INTRODUCTION some organisms functions as a bifunctional protein exhibiting activity of 5,10-methenyl-THF cyclohydro- Glycine betaine is accumulated as a compatible solute lase: in many bacteria [14]. This compound can be used 5,10-methenyl-THF+ + H O ↔ 10-formyl-THF + 2 by microorganisms as a carbon and nitrogen source + H+. [5, 6]. In the first catabolic step betaine is converted The bifunctional form was isolated from Clostri- by betainehomocysteine transmethylase to dimethyl- dium thermoaceticum [16] and Escherichia coli [17]. glycine [7]. Dimethylglycine dehydrogenase (or oxida- In eukaryotes [18, 19] the 5,10-methylene-THF de- se) further converts dimethylglycine into sarcosine, and hydrogenase is the part of a trifunctional enzyme, sarcosine dehydrogenase oxidises sarcosine into glyci- called C1-THF synthase, that also exhibits 10-for- ne [810]. Corynebacterial sarcosine oxidase, as well myl-THF synthetase activity: as mammalian sarcosine dehydrogenase, enzymes that HCOO + MgATP2 + THF ↔ 10-formyl THF exhibit many similarities with dimethylglycine dehyd- + MgADP + HPO2. 4 rogenase, have been shown to bind tetrahydrofolate Folates are supposedly involved in the catabo- (THF) and in the presence of THF form 5,10-methy- lism of glycine betaine, since the genes involved in lene THF as a second product of sarcosine oxidation C1-folate pathways are localized in sarcosine oxida- [11, 12]. 5,10-methylene-THF and 10-formyl-THF are se or dimethylglycine oxidase operons [20, 21]. The important intermediates in cellular pools of one-car- latter operon, cloned from Arthrobacter globiformis, bon units for catabolic and anabolic processes such as contains folD and fic genes localized downstream of purine and thymidine biosynthesis. Interconversion the dimethylglycine oxidase gene (dmg). Judging by between these forms is catalyzed by two enzymes the sequence similarity at the amino acid level, tho- 5,10-methylene-THF dehydrogenase and 5,10-methe- se genes encode putative 5,10-methylene-THF de- nyl-THF cyclohydrolase. The first enzyme participates hydrogenase/5,10-methenyl-THF cyclohydrolase and in the following reaction [13]: 5-formimino-THF cyclohydrolase (or 5,10-methenyl- 5,10-methylene-THF + NAD(P)+ ↔ 5,10-methe- THF cyclohydrolase), respectively [21]. nyl-THF+ + NAD(P)H. The aim of this work was to clone the folD and NAD(P)-dependent activity has been for the first fic genes, express them and elucidate the catalytic time detected in a vertebrate liver preparation [14]. properties of proteins encoded by these genes. Later such monofunctional dehydrogenase was purified from Clostridium cylindrosporum [15]. It has been found that 5,10-methylene-THF dehydrogenase from MATERIALS AND METHODS Cell strains, plasmids and growth conditions. E. * Corresponding author. E-mail: [email protected] coli DH5α and C41(DE3) were used for plasmid 2 V. Èasaitë, R. Meðkienë, R. Meðkys transformation and recombinant protein expression. a total volume of 1 ml at 30 °C. 5,10-Methenyl- Plasmid pEH1 harboring the entire dmg operon was THF cyclohydrolase activity was determined by me- described earlier [21]. Plasmids pTZ18R and pET15b asuring the decrease in absorbance at 356 nm (ε = were used for cloning experiments. Recombinant E. = 24.9 mM1 cm1). The standard assay contained coli strains were grown on nutrient agar (Oxoid) 2 M potassium maleate buffer pH 7.0, 0.1 M potas- containing ampicillin (50 µg/ml) at 37 °C or in a sium hydroxide, 0.4 M 2-mercaptoetanol and 5,10- TB medium (g/l: tryptone 12, yeast extract 24, gly- methenyl-THF in 0.12 N HCl (0.00730.0293 mM). cerol 4) containing 17 mM KH PO , 72 mM The reaction was started by the addition of protein. 2 4 K HPO ) and ampicillin (50 µg/ml) on a rotary sha- The rate of spontaneous hydrolysis of 5,10-methe- 2 4 ker at 30 °C. nyl-THF was considered. One unit is defined as the DNA manipulations. Restriction endonucleases, amount of enzyme that catalyzes the hydrolysis of the linear DNA fragments isolation kit, T4 DNA 1 µmol of 5,10-methenyl-THF per minute [24]. ligase, Taq polimerase were purchased from MBI Assay of 5,10-methylene-THF dehydrogenase ac- Fermentas (Lithuania) and used according to the tivity. The assay was performed in 1 cm cuvettes in manufacturers instructions. Competent cells were a total volume of 1 ml at 30 °C. The enzyme was prepared according to the method of Hanahan [22]. assayed