Both O4-methylthymine and O4-ethylthymine preferentially form alkyl T·G pairs that do not block in vitro replication in a defined sequence

M. K. Dosanjh, P. Menichini, R. Eritja, B. Singer

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Abstract

The mutagenic potential of O4-methylthymine (m4T) and O4-ethylthymine (e4T) was determined by a primer extension assay on a 25mer oligonucleotide containing a single site-specifically incorporated modified thy mine. The e4T-containing oligonucleotide was prepared by using a new synthetic procedure suitable for large alkyl groups on thymine. The second-order rate constants, Kapp m and Vrel max permitted calculation of the frequency of formation and extension of modified base pairs compared to Watson -Crick pairing. With both m4T and e4T, the T-G type pairing was formed at least 10-fold more frequently than the non-mutagenic alkyl T-A pairing. However, there was a small but reproducible preference for m4T·G pairing. In both cases T → C transitions would result. There was no evidence for formation of alkyl T·C or T·T. These data suggest that reported T → A transversions by ethylation are not likely to result from O4-alkylthymine. In contrast to insertion, extension beyond alkylthymine under kinetic conditions did not occur with alkyl T but only with the alkyl T · G termini. For this latter T-G type pairing, the larger ethyl group did not hinder extension compared to that of the methyl group, in the sequence studied. Under non-limiting conditions of dNTP concentration and time, complete replication could be demonstrated for both methyl- and ethyl-containing oligonucleotides. We conclude that the increase in size of the alkyl group from methyl to ethyl does not significantly affect the mutagenic potential and type of mutations of O4-alkylthymine in vitro.

Original languageEnglish
Pages (from-to)1915-1919
Number of pages5
JournalCarcinogenesis
Volume14
Issue number9
Publication statusPublished - Sep 1993

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ASJC Scopus subject areas

  • Cancer Research
  • Statistics, Probability and Uncertainty
  • Applied Mathematics
  • Physiology (medical)
  • Physiology
  • Behavioral Neuroscience

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