Nucleic Acids Research, 2001, Vol. 29, No. 16 3320-3326
© 2001 Oxford University Press
Initiation of DNA replication by DNA polymerases from primers forming a triple helix
EMI 99, 29 INSERM, Génétique Mitochondriale, Université Victor Segalen Bordeaux 2, 146 Rue Léo Saignat, 33076 Bordeaux Cedex, France and 1UMR 5471 CNRS, Unité de Biophysique Structurale, Université Bordeaux 1, Avenue des Facultés, Bât B8, 33405 Talence, France
Despite extensive studies on oligonucleotide-forming triple helices, which were discovered in 1957, their possible relevance in the initiation of DNA replication remains unknown. Using sequences forming triple helices, we have developed a DNA polymerisation assay by using hairpin DNA templates with a 3' dideoxynucleotide end and an unpaired 5'-end extension to be replicated. The T7 DNA polymerase successfully elongated nucleotides to the expected size of the template from the primers forming triple helices composed of 9–14 deoxyguanosine-rich residues. The triple helix-forming primer required for this reaction has to be oriented parallel to the homologous sequence of the hairpin DNA template. Substitution of the deoxyguanosine residues by N7 deazadeoxyguanosines in the hairpin of the template prevented primer elongation, suggesting that the formation of a triple helix is a prerequisite for primer elongation. Furthermore, DNA sequencing could be achieved with the hairpin template through partial elongation of the third DNA strand forming primer. The T4 DNA polymerase and the Klenow fragment of DNA polymerase I provided similar DNA elongation to the T7 polymerase–thioredoxin complex. On the basis of published crystallographic data, we show that the third DNA strand primer fits within the catalytic centre of the T7 DNA polymerase, thus underlying this new property of several DNA polymerases which may be relevant to genome rearrangements and to the evolution of the genetic apparatus, namely the DNA structure and replication processes.
* To whom correspondence should be addressed. Tel: +33 5 57 57 13 91; Fax: +33 5 57 57 13 91; Email: patrick.lestienne{at}phys-mito.u-bordeaux2.fr
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