An enzyme of unknown function within the amidohydrolase superfamily was discovered to catalyze the hydrolysis of N-6-substituted adenine derivatives, several of which are cytokinins. Cytokinins are a common type of plant hormone and N-6-substituted adenines are also found as modifications to tRNA. Patl2390, from Pseudoalteromonas atlantica T6c, was shown to hydrolytically deaminate N-6-isopentenyladenine to hypoxanthine and isopentenylamine with a kcat/Km of 1.2 × 107 M−1 s−1. Additional substrates include N-6-benzyl adenine, cis- and trans-zeatin, kinetin, O-6-methylguanine, N-6-butyladenine, N-6-methyladenine, N,N-dimethyladenine, 6-methoxypurine, 6-chloropurine, and 6-thiomethylpurine. This enzyme does not catalyze the deamination of adenine or adenosine. A comparative model of Patl2390 was computed using the three-dimensional crystal structure of Pa0148 (PDB code: 3PAO) as a structural template and docking was used to refine the model to accommodate experimentally identified substrates. This is the first identification of an enzyme that will hydrolyze an N-6 substituted side chain larger than methylamine from adenine.
Adenine deaminase (ADE) catalyzes the conversion of adenine to hypoxanthine and ammonia. The enzyme isolated from Escherichia coli using standard expression conditions was low for the deamination of adenine (kcat = 2.0 s−1; kcat/Km = 2.5 × 103 M−1 s−1). However, when iron was sequestered with a metal chelator and the growth medium was supplemented with Mn2+ prior to induction, the purified enzyme was substantially more active for the deamination of adenine with values of kcat and kcat/Km of 200 s−1 and 5 × 105 M−1s−1, respectively. The apo-enzyme was prepared and reconstituted with Fe2+, Zn2+, or Mn2+. In each case, two enzyme-equivalents of metal were necessary for reconstitution of the deaminase activity. This work provides the first example of any member within the deaminase sub-family of the amidohydrolase superfamily (AHS) to utilize a binuclear metal center for the catalysis of a deamination reaction. [FeII/FeII]-ADE was oxidized to [FeIII/FeIII]-ADE with ferricyanide with inactivation of the deaminase activity. Reducing [FeIII/FeIII]-ADE with dithionite restored the deaminase activity and thus the di-ferrous form of the enzyme is essential for catalytic activity. No evidence for spin-coupling between metal ions was evident by EPR or Mössbauer spectroscopies. The three-dimensional structure of adenine deaminase from Agrobacterium tumefaciens (Atu4426) was determined by X-ray crystallography at 2.2 Å resolution and adenine was modeled into the active site based on homology to other members of the amidohydrolase superfamily. Based on the model of the adenine-ADE complex and subsequent mutagenesis experiments, the roles for each of the highly conserved residues were proposed. Solvent isotope effects, pH rate profiles and solvent viscosity were utilized to propose a chemical reaction mechanism and the identity of the rate limiting steps.
The substrate specificities of two incorrectly annotated enzymes belonging to cog3964 from the amidohydrolase superfamily (AHS) were determined. This group of enzymes is currently misannotated as either dihydroorotase or adenine deaminase. Atu3266 from Agrobacterium tumefaciens C58 and Oant2987 from Ochrobactrum anthropi ATCC 49188 were determined to catalyze the hydrolysis of acetyl-R-mandelate and similar esters with values of kcat/Km that exceed 105 M−1 s−1. These enzymes do not catalyze the deamination of adenine or the hydrolysis of dihydroorotate. Atu3266 was crystallized and the structure determined to a resolution of 2.62 Å. The protein folds as a distorted (β/α)8-barrel and binds two zincs in the active site. The substrate profile was determined via a combination of computational docking to the three-dimensional structure of Atu3266 and screening of a highly focused library of potential substrates. The initial weak hit was the hydrolysis of N-acetyl-D-serine (kcat/Km = 4 M−1s−1). This was followed by the progressive identification of acetyl-R-glycerate (4 × 102 M−1s−1), acetyl glycolate (kcat/Km = 1.3 × 104 M−1 s−1) and ultimately acetyl-R-mandelate (kcat/Km =2.8 × 105 M−1 s−1).
