Formation of the triple-stranded polynucleotide helix, poly(A.A.U).

A polynucleotide helical structure containing two strands of poly(A) and one of poly(U) is reported. As shown by spectroscopic observations, the complex only forms when the poly(A) strands are of Mr between 9000 and 50,000 (degree of polymerization congruent to 28-150), whereas the size of the poly(U) strand has no effect. This limitation may explain why poly(A.A.U) was not seen in previous investigations. The potential of the poly(A) tails of mRNA for formation of this triple helix and of A.A.U or/and A.A.T triplet formation to contribute to the binding of specific RNA strands to gene-encoding nucleic acid double helices are noted.