Highly conserved region 1816-1831 of the 18S ribosomal RNA is close to the first nucleotide of the A-site codon in elongation and termination of translation

Two mRNA analogs, pUUCUAAA (with stop codon UAA) and pUUCUCAA (with Ser codon UCA) containing a perfluoroarylazido group at U4, were used to study the position relative to the 18S rRNA for the first nucleotide of the codon located in the A site of the human 80S ribosome. To place UAA or UCA in the A site, UCC-recognizing tRNA(Phe) was bound in the P site. With each analog, crosslinking was detected for highly conserved fragment 1816-1831, which contains invariant dinucleotide A1823/A1824 and is in helix 44 at the 3' end of the 18S rRNA. Since 18S rRNA modification did not depend on whether the U4 photoreactive group was in the sense or stop codon, it was assumed that polypeptide chain release factor I directly recognizes the trinucleotide of a stop codon located in the A site.