An enzyme from Pseudomonas aeruginosa, Pa0142 (gi|9945972) has been identified for the first time that is able to catalyze the deamination of 8-oxoguanine (8-oxoG) to uric acid. 8-Oxoguanine is formed by the oxidation of guanine residues within DNA by reactive oxygen species and this lesion results in the G:C to T:A transversions. The value of kcat/Km for the deamination of 8-oxoG by Pa0142 at pH 8.0 and 30 °C is 2.0 × 104 M−1 s−1. This enzyme can also catalyze the deamination of isocystosine and guanine at rates that are approximately an order of magnitude slower. The three-dimensional structure of a homologous enzyme (gi|44264246) from the Sargasso Sea has been determined by x-ray diffraction methods to a resolution of 2.2Å (PDB code: 3h4u). The enzyme folds as a (β/α)8− barrel and it is a member of the amidohydrolase superfamily with a single zinc in the active site. This enzyme catalyzes the deamination of 8-oxoG with a value of kcat/Km of 2.7 × 105 M−1 s−1. Computational docking of potential high energy intermediates for the deamination reaction to the x-ray crystal structure suggests that the active site binding of 8-oxoG is facilitated by hydrogen bond interactions from a conserved glutamine that follows β-strand 1 with O6, a conserved tyrosine that follows β-strand 2 with N7, and a conserved cysteine residue that follows β-strand 4 with O8. A bioinformatic analysis of available protein sequences suggest that approximately 200 other bacteria possess an enzyme capable of catalyzing the deamination of 8-oxoG.
Four proteins from NCBI cog1816, previously annotated as adenosine deaminases, have been subjected to structural and functional characterization. Pa0148 (Pseudomonas aeruginosa PAO1), AAur1117 (Arthrobacter aurescens TC1), Sgx9403e, and Sgx9403g, have been purified and their substrate profiles determined. Adenosine is not a substrate for any of these enzymes. All of these proteins will deaminate adenine to produce hypoxanthine with values of kcat/Km that exceed 105 M−1s−1. These enzymes will also accept 6-chloropurine, 6-methoxypurine, N-6-methyladenine, and 2,6-diaminopurine as alternate substrates. X-ray structures of Pa0148 and AAur1117 have been determined and reveal nearly identical distorted (β/α)8-barrels with a single zinc ion that is characteristic of members of the amidohydrolase superfamily. Structures of Pa0148 with adenine, 6-chloropurine and hypoxanthine were also determined thereby permitting identification of the residues responsible for coordinating the substrate and product.
Two previously uncharacterized proteins have been identified that efficiently catalyze the deamination of isoxanthopterin and pterin-6-carboxylate. The genes encoding these two enzymes, NYSGXRC-9339a (gi|44585104) and NYSGXRC-9236b (gi|44611670), were first identified from DNA isolated from the Sargasso Sea as part of the Global Ocean Sampling Project. The genes were synthesized, and the proteins were subsequently expressed and purified. The X-ray structure of Sgx9339a was determined at 2.7 Å resolution (PDB code: 2PAJ). This protein folds as a distorted (β/α)8-barrel and contains a single zinc ion in the active site. These enzymes are members of the amidohydrolase superfamily and belong to cog0402 within the clusters of orthologous groups (COG). Enzymes in cog0402 have previously been shown to catalyze the deamination of guanine, cytosine, S-adenosyl homocysteine, and 8-oxoguanine. A small compound library of pteridines, purines, and pyrimidines was used to probe catalytic activity. The only substrates identified in this search were isoxanthopterin and pterin-6-carboxylate. The kinetic constants for the deamination of isoxanthopterin with Sgx9339a were determined to be 1.0 s−1, 8.0 μM, and 1.3 × 105 M−1 s−1 for kcat, Km, and kcat/Km, respectively. The active site of Sgx9339a most closely resembles the active site for 8-oxoguanine deaminase (PDB code: 2UZ9). A model for substrate recognition of isoxanthopterin by Sgx9339a was proposed based upon the binding of guanine and xanthine in the active site of guanine deaminase. Residues critical for substrate binding appear to be conserved glutamine and tyrosine residues that hydrogen bond with the carbonyl oxygen at C4, a conserved threonine residue that hydrogen bonds with N5, and another conserved threonine residue that hydrogen bonds with the carbonyl group at C7. These conserved active site residues were used to identify 24 other genes which are predicted to deaminate isoxanthopterin.
