Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

---

Query: EC:2.1.1.37 (DNA methyltransferase)
4,983 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

The gene encoding the Neisseria lactamica III DNA methyltransferase (M.NlaIII) which recognizes the sequence CATG has been cloned and expressed in Escherichia coli. DNA sequencing of a 3.125 kb EcoRI-PstI fragment localizes the M. NlaIII gene to a 334 codon open reading frame (ORF) and identifies, 468 bp downstream, a second ORF of 313 amino acids, which is referred to as M.NlaX. Both proteins are detectable in the E. coli coupled in vitro transcription-translation system; they are apparently expressed from separate N. lactamica promoters. The N-terminal half of the previously characterized M.FokI, which methylates adenine in one of the DNA strands with its asymmetric recognition sequence (GGATG), is found to have 41% sequence identity and a further 11.7% sequence similarity with M.NlaIII. Among the conserved amino acids is the wellknown DPPY sequence motif. With one exception, analysis of the nucleotides coding for the DP dipeptide in all known DPPY sequences shows the presence of an inherent DNA adenine methylation (dam) recognition site of GATC. A low level of expression of M.NlaX in E. coli prevents the elucidation of its sequence recognition specificity. Sequence analysis of M.NlaX shows that it is closely related to the group of monospecific 5-methylcytosine DNA methyltransferases (M.EcoRII, Dcm, M.HpaII and M.HhaI) which all have a modified cytosine at the second position of the recognition sequences. Both M.EcoRII and Dcm amino acid sequences are about 50% identical with M.NlaX; a considerable degree of sequence identity is found in the so-called variable region which is believed to be responsible for sequence recognition specificity. M.NlaX is probably the counterpart to the E. coli Dcm in N. lactamica.
...
PMID:Cloning and characterization of two tandemly arranged DNA methyltransferase genes of Neisseria lactamica: an adenine-specific M.NlaIII and a cytosine-type methylase. 227 28

To assess the biological role of DNA methylation at the O6 position of guanine (O6MeG) a human cell line was created that contains a regulatable gene of the O6MeG-DNA methyltransferase (MT), a repair activity that removes O6MeG adducts from the DNA. MT-deficient HeLa MR cells were transformed with an SV40-based expression vector in which the bacterial MT gene ada was put under the control of a glucocorticoid-inducible MMTV promoter. In response to dexamethasone (Dex), pSV MTV ada cells actively accumulated MT protein to reach a constant level after 10-12 h of approximately 15,000 MT molecules per cell. Co-induction by Dex and 12-O-tetradecanoylphorbol-13-acetate (TPA) further accelerated this synthesis approximately 2-fold and, as a result, higher final MT levels were achieved. The inducers were added to exponentially growing cells either before or at the time of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) exposure and the kinetics of MT synthesis was studied. MNNG affected in a dose-dependent manner (i) the loss of the pre-existing MT activity; (ii) the lag before newly synthesized MT appeared; (iii) the final level of MT accumulated by the cells; and (iv) to a lesser extent the rate of MT synthesis. In cells with a down-regulated MT gene (no inducer) even small MNNG doses lead to an irreversible loss of the pre-existing MT activity, i.e. to incomplete repair, whereas an up-regulated MT gene supported the restoration of a pool of active MT molecules in the cells, i.e. an O6MeG repair that has gone to completion. Hence, effective O6MeG repair relies not only on the pre-existing MT level, but depends to an even greater extent on the state of expression of the MT gene. The activity of the MT gene also correlated with cell survival, which confirms our earlier finding that O6MeG adducts are cytotoxic for the cell.
...
PMID:Effect of O6-methylguanine-DNA methyltransferase gene activity on repair in human cells transformed by a regulatable ada gene. 229 24

In a previous communication, we proposed that N-methyl-N'-nitro-N-nitrosoguanidine (MNNG)-induced cytotoxicity in an O6-methylguanine (O6-MeG)-DNA methyltransferase-deficient (MT-) HeLa cell mutant was mainly the consequence of DNA strand breaks resulting from the failure to remove O6-MeG lesions. If MNNG-induced cytotoxicity, DNA strand breaks and O6-MeG lesions are related, there should be a corresponding relationship of these properties in MNNG-resistant clones derived from the MT- strain. A study of such revertants indicated that they were a heterogeneous group with increased repair of DNA strand breaks and O6-MeG lesions and increased resistance to the cytotoxic effects of MNNG. These observations support the hypothesis relating O6-MeG, DNA strand breaks and cytotoxicity. The relationship of these 'revertants' to the MT- and wild-type strains is discussed.
...
PMID:O6-methylguanine (O6-MeG) and cytotoxicity: reversion analysis involving an N-methyl-N'-nitro-N-nitrosoguanidine-sensitive, O6-MeG-DNA methyltransferase-deficient HeLa cell mutant. 233 6

