The 3′-terminal primary structure of five eukaryotic 18S rRNAs determined by the direct chemical method of sequencing. The highly conserved sequence include an invariant region complementary to eukaryotic 5S rRNA

The 3′-terminal sequences of 18S rRNA from chicken reticulocyte, mouse sarcoma, rat liver, rabbit reticulocyte and barley embryo were determined by the direct chemical sequencing method. The regions sequenced show complete homology for the first 73 nucleotides. A sequence 5′-proximal to the m26Am26A residues that is complementary to eukaryotic 5S RNAs is totally conserved. This supports the hypothesis that base-paired interaction between 5S and 18S rRNA, which are present in the large and small ribosomal subunits respectively, may be involved in the reversible association of ribosomal subunits during protein synthesis.