Nucleic Acids Research, Vol 25, Issue 2 297-303, Copyright © 1997 by Oxford University Press
N Baran, L Pucshansky, Y Marco, S Benjamin and H Manor
We describe a novel activity of the SV40 large T-ag helicase, the unwinding
of four stranded DNA structures linked by stacked G-quartets, namely
stacked groups of four guanine bases bound by Hoogsteen hydrogen bonds. The
structures unwound by the helicase were of two types: (i) quadruplexes
comprising four parallel strands that were generated by annealing
oligonucleotides including clustered G residues in a buffer containing
Na+ions. Each parallel quadruplex consisted of four oligonucleotide
molecules. (ii) Complexes comprising two parallel and two antiparallel
strands that were generated by annealing the above oligonucleotides in a
buffer containing K+ions. Each antiparallel complex consisted of two folded
oligonucleotide molecules. Unwinding of these unusual DNA structures by the
T-ag was monitored by gel electrophoresis. The unwinding process required
ATP and at least one single stranded 3'-tail extending beyond the four
stranded region. These data indicated that the T-ag first binds the 3'-tail
and moves in a 3'-->5'direction, using energy provided by ATP
hydrolysis; then it unwinds the four stranded DNA into single strands. This
helicase activity may affect processes such as recombination and telomere
extension, in which four stranded DNA could play a role.
ARTICLES
The SV40 large T-antigen helicase can unwind four stranded DNA structures linked by G-quartets
Department of Biology, Technion-Israel Institute of Technology, Haifa 32,000, Israel.
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