Gene/Protein Disease Symptom Drug Enzyme Compound
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Query: EC:6.5.1.2 (DNA ligase)
 2,749 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

An Indian muntjac cell line, SVM, is unusually sensitive to cell killing induced by a range of alkylating agents. Cells transfected with the Escherichia coli ada gene or human genomic DNA have allowed the response of SVM to alkylating agents to be dissociated into two distinct components. Thus, in SVM, which expresses very low levels of alkyltransferase (AT), O6-alkylguanine appears to be the major cytotoxic, clastogenic, and recombinogenic lesion following exposure to agents such as methylnitrosourea (MNU). However, SVM is also very sensitive to agents such as dimethylsulfate (DMS), which produce only very low levels of O6-methylguanine damage. Sensitivity to DMS resides in an inability to complete base excision repair, with the appearance of persistent single-strand DNA breaks (SSBs), and does not appear to involve defects in glycosylase, apurinic/apyrimidinic endonuclease, or DNA ligase activities. Another, possibly related, phenotypic trait in SVM is its limited ability to ligate transfected linear plasmid DNA. Transfectants of SVM, harboring human DNA sequences, show a significant correction of DMS-induced cytotoxicity and clastogenicity and a reduction in the levels of DMS-induced DNA SSBs. The DMS-resistant transfectants have an increased ability to ligate linear plasmid DNA, and also express AT, making these lines resistant to alkylating agents such as MNU. These results suggest that cells possess a mechanism that regulates AT expression, plasmid break-joining ability, and certain aspects of base excision repair. Transfectants of SVM containing human DNA provide a means to isolate genes involved in a coordinate response to alkylation damage.
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PMID:Characterisation and correction of a mammalian cell mutant defective in late step of base excision repair. 128 51

The ada gene of Escherichia coli encodes O6-methylguanine-DNA methyltransferase, which serves as a positive regulator of the adaptive response to alkylating agents and as a DNA repair enzyme. The gene which can make an ada-deficient strain of E. coli resistant to the cell-killing and mutagenic effects of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) has been cloned from Salmonella typhimurium TA1538. DNA sequence analysis indicated that the gene potentially encoded a protein with a calculated molecular weight of 39,217. Since the nucleotide sequence of the cloned gene shows 70% similarity to the ada gene of E. coli and there is an ada box-like sequence (5'-GAATTAAAACGCA-3') in the promoter region, we tentatively refer to this cloned DNA as the adaST gene. The gene encodes Cys-68 and Cys-320, which are potential acceptor sites for the methyl group from the damaged DNA. The multicopy plasmid carrying the adaST gene significantly reduced the frequency of mutation induced by MNNG both in E. coli and in S. typhimurium. The AdaST protein encoded by the plasmid increased expression of the ada'-lacZ chromosome fusion about 5-fold when an E. coli strain carrying both the fusion operon and the plasmid was exposed to a low concentration of MNNG, whereas the E. coli Ada protein encoded by a low-copy-number plasmid increased it about 40-fold under the same conditions. The low ability of AdaST to function as a positive regulator could account for the apparent lack of an adaptive response to alkylation damage in S. typhimurium.
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PMID:Cloning and characterization of the Salmonella typhimurium ada gene, which encodes O6-methylguanine-DNA methyltransferase. 190 55

The ada gene of Escherichia coli encodes a 39-kDa protein which serves both as a transcriptional activator of the adaptive response to alkylating agents and as a DNA repair enzyme demethylating O6-methyl-guanine and phosphotriester residues. Here, the isolated Ada protein was found to be readily cleaved into two fragments of similar size by treatment with trypsin, chymotrypsin, subtilisin, or V8 protease. The fragments retained their respective methyltransferase activities. The Ada protein is, therefore, comprised of two stable active domains united by a central hinge region of about 10 amino acids. Post-translational modification of the Ada protein by methylation of a specific cysteine residue in the NH2-terminal domain is known to convert it to an efficient transcriptional activator. This residue has now been identified as Cys-69.
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PMID:Functional domains and methyl acceptor sites of the Escherichia coli ada protein. 316 36

The inducible resistance to alkylation mutagenesis and killing in Escherichia coli (the adaptive response) is controlled by the ada gene. The Ada protein acts both as a positive regulator of the response and as a DNA repair enzyme, correcting premutagenic O6-alkylguanine in DNA by suicidal transfer of the alkyl group to one of its own cysteine residues. We have determined the DNA sequence of the cloned ada+ gene and its regulatory region. The data reveal potential sites of ada autoregulation. Amino acid sequence determinations show that the active center for the O6-methylguanine-DNA methyltransferase is located close to the polypeptide COOH terminus and has the unusual sequence -Pro-Cys-His-, preceded by a very hydrophobic region. These same structural features are present at the active site of thymidylate synthase, suggesting a common chemical mechanism for activation of the cysteine.
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PMID:Active site and complete sequence of the suicidal methyltransferase that counters alkylation mutagenesis. 388 9