The Effects of Thioketo Substitution upon Uracil‐Adenine Interactions in Polyribonucleotides

Abstract

The physical and biological properties of two new poly(U) analogs, poly(2‐thiouridylic acid) or poly(s 2 U) and poly(2,4‐dithiouridylic acid) or poly(s 2 s 4 U) have been investigated. Ultraviolet absorption and circular dichroism spectra of poly(s 2 U) and poly(s 2 s 4 U) as well as those of the corresponding monomers were measured. Absorption‐temperature and absorption‐pH profiles of both poly(s 2 U) and poly(s 2 s 4 U) displayed sharp cooperative transitions. The phase transition diagrams of poly(s 2 U) and poly(s 2 s 4 U) show the high stability of their secondary structures. Neither poly(s 2 U) nor poly(s 2 s 4 U) formed helical complexes with poly(A). It was concluded from the corresponding phase transition diagrams that the reason for this behaviour is the larger thermodynamical stability of the poly(s 2 U) or poly(s 2 s 4 U) helices over the hypothetical complexes poly(s 2 U) · poly(A) or poly(s 2 s 4 U) – poly(A). Sedimentation velocity analysis of poly(s 2 U) and poly(s 2 s 4 U) revealed sedimentation coefficients S 20,w of 28 S and 12 S, respectively. A more detailed analysis of poly(s 2 U) lead to the conclusion that poly(s 2 U) exists as an intra‐molecular associate of two anti‐parallel poly(s 2 U) strands in the form of a large hairpin. Consistent with their physical properties, poly(s 2 U) and poly(s 2 s 4 U) are not able to replace poly(U) as messengers in the cell‐free synthesis of poly(phenylalanine). Poly(s 2 U) is only poorly, poly(s 2 s 4 U) not at all phosphorolyzed by polynucleotide phosphorylase.