Modification of ribonucleic acid by chemical carcinogens: VI. Effect of N-2-acetylaminofluorene modification of guanosine on the codon function of adjacent nucleosides in oligonucleotides

Abstract

Oligonucleotides containing a guanosine residue on the 5′ or the 3′ side of tri- and tetranucleotides were prepared. The guanosine residue was modified with the chemical carcinogen N-2-acetylaminofluorene and the control and modified oligonucleotides were tested for their ability to stimulate 14C-labeled amino-acyl-tRNA binding to ribosomes. The effects of the modification are twofold. The first is that if the guanosine residue to which the drug is eovalently bound is part of a codon the oligonucleotide is completely inactive in the ribosomal binding assay. The second is that if an adenosine residue is adjacent to either the 5′ or 3′ side of the modified guanosine, as in (Ap) 3G or G(pA) 3, there is partial inhibition of 14C-labeled lysyl-tRNA binding to ribosomes. This inhibitory effect extends only to the function of the immediately adjacent adenosine since the chemical modification of guanosine residues in (Ap) 4G or G(pA) 4 did not impair their ability to code for lysine. In contrast to these findings if there is a uridine residue adjacent to the modified guanosine, as in (Up) 3G or G(pU) 3 there is no effect on 14C-labeled phenylalanyl-tRNA binding to ribosomes. Proton magnetic resonance spectra of UpG, GpU and the corresponding dinners in which the guanosine residue was modified with the drug failed to indicate a stacking interaction between the fluorene moiety and the adjacent uridine residue. This is in contrast to previous studies demonstrating a strong stacking interaction between fluorene and adjacent adenosine residues. Taken together these results indicate that acetylaminofluorene modification of guanosine next to an adenosine residue in oligonucleotide inhibits its ribosomal binding capacity. The stacking interaction with adjacent adenosine, and not with adjacent uridine residues, in oligonucleotides probably accounts for the effects observed in the ribosomal binding assay. These data are consistent with our previously described “base displacement” model.