The triple helix: a potential mechanism for gene regulation.

Nature has provided the cell with an exquisite recognition mechanism in the ability of oligoribonucleotide polypyrimidine sequences to specifically bind to native duplex DNA polypurine X polypyrimidine stretches in a sequence specific fashion. This binding does not require DNA strand separation, since the oligoribonucleotide inserts into the DNA major grove where sequence-specific determinants are recognized. Thus far, there is no evidence in either prokaryotes or eukaryotes that this specificity plays a role in regulation of nucleic acid functions. Current understanding of prokaryotic transcription regulatory mechanisms is fairly detailed and appears to exclude this possibility. Our understanding of regulation of eukaryotic transcription is less advanced; possible sites for transcriptional inhibition by triple helix formation in eukaryotes include polypyrimidine stretches involved in termination of tRNA transcripts and premature termination of SV40 late mRNA transcripts.