A substantial challenge for genomic enzymology is the reliable annotation for proteins of unknown function. Described here is an interrogation of uncharacterized enzymes from the amidohydrolase superfamily using a structure-guided approach that integrates bioinformatics, computational biology and molecular enzymology. Previously, Tm0936 from Thermotoga maritima was shown to catalyze the deamination of S-adenosylhomocysteine (SAH) to Sinosylhomocysteine (SIH). Homologues of Tm0936 homologues were identified, and substrate profiles were proposed by docking metabolites to modeled enzyme structures. These enzymes were predicted to deaminate analogues of adenosine including SAH, 5’-methylthioadenosine (MTA), adenosine (Ado), and 5’-deoxyadenosine (5’-dAdo). Fifteen of these proteins were purified to homogeneity and the three-dimensional structures of three proteins were determined by X-ray diffraction methods. Enzyme assays supported the structure-based predictions and identified subgroups of enzymes with the capacity to deaminate various combinations of the adenosine analogues, including the first enzyme (Dvu1825) capable of deaminating 5’-dAdo. One subgroup of proteins, exemplified by Moth1224 from Moorella thermoacetica, deaminates guanine to xanthine and another subgroup, exemplified by Avi5431 from Agrobacterium vitis S4, deaminates two oxidatively damaged forms of adenine: 2-oxoadenine and 8-oxoadenine. The sequence and structural basis of the observed substrate specificities was proposed and the substrate profiles for 834 protein sequences were provisionally annotated. The results highlight the power of a multidisciplinary approach for annotating enzymes of unknown function.
Human alkyladenine-DNA glycosylase (AAG) initiates base excision repair (BER) of alkylated and deaminated bases in DNA. Here, we assessed the mutability of the AAG substrate binding pocket, and the essentiality of individual binding pocket amino acids for survival of methylation damage. We used oligonucleotide-directed mutagenesis to randomize 19 amino acids, 8 of which interact with substrate bases, and created more than 4.5 million variants. We expressed the mutant AAG's in repair-deficient E. coli and selected for protection against the cytotoxicity of either methylmethane sulfonate (MMS) or methyl-lexitropsin (Me-lex), an agent that produces 3-methyladenine as the predominant base lesion. Sequence analysis of 116 methylation-resistant mutants revealed no substitutions for highly conserved Tyr127and His136. In contrast, one mutation, L180F, was greatly enriched in both the MMS- and Me-lex-resistant libraries. Expression of the L180F single mutant conferred 4.4-fold enhanced survival at the high dose of MMS used for selection. The homogeneous L180F mutant enzyme exhibited 2.2-fold reduced excision of 3-methyladenine and 7.3-fold reduced excision of 7-methylguanine from methylated calf thymus DNA. Decreased excision of methylated bases by the mutant glycosylase could promote survival at high MMS concentrations, where the capacity of downstream enzymes to process toxic BER intermediates may be saturated. The mutant also displayed 6.6-, and 3.0-fold reduced excision of 1,N6-ethenoadenine and hypoxanthine from oligonucleotide substrates, respectively, and a 1.7-fold increase in binding to abasic site-containing DNA. Our work provides in vivo evidence for the substrate binding mechanism deduced from crystal structures, illuminates the function of Leu180 in wild-type human AAG, and is consistent with a role for balanced expression of BER enzymes in damage survival.
Alkylating agents; base excision repair; 3-methyladenine; 7-methylguanine; methyl-lexitropsin; random mutagenesis
Melamine toxicity in mammals has been attributed to the blockage of kidney tubules by insoluble complexes of melamine with cyanuric acid or uric acid. Bacteria metabolize melamine via three consecutive deamination reactions to generate cyanuric acid. The second deamination reaction, in which ammeline is the substrate, is common to many bacteria, but the genes and enzymes responsible have not been previously identified. Here, we combined bioinformatics and experimental data to identify guanine deaminase as the enzyme responsible for this biotransformation. The ammeline degradation phenotype was demonstrated in wild-type Escherichia coli and Pseudomonas strains, including E. coli K12 and Pseudomonas putida KT2440. Bioinformatics analysis of these and other genomes led to the hypothesis that the ammeline deaminating enzyme was guanine deaminase. An E. coli guanine deaminase deletion mutant was deficient in ammeline deaminase activity, supporting the role of guanine deaminase in this reaction. Two guanine deaminases from disparate sources (Bradyrhizobium japonicum USDA 110 and Homo sapiens) that had available X-ray structures were purified to homogeneity and shown to catalyze ammeline deamination at rates sufficient to support bacterial growth on ammeline as a sole nitrogen source. In silico models of guanine deaminase active sites showed that ammeline could bind to guanine deaminase in a similar orientation to guanine, with a favorable docking score. Other members of the amidohydrolase superfamily that are not guanine deaminases were assayed in vitro, and none had substantial ammeline deaminase activity. The present study indicated that widespread guanine deaminases have a promiscuous activity allowing them to catalyze a key reaction in the bacterial transformation of melamine to cyanuric acid and potentially contribute to the toxicity of melamine.
