Triplex-Forming Sequences in Regulatory Regions

Triplexes are formed when a triplex-forming oligonucleotide (TFO), typically a short single stranded oligonucleotide recognizes a triplex-forming sequence (TFS) in a duplex DNA by means of specific hydrogen-bond interactions between the central Watson-Crick and the third nucleobase. Two types of triplexes exist: i) parallel triplexes based on the pyrimidine motif and Hoogsteen-type recognition patterns, and ii) antiparallel triplexes based on the purine motif and reverse Hoogsteen recognition patterns (1). The two families of triplex-recognition modes lead to very different helix topology, but in both cases: a) the recognition is specific, and b) the hydrogen bonds are localized on the purine, which implies that triplexes are possible only in poly-purine tracks (1).