[Part of the matrix on the 5' side from codon in the E-segment is close to protein S26 on the human 80S ribosome].

Positioning of mRNA on the 80S ribosome upstream the E site bound codon was studied using derivatives of nona- and dodecaribonucleotides containing the triplet UUU coding for Phe at the 3'-terminus, and a perfluorophenylazide cross-linker on either the first or the third nucleotide. Two sets of the mRNA analogues were used, with the photoactivatable groups on either the C5 atom of the uridine or the N7 atom of the guanosine. The modified nucleotides were directed to positions from -4 to -9 with respect to the first nucleotide of the P site bound codon by tRNA(Phe) cognate to the triplet UUU targeted to the P site. Mild UV-irradiation of ribosomecomplexes with tRNA(Phe) and mRNA analogues resulted in the cross-linking to the 40S subunits preferentially, mainly to the proteins. The principal target for the cross-linking was protein S26 in all cases. Location of the photoactivatable group on the nucleotide at position -4 lead also to the minor cross-linking to protein S3, and at position -6 to protein S14. In the absence of tRNA, all mRNA analogues cross-linked to protein S3.