A human cDNA coding sequence for a 3-methyladenine-DNA glycosylase was expressed in Escherichia coli. In addition to the full-length 3-methyladenine-DNA glycosylase coding sequence, two other sequences (resulting from differential RNA splicing and the truncated anpg cDNA) derived from that sequence were also expressed. All three proteins were purified to physical homogeneity and their N-terminal amino acid sequences are identical to those predicted by the nucleic acid sequences. The full-length protein has 293 amino acids coding for a protein with a molecular mass of 32 kDa. Polyclonal antibodies against one of the proteins react with the other two proteins, and a murine 3-methyladenine-DNA glycosylase, but not with several other E. coli DNA repair proteins. All three proteins excise 3-methyl-adenine, 7-methylguanine, and 3-methylguanine as well as ethylated bases from DNA. The activities of the proteins with respect to ionic strength (optimum 100 mM KCl), pH (optimum 7.6), and kinetics for 3-methyladenine and 7-methylguanine excision (average values: 3-methyladenine: Km 9 nM and kcat 10 min-1, 7-methylguanine: Km 29 nM and kcat 0.38 min-1) are comparable. In contrast to these results, however, the thermal stability of the full-length and splicing variant proteins at 50 degrees C is less than that of the truncated protein.
Cytosine deaminase (CDA) from E. coli is a member of the amidohydrolase superfamily. The structure of the zinc-activated enzyme was determined in the presence of phosphonocytosine, a mimic of the tetrahedral reaction intermediate. This compound inhibits the deamination of cytosine with a Ki of 52 nM. The zinc and iron containing enzymes were characterized to determine the effect of the divalent cations on activation of the hydrolytic water. Fe-CDA loses activity at low pH with a kinetic pKa of 6.0 and Zn-CDA has a kinetic pKa of 7.3. Mutation of Gln-156 decreased the catalytic activity by more than 5 orders of magnitude, supporting its role in substrate binding. Mutation of Glu-217, Asp-313, and His-246 significantly decreased catalytic activity supporting the role of these three residues in activation of the hydrolytic water molecule and facilitation of proton transfer reactions. A library of potential substrates was used to probe the structural determinants responsible for catalytic activity. CDA was able to catalyze the deamination of isocytosine and the hydrolysis of 3-oxauracil. Large inverse solvent isotope effects were obtained on kcat and kcat/Km, consistent with the formation of a low-barrier hydrogen bond during the conversion of cytosine to uracil. A chemical mechanism for substrate deamination by CDA was proposed.
An enzyme of unknown function within the amidohydrolase superfamily was discovered to catalyze the hydrolysis of the universal second messenger, cyclic-3’, 5’-adenosine monophosphate (cAMP). The enzyme, which we have named CadD, is encoded by the human pathogenic bacterium Leptospira interrogans. Although CadD is annotated as an adenosine deaminase, the protein specifically deaminates cAMP to cyclic-3’, 5’-inosine monophosphate (cIMP) with a kcat/Km of 2.7 ± 0.4 × 105 M−1 s−1 and has no activity on adenosine, adenine, or 5’-adenosine monophosphate (AMP). This is the first identification of a deaminase specific for cAMP. Expression of CadD in Escherichia coli mimics the loss of adenylate cyclase in that it blocks growth on carbon sources that require the cAMP-CRP transcriptional activator complex for expression of the cognate genes. The cIMP reaction product cannot replace cAMP as the ligand for CRP binding to DNA in vitro and cIMP is a very poor competitor of cAMP activation of CRP for DNA binding. Transcriptional analyses indicate that CadD expression represses expression of several cAMP-CRP dependent genes. CadD adds a new activity to the cAMP metabolic network and may be a useful tool in intracellular study of cAMP-dependent processes.
