EVIDENCE for a triple-stranded complex of polynucleotides was reported for homopolymer RNAs by Felsenfeld et al.1 in 1957. They detected a drop in absorbancy at 260 nm for a mixture consisting of two parts polyadenylic acid (poly(rA)) and one part polyuridylic acid (poly(rU)), during a continuous variation mixing study of the two homopolymers. Addition of 0.1 M MgCl 2 produced the greatest triple-strand formation with a minimum absorbance at 67 mol % poly(rU), 33 mol % poly(rA)2. Since this early work, many triple-stranded complexes have been identified, including combinations of homopolymers of deoxyribonucleotides and ribonucleotides; Riley et al.3 reported a triple strand formed by the homopolymers dA: dT: dT. They also reported the caesium sulphate buoyant densities of various double-stranded and triple-stranded homopolymers. In all cases, the triple-stranded homopolymer had a greater buoyant density than the analogous double-stranded homopolymer. We now report evidence for the formation of triple-stranded bovine DNA in vitro.
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