[Structural elements of 80S ribosomes located near the 5'-region of the mRNA-binding center]

Affinity labeling of 80S ribosomes with 4-(N-2-chloroethyl-N-methylamino)benzylmethylphosphoramides of oligoribonucleotides [32P]AUGUn--mRNA analogs--was studied in three model complexes: 80S.ClRCH2N(CH3)-pAUGU6.Met(Phe)2-trRNA(Phe), 80S.ClRCH2N(CH3)pAUGU3.MetPhe-tRNA(Phe), and 80S.ClRCH2N(CH3)-pAUG.Met- tRNA(Met). Two of these complexes imitate the posttranslocational state of 80S ribosomes. Small subunits were labeled preferentially; both 18S rRNA and ribosomal proteins were modified by the mRNA analogs. The relative modification extents of proteins and rRNA depended on the length of the reagent oligoribonucleotide moiety. Extension of the latter resulted in decrease in the relative extent of 18S rRNA modification from 95 a to 16% (for proteins, increase from 5 to 84%, respectively). Fragments of 18S rRNA containing cross-linking sites were identified using blot hybridization. In all cases, fragment 976-1164 was found to be modified. In the case of ClRCH2N(CH3)pAUGU6, labeling occurred also within fragments 593-673 and 1748-1869. Analysis of the modified proteins revealed that proteins S14/S15 were labeled with all three reagents and were the single target of modification with ClRCH2N(CH3)pAUGU6. Proteins S3/S3a, S6, and S16/S18 were modified only with ClRCH2N(CH3)pAUGU3; protein S20 only with ClRCH2N(CH3)pAUG; and proteins S5 and S17 were labeled with both reagents (n = 0, 3).