DNA methylation is an epigenetic mechanism involved in many biological functions in prokaryotes and eukaryotes. Contrary to mammalian DNA, which is thought to contain only 5-methylcytosine (m5C), bacterial DNA contains two additional methylated bases, namely N6-methyladenine (m6A), and a more recently discovered minor base N4-methylcytosine (m4C). These modified bases are involved in the protection of bacterial DNA from the action of specific endonucleases via the host-specific restriction-modification system which is regarded as a defense mechanism against bacteriophage infection. However, if the main function of m5C and m4C in bacteria is the protection against restriction enzymes, the roles of m6A are multiple and include for example the regulation of virulence and the control of many bacterial DNA functions such as the replication, repair, expression and transposition of DNA. Hence, in regard to the multiple roles of m6A in bacteria, and to the well known tendency for m5C to deaminate in thymine, the selection of the mutagenic m5C instead of m6A in mammals as the only methylated base may seem surprising. However, even if adenine methylation is usually considered as a bacterial DNA feature, the presence of m6A is not restricted to prokaryotic DNA since this methylated base has been found in protist and plant DNAs. Furthermore, indirect evidence suggests the presence of m6A in mammal DNA, raising the possibility that this base has remained undetected due to the low sensitivity of the analytical methods used. This points to the importance to consider m6A as the sixth element of DNA.
Adenine; analogs & derivatives; classification; metabolism; Animals; DNA; chemistry; DNA Methylation; Eukaryotic Cells; Phylogeny; DNA methylation; epigenetics; N6-methyladenine; 5-methylcytosine
The Escherichia coli 3-methyladenine DNA glycosylase II protein (AlkA) recognizes a broad range of oxidized and alkylated base lesions and catalyzes the hydrolysis of the N-glycosidic bond to initiate the base excision repair pathway. Although the enzyme was one of the first DNA repair glycosylases to be discovered more than 25 years ago, and there are multiple crystal structures, the mechanism is poorly understood. Therefore, we have characterized the kinetic mechanism for the AlkA-catalyzed excision of the deaminated purine, hypoxanthine. The multiple turnover glycosylase assays are consistent with Michaelis-Menten kinetics. However, under single turnover conditions that are commonly employed to study other DNA glycosylases, we observe an unusual biphasic protein saturation curve. Initially the observed rate constant for excision increases with increasing AlkA protein, but at higher concentrations of protein the rate constant decreases. This behavior can be most easily explained by tight binding to DNA ends and by crowding of multiple AlkA protamers on the DNA. Consistent with this model, crystal structures have shown the preferential binding of AlkA to DNA ends. By varying the position of the lesion, we identified an asymmetric substrate that does not show inhibition at higher concentrations of AlkA and we performed pre-steady state and steady state kinetic analysis. Unlike other glycosylases, release of the abasic product is faster than N-glycosidic bond cleavage. Nevertheless, AlkA exhibits significant product inhibition under multiple-turnover conditions, and it binds approximately 10-fold more tightly to an abasic site than to a hypoxanthine lesion site. This tight binding could help protect abasic sites when the adaptive response to DNA alkylation is activated and very high levels of AlkA protein are present.
The mom gene of bacteriophage Mu encodes an enzyme that converts adenine to N6-(1-acetamido)-adenine in the phage DNA and thereby protects the viral genome from cleavage by a wide variety of restriction endonucleases. Mu-like prophage sequences present in Haemophilus influenzae Rd (FluMu), Neisseria meningitidis type A strain Z2491 (Pnme1) and H. influenzae biotype aegyptius ATCC 11116 do not possess a Mom-encoding gene. Instead, at the position occupied by mom in Mu they carry an unrelated gene that encodes a protein with homology to DNA adenine N6-methyltransferases (hin1523, nma1821, hia5, respectively). Products of the hin1523, hia5 and nma1821 genes modify adenine residues to N6-methyladenine, both in vitro and in vivo. All of these enzymes catalyzed extensive DNA methylation; most notably the Hia5 protein caused the methylation of 61% of the adenines in λ DNA. Kinetic analysis of oligonucleotide methylation suggests that all adenine residues in DNA, with the possible exception of poly(A)-tracts, constitute substrates for the Hia5 and Hin1523 enzymes. Their potential ‘sequence specificity’ could be summarized as AB or BA (where B = C, G or T). Plasmid DNA isolated from Escherichia coli cells overexpressing these novel DNA methyltransferases was resistant to cleavage by many restriction enzymes sensitive to adenine methylation.
The structure of 3-methyladenine DNA glycosylase I in complex with 3-methyladenine is reported.
