Nucleic Acids Research, Vol 24, Issue 22 4506-4512, Copyright © 1996 by Oxford University Press
C Dagneaux, H Gousset, AK Shchyolkina, M Ouali, R Letellier, J Liquier, VL Florentiev and E Taillandier
Intramolecular triple helices have been obtained by folding back twice
oligonucleotides formed by decamers bound by non-nucleotide linkers:
dA10-linker-dA10-linker-dT10 and dA10-linker-dT10-linker-dA10. We have thus
prepared two triple helices with forced third strand orientation,
respectively antiparallel (apA*A-T) and parallel (pA*A-T) with respect to
the adenosine strand of the Watson-Crick duplex. The existence of the
triple helices has been shown by FTIR, UV and fluorescence spectroscopies.
Similar melting temperatures have been obtained in very different oligomer
concentration conditions (micromolar solutions for thermal denaturation
classically followed by UV spectroscopy, milimolar solutions in the case of
melting monitored by FTIR spectroscopy) showing that the triple helices are
intramolecular. The stability of the parallel triplex is found to be
slightly lower than that of the antiparallel (deltaT(m) = 6 degrees C). The
sugar conformations determined by FTIR are different for both triplexes.
Only South-type sugars are found in the antiparallel triplex whereas both
South- and North-type sugars are detected in the parallel triplex. In this
case, thymidine sugars have a South-type geometry, and the adenosine strand
of the Watson-Crick duplex has North-type sugars. For the antiparallel
triplex the experimental results and molecular modeling data are consistent
with a reverse-Hoogsteen like third-strand base pairing and South-type
sugar conformation. An energetically optimized model of the parallel A*A-T
triple helix with a non-uniform distribution of sugar conformations is
discussed.
ARTICLES
Parallel and antiparallel A*A-T intramolecular triple helices
Laboratoire CSSB - URA CNRS 1430, UFR de Sante-Medecine-Biologie- Humaine, Universite Paris XIII, Bobigny, France.
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