FULL NAME: methylphosphotriester (mePT)

Methylation damage can occur on the DNA sugar-phosphate backbone to form MePT. The physical accessibility and negative charge of the phosphate oxygens make them a favorable site for chemical reaction. When double-stranded DNA is treated with MNU, 17% of the total methylation occurs on the backbone to yield MePT. These adducts react with water and other nucleophiles much faster than the common diester form of phosphate linking adjacent nucleosides, leading to facile cleavage of the backbone. Of the two diastereomers formed, only the Sp-MePT is repaired by the Cys-38 residue in the N-terminal domain of Ada. This selective repair results because the oxygen atom on the phosphate in the Sp diastereomer is only 3.5Å away from the acceptor cysteine residue versus 5Å in the Rp configuration. In vivo studies using wild-type Ada and truncated Ada (lacking MePT repair capability) transfected into HeLa cells showed the same extent of resistance to the cytotoxic effects of alkylating agents, similar sister chromatid exchange induction, as well as host-cell reactivation of adenovirus. This observation suggests that MePT may not have cytotoxic effects in cells. The role of MePT seems to be a chemosensor for detection of methylation damage and induction of the adaptive response in E.coli, but their role, if any, in eukaryotes is unknown. The alkyl groups from O6meG and O4meT are transferred to Cys-321 at the C-terminus of Ada E.coli protein, whereas those from a third substrate, methylphosphotriester (MePT), is transferred to the N-terminus of Ada. It was initially believed that the methyl group from MePT was transferred to Cys-69 on the protein but recent evidence identifies Cys-38 as the acceptor residue. Methylation of Cys-38 of Ada converts it to a transcriptional activator of the genes encoding the ‘adaptive response’ to alkylating agents, namely, ada, alkA, alkB and aidB. This is the most nucleophilic of all available cysteine residues in Ada since it is not part of a network of hydrogen bonds. Methylation at this site reduces the overall negative charge on Ada. Reduction in charge density is important for the role of Ada as a transcription factor as it enhances its interaction with negatively charged DNA by 1000-fold. The number of Ada molecules is estimated to rise from one to two molecules in an unadapted state to 3000 molecules in a fully adapted cell. It was initially found that Ada preferentially repairs O6meG as compared with O4-meT but recent evidence suggests that it repairs both lesions with equal efficiency. As discussed earlier, N-terminal domain of Ada has an inherent electrostatic switch that works in a methylation-dependent fashion to modulate its affinity for DNA and ability to act as a transcription activator. There is no known homolog of N-terminal domain of Ada in eukaryotes, thus making the repair of MePT in mammalian cells uncertain.

DAMAGE TYPE: methylation damage

N-methyl-N-nitrosourea (NMU)
alkylating agents

stalled replication fork

direct reversal (DR)

DNA repair protein(s) related to damage:

Last modification date: Sept. 16, 2011