The removal of chemically damaged DNA bases such as 3-methyladenine (3-MeA) is an essential process in all living organisms and is catalyzed by the enzyme 3-MeA DNA glycosylase I. A key question is how the enzyme selectively recognizes the alkylated 3-MeA over the much more abundant adenine. The crystal structures of native and Y16F-mutant 3-MeA DNA glycosylase I from Staphylococcus aureus in complex with 3-MeA are reported to 1.8 and 2.2 Å resolution, respectively. Isothermal titration calorimetry shows that protonation of 3-MeA decreases its binding affinity, confirming previous fluorescence studies that show that charge–charge recognition is not critical for the selection of 3-MeA over adenine. It is hypothesized that the hydrogen-bonding pattern of Glu38 and Tyr16 of 3-MeA DNA glycosylase I with a particular tautomer unique to 3-MeA contributes to recognition and selection.
3-methyladenine DNA glycosylase I; fluorescence measurements; ITC; DNA repair; recognition
Cytosine deaminase (CDA) from Escherichia coli was shown to catalyze the deamination of isoguanine (2-oxoadenine) to xanthine. Isoguanine is an oxidation product of adenine in DNA that is mutagenic to the cell. The isoguanine deaminase activity in E. coli was partially purified by ammonium sulfate fractionation, gel filtration and anion exchange chromatography. The active protein was identified by peptide mass fingerprint analysis as cytosine deaminase. The kinetic constants for the deamination of isoguanine at pH 7.7 are kcat = 49 s-1, Km = 72 μM, and kcat/Km = 6.7 × 105 M-1 s-1. The kinetic constant for the deamination of cytosine are kcat = 45 s-1, Km = 302 μM, and kcat/Km = 1.5 × 105 M-1 s-1. Under these reaction conditions isoguanine is the better substrate for cytosine deaminase. The three dimensional structure of CDA was determined with isoguanine in the active site.
orphan enzymes; isoguanine deaminase
Hydroxyatrazine [2-(N-ethylamino)-4-hydroxy-6-(N-isopropylamino)-1,3,5-triazine] N-ethylaminohydrolase (AtzB) is the sole enzyme known to catalyze the hydrolytic conversion of hydroxyatrazine to N-isopropylammelide. AtzB, therefore, serves as the point of intersection of multiple s-triazine biodegradative pathways and is completely essential for microbial growth on s-triazine herbicides. Here, atzB was cloned from Pseudomonas sp. strain ADP and its product was purified to homogeneity and characterized. AtzB was found to be dimeric, with subunit and holoenzyme molecular masses of 52 kDa and 105 kDa, respectively. The kcat and Km of AtzB with hydroxyatrazine as a substrate were 3 s−1 and 20 μM, respectively. Purified AtzB had a 1:1 zinc-to-subunit stoichiometry. Sequence analysis revealed that AtzB contained the conserved mononuclear amidohydrolase superfamily active-site residues His74, His76, His245, Glu248, His280, and Asp331. An intensive in vitro investigation into the substrate specificity of AtzB revealed that 20 of the 51 compounds tested were substrates for AtzB; this allowed for the identification of specific substrate structural features required for catalysis. Substrates required a monohydroxylated s-triazine ring with a minimum of one primary or secondary amine substituent and either a chloride or amine leaving group. AtzB catalyzed both deamination and dechlorination reactions with rates within a range of one order of magnitude. This differs from AtzA and TrzN, which do not catalyze deamination reactions, and AtzC, which is not known to catalyze dechlorination reactions.
In murine cells expressing the PaeR7 endonuclease and methylase genes, the recognition sites (CTCGAG) of these enzymes can be methylated at the adenine residue by the PaeR7 methylase and at the internal cytosine by the mouse DNA methyltransferase. Using nonadecameric duplex deoxyoligonucleotide substrates, the specificity of the PaeR7 endonuclease for unmethylated, hemi-methylated, and fully methylated N6-methyladenine (m6A) and C5-methylcytosine (m5C) versions of these substrates has been studied. The Km, Kcat, and Ki values for these model substrates have been measured and suggest that fully or hemi-m6A-methylated PaeR7 sites in the murine genome are completely protected. However, the reactivity of fully or hemi-m5C-methylated PaeR7 sites is depressed 2900- and 100-fold respectively, compared to unmodified PaeR7 sites. The implications of the kinetic constants of the PaeR7 endonuclease for these methylated recognition sites as they occur in murine cells expressing this endonuclease gene are discussed.