in the direction of NADPH and 5,10-methe- Standard recombinant DNA techniques were em- nyl-THF formation. Both products absorb at 356 nm, ployed for transformation and isolation of plasmid so the combined molar absorption coefficient ε = DNA [23]. Electrophoresis of DNA fragments was = 29.7 mM1 cm1 was used for calculations. The carried out in 1% agarose gels in TAE buffer standard assay mixture contained 1 M potassium ma- (pH8.3). leate, pH 7.0, 12 mM NADP and 3.75 mM 5,10- Subcloning of folD gene. For incorporation of methylen-THF. The reaction was started by the ad- folD into the expression vector pET15b, the gene dition of protein. One unit of enzyme is defined as was amplified by PCR using pEH1 as a DNA tem- the amount that catalyzes the formation of 1 µmol plate. The reaction was carried out using the follo- NADPH and 5,10-methenyl THF per minute [24]. wing primers: FOLPAG 5'-ACGGGAAAGGATCC 5,10-methenyl-THF and 5,10-methylene-THF we- TCTCATGACGGCATC-3 and FOLXHO 5'-ACCA re synthesised from tetrahydrofolate and 5-formyl- GGCTCGAGCGCGGTCCAGTC-3. The sense pri- tetrahydrofolic acid respectively, as described pre- mer was designed with PagI restriction site so that viously [24, 25]. the amplified gene started with the ATG start co- don and the antisense primer was designed with a RESULTS AND DISCUSSION XhoI restriction site. The PCR product (981 bp) was digested with PagI and XhoI and ligated into Expression of the fic and folD genes. The fic and the pET15b expression vector previously digested folD genes were subcloned from pEH1 into pTZ18R with XhoI and NcoI restriction enzymes. The resul- and pET15b vectors resulting in the pMP5 and ting expression plasmid was named pFOL and mai- pFOL plasmids, respectively, by the procedures de- tained in E. coli DH5α. pFOL was then used to scribed in Materials and Methods. Cell-free extracts transform E. coli C41(DE3) for protein expression. of IPTG-induced E. coli (pMP5) and E. coli (pFOL) Subcloning of fic gene. DNA fragment contai- cells were subjected to SDS-PAGE to detect fic and ning fic gene was prepared by digestion of pEH1 folD gene products. In both cases a good expres- plasmid with MbiI and PstI restriction enzymes, se- sion of the recombinant protein was achieved ac- paration by electrophoresis and recovering of the cording to the analysis (Fig. 1). The cell-free extract 0.85 kb DNA fragment. The obtained fragment was of E. coli DH5α harboring the recombinant plas- inserted into pTZ18R, linearized with SmaI and PstI mid pMP5 formed an additional protein band on restriction enzymes. The construct thus obtained was SDS-PAGE in comparison with E. coli (pTZ18R) designated as pMP5 and used to transform E. coli (Figure 1, lanes 1 and 2). The determined molecu- DH5α. lar mass of this band is 22 kDa, which is in agree- Preparation of cell-free extract. Bacterial strains ment with the molecular mass calculated using the grown in TB media were harvested, washed with known sequence of the fic gene. In the cell-free ex- 0.9% NaCl, resuspended in 50 mM Tris-HCl buffer tract of E.coli (pFOL) an additional protein was pH 7.5 and disrupted by sonication. Cell debris was found. The cell-free extract of E. coli (pET15b) removed by centrifugation at 14000 g for 10 min. served as a control. The molecular mass (31 kDa) The resulting cell-free extract was used to assay the of this protein determined by SDS-PAGE is in fair enzymes. agreement with the molecular mass calculated using Assay of 5,10-methenyl-THF cyclohydrolase acti- the nucleotide sequence data of folD gene (Figure, vity. The assay was performed in 1 cm cuvettes in lanes 4 and 5). Elucidation of enzymatic function encoded by folD and fic genes from dimethylglycine operon 3 folD gene was specific for NADP instead of NAD. Most of prokaryotic 5,10-methylene-THF dehydrogenases as well as the mitochondrial enzyme (for example, from Clostridium thermoaceticum, Escheri- chia coli, Photobacterium phosphoreum [27]) are bifunctional and catalyze the cyclohydrolytic cleava- ge of 5,10-methenyl-THF, too. Among the enzymes that have been purified to homogeneity, monofunctional 5,10-methylene-THF dehydrogenases were from Clostridium cylindrosporum [15], Peptostrepto- cocus productus [28], both specific for NADP, and from C. formicoaceticum [24], A. woodii [29], both NAD-dependent.

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