Three-stranded (triplex) DNAs (RNAs): do they have a role in biology?

The first triplexes were homopolymer mixtures, e.g. dTn.dAn.dTn. More complex triplexes could be made on the basis that the base triads [symbol: see text] and [symbol: see text] containing Watson-Crick and Hoogsteen base pairs are isomorphous and so could be expected to give regular triple helices. It follows that such triplexes can only be formed from asymmetric DNAs with pyrimidines (Yn) in one strand and purines (Rn) in the complementary strand. Such triplexes Yn.Rn.Yn are formed with the above rules for pairing of triads with the Hoogsteen C protonated, [symbol: see text]. There are also triplexes built on the theme Yn.Rn.Rn (with triads T:A:A and [symbol: see text]). Here the triads are nearly isomorphous. Recently other triplexes without isomorphous triads at all have been obtained. Also RecA protein can promote triplex formation between a duplex DNA of any sequence and an homologous single-stranded DNA. The latter triplex is evidently important in recombination. The other possible roles for triplexes include transcriptional control, and roles in origins of replication and DNA condensation.