Immunological recognition of three polynucleotide structures associating adenylic and uridylic acids

The reactions of antibodies directed against the two double-stranded synthetic polyribonucleotide complexes poly(A).poly(U) or poly(I). poly(C) with three polynucleotide structures associating equimolar amounts of adenylic and uridylic acids were studied by immunodiffusion, quantitative precipitation and readioimmunoassay. The three sequence isomers of the same base composition but different nucleotide distribution were: the double helical complex poly(A).poly(U) containing two homopolymeric strands; the copolynucleotide poly(A-U) composed of a strictly repeating riboadenylic acid and ribouridylic acid sequence in both strands; and the copolymer poly(A,U) where the two strands contain both residues but in a random distribution. The three antigens reacted with anti-poly(A).poly(U) and antipoly(I).poly(C) antisera but not to the same extent. The random copolymer poly(A,U) reacted poorly with sera of both specificity. In contrast the reactivity of the alternating copolnucleotide poly(A-U) was widely different according to the specificity of the sera used. Whereas it was recognized by the anti-poly(A).poly(U) antibodies almost to the same extent than the homologous complex, its activity was much lower with anti-poly(I).poly(C) antisera. In addition, the relative efficiency of poly(A-U) was approximately 50 times lower than that of poly(A).poly(U) when tested with anti-poly(I).poly(C) antibody, thereby indicating recognition of different antigenic determinants in both duplexes. Antibodies directed against defined conformational determinants are therefore able to distinguish between three polynucleotide structures of the same base composition but different sequence.