Nucleic Acids Research, Vol 24, Issue 9 1702-1709, Copyright © 1996 by Oxford University Press
C Pfannschmidt, A Schaper, G Heim, TM Jovin and J Langowski
Site-specific labeling of covalently closed circular DNA was achieved by
using triple helix-forming oligonucleotides 10, 11 and 27 nt in length. The
sequences consisted exclusively of pyrimidines (C and T) with a reactive
psoralen at the 5'-end and a biotin at the 3'-end. The probes were directed
to different target sites on the plasmids pUC18 (2686 bp), pUC18/4A (2799
bp) and pUC1 8/4A-H 1 (2530 bp). After triple helix formation at acid pH
the oligonucleotides were photocrosslinked to the target DNAs via the
psoralen moiety, endowing the covalent adduct with unconditional stability,
e.g. under conditions unfavorable for preservation of the triplex, such as
neutral pH. Complex formation was monitored after polyacrylamide gel
electrophoresis by streptavidin- alkaline phosphatase (SAP)-induced
chemiluminescence. The yield of triple helix increased with the molar ratio
of oligonucleotide to target and the length of the probe sequence (27mer
> 11mer). The covalent adduct DNA were visualized by scanning force
microscopy (SFM) using avidin or streptavidin as protein tags for the
biotin group on the oligonucleotide probes. We discuss the versatility of
triple helix DNA complexes for studying the conformation of superhelical
DNA.
ARTICLES
Sequence-specific labeling of superhelical DNA by triple helix formation and psoralen crosslinking
German Cancer Research Center, Heidelberg, Germany.
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