We have purified 3-methyladenine DNA glycosylase I from Escherichia coli to apparent physical homogeneity. The enzyme preparation produced a single band of Mr 22,500 upon sodium dodecyl sulphate/polyacrylamide gel electrophoresis in good agreement with the molecular weight deduced from the nucleotide sequence of the tag gene (Steinum, A.-L. and Seeberg, E. (1986) Nucl. Acids Res. 14, 3763-3772). HPLC confirmed that the only detectable alkylation product released from (3H)dimethyl sulphate treated DNA was 3-methyladenine. The DNA glycosylase activity showed a broad pH optimum between 6 and 8.5, and no activity below pH 5 and above pH 10. MgSO4, CaCl2 and MnCl2 stimulated enzyme activity, whereas ZnSO4 and FeCl3 inhibited the enzyme at 2 mM concentration. The enzyme was stimulated by caffeine, adenine and 3-methylguanine, and inhibited by p-hydroxymercuribenzoate, N-ethylmaleimide and 3-methyladenine. The enzyme showed no detectable endonuclease activity on native, depurinated or alkylated plasmid DNA. However, apurinic sites were introduced in alkylated DNA as judged from the strand breaks formed by mixtures of the tag enzyme and the bacteriophage T4 denV enzyme which has apurinic/apyrimidinic endonuclease activity. It was calculated that wild-type E. coli contains approximately 200 molecules per cell of 3-methyladenine DNA glycosylase I.
We have determined the nature of the deoxyribonucleic acid (DNA) modification governed by the SA host specificity system of Salmonella typhimurium. Two lines of evidence indicate that SA modification is based on methylation of DNA-adenine residues. (i) The SA+ locus of Salmonella was transferred into Escherichia coli B, a strain that does not contain 5-methylcytosine in its DNA; although the hybrid strain was able to confer SA modification, its DNA still did not contain 5-methylcytosine. (ii) the N6-methyladenine content of phage L DNA was measured after growth in various host strains; phage lacking SA modification contained fewer N6-methyladenine residues per DNA. We also investigated the possibility, suggested by others (32), that SA modification protects phage DNA against restriction by the RII host specificity system. Phages lambda, P3, and L were grown in various SA+ and SA- hosts and tested for their relative plating ability on strains containing or lacking RII restriction; the presence or absence of SA modification had no effect on RII restriation. In vitro studies revealed, however, that Salmonella DNA is protected against cleavage by purified RII restriction endonuclease (R-EcoRII). This protection is not dependent on SA modification; rather, it appears to be due to methylation by a DNA-cytosine methylase which has overlapping specificity with the RII modification enzyme, but which is not involved in any other known host specificity system.
Proteins of unknown function belonging to cog1816 and cog0402 were characterized. Sav2595 from Steptomyces avermitilis MA-4680, Acel0264 from Acidothermus cellulolyticus 11B, Nis0429 from Nitratiruptor sp. SB155-2 and Dr0824 from Deinococcus radiodurans R1 were cloned, purified, and their substrate profiles determined. These enzymes were previously incorrectly annotated as adenosine deaminases or chlorohydrolases. It was shown here that these enzymes actually deaminate 6-aminodeoxyfutalosine. The deamination of 6-aminodeoxyfutalosine is part of an alternative menaquinone biosynthetic pathway that involves the formation of futalosine. 6-Aminodeoxyfutalosine is deaminated by these enzymes with catalytic efficiencies greater than 105 M−1 s−1, Km values of 0.9 to 6.0 μM and kcat values of 1.2 to 8.6 s−1. Adenosine, 2′-deoxyadenosine, thiomethyladenosine, and S-adenosylhomocysteine are deaminated at least an order of magnitude slower than 6-aminodeoxyfutalosine. The crystal structure of Nis0429 was determined and the substrate, 6-aminodeoxyfutalosine, was positioned in the active site, based on the presence of adventitiously bound benzoic acid. In this model Ser-145 interacts with the carboxylate moiety of the substrate. The structure of Dr0824 was also determined, but a collapsed active site pocket prevented docking of substrates. A computational model of Sav2595 was built based on the crystal structure of adenosine deaminase and substrates were docked. The model predicted a conserved arginine after β-strand 1 to be partially responsible for the substrate specificity of Sav2595.