O-6-Alkylguanine is a mutagenic and carcinogenic DNA lesion induced by a variety of alkylating agents, including the chloroethylnitrosoureas. The lesion is repaired by the alkyl-accepting suicide enzyme O-6-methylguanine DNA methyltransferase (MGMT). Approximately 25% of cell lines derived from human tumors are phenotypically deficient in this enzyme and are described as Mer-. Recent cloning of the human MGMT cDNA (Tano, K.; Shiota, S.; Collier, J.; Foote, R.S.; Mitra, S. Proc. Natl. Acad. Sci. USA 87:686-690; 1990) has allowed for a more detailed analysis of the basis of the Mer- phenotype in human Mer- tumor cell lines. Using the polymerase chain reaction (PCR) technique, an MGMT cDNA probe based on the published sequence was generated. The probe and the PCR technique were then used to analyze the presence and expression of the MGMT gene in two Mer+ and four Mer- lines, including one SV40-transformed Mer- line and three Mer- human tumor cell lines. The data demonstrate that while all six cell lines contained a relatively nonamplified, nonrearranged MGMT gene, Mer- lines contained levels of MGMT mRNA detectable only by PCR analysis. Of the three Mer- tumor cell lines examined, two (COLO 320 HSR, A1235) contained MGMT mRNA levels that were four to five orders of magnitude lower than that of the prototype Mer+ tumor line (HT-29), while one (BE) contained no consistently detectable MGMT mRNA. These results suggest that in the human Mer- tumor lines tested, the Mer- phenotype was mediated by a severe reduction in MGMT mRNA levels, despite the presence of the MGMT gene.
...
PMID:Comparison of O-6-methylguanine DNA methyltransferase (MGMT) mRNA levels in Mer+ and Mer- human tumor cell lines containing the MGMT gene by the polymerase chain reaction technique. 236 49

Distinct species differences exist between BDIV rats and Syrian Golden hamsters in the repair of methylated DNA lesions, after single exposures to dimethylnitrosamine (DMN). The promutagenic lesions O6-methylguanine (O6-MeG) and O4-methylthymidine were actively repaired in rat liver; in contrast, in hamster liver the levels of O6-MeG remained relatively stable while O4-methylthymidine levels were reduced. Species differences in the levels of two enzymes involved in the repair of DNA alkylation damage were also noted. An increase in the methylpurine-DNA glycosylase levels was seen in both species following DMN exposure; however, significant species differences in the inactivation and subsequent time course of recovery of the "suicide protein" O6-MeG-DNA methyltransferase were observed. In the rat a rapid recovery of activity began within 24 h of DMN exposure (20 mg/kg) and an approximately 3-fold induction in enzyme levels was observed at 96 h. In hamster liver, in which the constitutive level of expression of this enzyme is similar, no activity was detectable up to 96 h after treatment (25 mg/kg DMN). Only in animals in the lowest treatment group (2.5 mg/kg DMN) was a significant recovery seen, 264 h after treatment. The data presented suggest that the schedule of DMN treatment, in particular the time between doses of the carcinogen and the regeneration of the O6-MeG-DNA methyltransferase, would evoke different carcinogenic responses in hamster and rat liver following chronic exposure to alkylating agents.
...
PMID:Modulation of O6-methylguanine-DNA methyltransferase in rat and hamster liver after treatment with dimethylnitrosamine. 238 47

The Mex- (Mer-) phenotype of human cells is characterised by a sensitivity to agents such as N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) and N-methyl-N-nitrosourea (MNU). The hypersensitivity of Mex- cells is a consequence of their failure to express the DNA-repair enzyme m6-Gua-DNA methyltransferase. Resistance to MNNG and MNU may be acquired by Mex- cells either by reexpression of a methyltransferase function or by an ill-defined process of tolerance in which the cytotoxic potential of m6-Gua is circumvented without the altered base being removed from DNA. It has been suggested that tolerance might involve an altered mismatch correcting function. We have investigated proteins which recognise and bind specifically to DNA fragments containing single-base mismatches. Cell-free extracts of a Burkitt's lymphoma cell line (Raji) contain two such mismatch binding activities. Neither protein appears to have a high affinity for m6-Gua-containing base pairs. The data indicate that m6-Gua-containing base pairs might be poor substrates for mismatch repair processes in human cells.
...
PMID:Mismatch binding proteins and tolerance to alkylating agents in human cells. 239 14

