title: O-Alkyl deoxythymidines are recognized by DNA polymerase I as deoxythymidine or deoxycytidine.
abstract: The O2- and O4-methyldeoxythymidine triphosphates (O-alkyl dTTP) can be used to substitute for dTTP in Escherichia coli DNA polymerase I (Pol I)-catalysed synthesis of poly[deoxyadenosine-deoxythymidine] (dA-dT). When incorporated into the polynucleotide, no detectable perturbation of structure occurred with even 20% O-methyldeoxythymidine in place of dT. However, on replication of such polymers with Pol I, significant amounts of deoxyguanosine triphosphate (dGTP) were incorporated, as well as high levels of deoxyadenosine triphosphate (dATP), indicating tautomer-like behaviour. Higher homologues, such as O4-ethyl (e4) dTTP or O4-isopropyl (ip4) dTTP, could also replace dTTP, but with lower efficiency. Nevertheless, their presence, like O4-methyl (m4) dT substitutions, caused transitions as well as inhibiting enzyme digestion with a variety of 3' nucleases, particularly to the 3'----5' exonuclease activity (proofreading) of polymerases. Further proof of mutagenicity comes from site-directed experiments placing m4dT or e4dT in place of dT at position 587 in am3 of phi X174, in which all revertants sequenced had A----G transitions. This implies that, since m4dT and e4dT are poorly repaired in eukaryotes, it is likely that they will remain in the DNA and lead to effects on enzyme activity, as well as mutations which contribute to the carcinogenicity of N-nitroso compounds.
authors.affiliation: Donner Laboratory, University of California, Berkeley 94720.
authors.forename: B
authors.initials: B
authors.lastname: Singer
authors.forename: S J
authors.initials: SJ
authors.lastname: Spengler
authors.forename: F
authors.initials: F
authors.lastname: Chavez
authors.forename: J
authors.initials: J
authors.lastname: Sagi
authors.forename: J T
authors.initials: JT
authors.lastname: Kúsmierek
authors.forename: B D
authors.initials: BD
authors.lastname: Preston
authors.forename: L A
authors.initials: LA
authors.lastname: Loeb
mesh_terms: D000478:Alkylation; D004256:DNA Polymerase I / Q000378:metabolism*; D003841:Deoxycytidine / Q000378:metabolism*; D003848:Deoxyguanine Nucleotides / Q000378:metabolism; D004926:Escherichia coli / Q000201:enzymology; D009154:Mutation; D009690:Nucleic Acid Conformation; D011067:Poly dA-dT / Q000378:metabolism; D011108:Polymers; D013329:Structure-Activity Relationship; D013379:Substrate Specificity; D013936:Thymidine / Q000031:analogs & derivatives / Q000378:metabolism*
chemical_list: D003848:Deoxyguanine Nucleotides; D011108:Polymers; D003841:Deoxycytidine; D011067:Poly dA-dT; C008505:O-methylthymidine; C029603:deoxyguanosine triphosphate; D004256:DNA Polymerase I; D013936:Thymidine