Two enzymes of unknown function from the cog1735 subset of the amidohydrolase superfamily (AHS), LMOf2365_2620 (Lmo2620) from Listeria monocytogenes str. 4b F2365 and Bh0225 from Bacillus halodurans C-125, were cloned, expressed and purified to homogeneity. The catalytic functions of these two enzymes were interrogated by an integrated strategy encompassing bioinformatics, computational docking to three-dimensional crystal structures, and library screening. The three-dimensional structure of Lmo2620 was determined at a resolution of 1.6 Å with two phosphates and a binuclear zinc center in the active site. The proximal phosphate bridges the binuclear metal center and is 7.1 Å away from the distal phosphate. The distal phosphate hydrogen bonds with Lys-242, Lys-244, Arg-275 and Tyr-278. Enzymes within cog1735 of the AHS have previously been shown to catalyze the hydrolysis of substituted lactones. Computational docking of the high energy intermediate (HEI) form of the KEGG database to the three-dimensional structure of Lmo2620 highly enriched anionic lactones versus other candidate substrates. The active site structure and the computational docking results suggested that probable substrates would likely include phosphorylated sugar lactones. A small library of diacid sugar lactones and phosphorylated sugar lactones was synthesized and tested for substrate activity with Lmo2620 and Bh0225. Two substrates were identified for these enzymes, d-lyxono-1,4-lactone-5-phosphate and l-ribono-1,4-lactone-5-phosphate. The kcat/Km values for the cobalt-substituted enzymes with these substrates are ~105 M−1 s−1.
MmeI from Methylophilus methylotrophus belongs to the type II restriction-modification enzymes. It recognizes an asymmetric DNA sequence, 5′-TCCRAC-3′ (R indicates G or A), and cuts both strands at fixed positions downstream of the specific site. This particular feature has been exploited in transcript profiling of complex genomes (using serial analysis of gene expression technology). We have shown previously that the endonucleolytic activity of MmeI is strongly dependent on the presence of S-adenosyl-l-methionine (J. Nakonieczna, J. W. Zmijewski, B. Banecki, and A. J. Podhajska, Mol. Biotechnol. 37:127-135, 2007), which puts MmeI in subtype IIG. The same cofactor is used by MmeI as a methyl group donor for modification of an adenine in the upper strand of the recognition site to N6-methyladenine. Both enzymatic activities reside in a single polypeptide (919 amino acids [aa]), which puts MmeI also in subtype IIC of the restriction-modification systems. Based on a molecular model, generated with the use of bioinformatic tools and validated by site-directed mutagenesis, we were able to localize three functional domains in the structure of the MmeI enzyme: (i) the N-terminal portion containing the endonucleolytic domain with the catalytic Mg2+-binding motif D70-X9-EXK82, characteristic for the PD-(D/E)XK superfamily of nucleases; (ii) a central portion (aa 310 to 610) containing nine sequence motifs conserved among N6-adenine γ-class DNA methyltransferases; (iii) the C-terminal portion (aa 610 to 919) containing a putative target recognition domain. Interestingly, all three domains showed highest similarity to the corresponding elements of type I enzymes rather than to classical type II enzymes. We have found that MmeI variants deficient in restriction activity (D70A, E80A, and K82A) can bind and methylate specific nucleotide sequence. This suggests that domains of MmeI responsible for DNA restriction and modification can act independently. Moreover, we have shown that a single amino acid residue substitution within the putative target recognition domain (S807A) resulted in a MmeI variant with a higher endonucleolytic activity than the wild-type enzyme.
We have investigated the occurrence of methylated adenine residues in the macronuclear ribosomal RNA genes of Tetrahymena thermophila. It has been shown previously that macronuclear DNA, including the palindromic ribosomal RNA genes (rDNA), of Tetrahymena thermophila contains the modified base N-6-methyladenine, but no 5-methylcytosine. Purified rDNA was digested with restriction enzymes Sau 3AI, MboI and DpnI to map the positions and levels of N-6-methyladenine in the sequence 5' GATC 3'. A specific pattern of doubly methylated GATC sequences was found; hemimethylated sites were not detected. The patterns and levels of methylation of these sites did not change significantly in different physiological states. A molecular form of the rDNA found in the newly developing macronucleus and for several generations following the sexual process, conjugation, contained no detectably methylated GATC sites. However, both the bulk macronuclear DNA and palindromic rDNA from the same macronuclei were methylated. Possible roles for N-6-methyladenine in macronuclear DNA are discussed in light of these findings.