Methylation of cytosine in the DNA inhibits the transcription by RNA polymerase II in higher eukaryotes, but has no influence on RNA polymerase I transcription. The effect on RNA polymerase III was unknown, so far. Two polymerase III genes: a type 1 5S rRNA gene and a type 2 tRNA gene were methylated in vitro with a purified eukaryotic DNA methyltransferase (EC2.1.1.37) and their transcription was analyzed in Xenopus oocytes. The 5S rRNA gene, an oocyte 5S rRNA gene from X. laevis which is subject to developmental inactivation, was not affected by methylation. Conversely, transcription of the tRNA gene was 80% inhibited by methylation with the eukaryotic methyltransferase. HhaI and HpaII methylation left its transcription unaffected.
...
PMID:DNA methylation inhibits transcription by RNA polymerase III of a tRNA gene, but not of a 5S rRNA gene. 240 61

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.
...
PMID:Analysis of substrate specificity of the PaeR7 endonuclease: effect of base methylation on the kinetics of cleavage. 240 35

O6-Methylguanine-DNA methyltransferase (MGMT; DNA-O6-methylguanine:protein-L-cysteine S-methyltransferase, EC 2.1.1.63), a unique DNA repair protein present in most organisms, removes the carcinogenic and mutagenic adduct O6-alkylguanine from DNA by stoichiometrically accepting the alkyl group on a cysteine residue in a suicide reaction. The mammalian protein is highly regulated in both somatic and germ-line cells. In addition, the toxicity of certain alkylating drugs in tumor and normal cells is inversely related to the levels of this protein. The cDNA of the human gene, henceforth named MGMT, has been cloned in an expression vector on the basis of its rescue of a methyltransferase-deficient (ada-) Escherichia coli host. A 22-kDa active methyltransferase encoded entirely by the cDNA contains an amino acid sequence of 61 residues that bears 60-65% similarity with segments of E. coli methyltransferase (products of the ada and ogt genes), which encompass the alkyl-acceptor residues. The human cDNA has no sequence similarity with the ada and ogt genes, due in part to differences in codon usage, and shows no detectable homology with E. coli genomic DNA. However, it hybridizes with distinct restriction fragments of human, mouse, and rat DNAs. The lack of methyltransferase observed in many human cell lines is due to the absence of the MGMT gene or to lack of synthesis and/or stability of its 0.95-kilobase poly(A)+ RNA transcript.
...
PMID:Isolation and structural characterization of a cDNA clone encoding the human DNA repair protein for O6-alkylguanine. 240 87

The levels of DNA methyltransferase in nuclei from 9 tumorigenic and 9 nontumorigenic cell lines were examined. In all but 2 cases, the extractable methyltransferase activity was 4-3000-fold higher in tumorigenic than in nontumorigenic cells. Tumorigenic and nontumorigenic cells from four species were grown in the presence of various concentrations (10(-8)-10(-6) M) of an inhibitor of the methylase enzyme, 5-aza-2'-deoxycytidine (5-aza-dCyd). The reduction of 5-methylcytosine content in newly replicated DNA in the presence of 5-aza-dCyd was used to determine the relative methylase activity in each cell line. In all 4 cases, tumorigenic cells required larger doses of drug to inhibit DNA methylation to the same extent as their nontumorigenic counterparts. The relative rates of incorporation of [3H]5-aza-dCyd were determined for each cell line, and tumorigenic cells were shown to incorporate equal or greater amounts of 5-aza-dCyd into DNA compared to nontumorigenic cells. These results showed that the differences in the inhibition of DNA methylation in response to 5-aza-dCyd were not due to differences in the ability of these cells to incorporate the drug. Thus, it was demonstrated by two independent methods that tumorigenic cells contained higher levels of methylating capacity than nontumorigenic cells. This overabundance of methyltransferase may alter DNA methylation patterns and affect phenotypic stability.
...
PMID:DNA methyltransferase levels in tumorigenic and nontumorigenic cells in culture. 241 16


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>

---