Strand-invasion of duplex DNA by peptide nucleic acid oligomers.

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Abstract
Polyamide oligomers, termed peptide nucleic acids (PNAs), bind with high affinity to both DNA and RNA and offer both antisense and antigene approaches for regulating gene expression. When a PNA binds to a complementary sequence in a double-stranded DNA, one strand of the duplex is displaced, and a stable D-loop is formed. Unlike oligodeoxynucleotides for which binding polarity is determined by the deoxyribose sugar, the unrestrained polyamide backbone of the PNA could permit binding to a DNA target in an orientation-independent manner. We now provide evidence that PNAs can, in fact, bind to their complementary sequence in DNA independent of the DNA-strand polarity--that is, a PNA binds to DNA in both "parallel" and "antiparallel" fashion. With a mixed-sequence 15-mer PNA, kinetic studies of PNA.DNA interactions revealed that D-loop formation was rapid and the complex was stable for several hours. However, when measured either by gel-mobility-shift analysis or RNA polymerase II-elongation termination, D-loop formation was salt dependent, but PNA-strand dissociation was not salt dependent. We observed that D-loop-containing DNA fragments had anomalous gel mobilities that varied as a function of the position of the D-loop relative to the DNA termini. On the basis of permutation analysis, the decreased mobility of the PNA.DNA complex was attributed to a bend in the DNA at or near the D-loop.