To study positioning of the polypeptide release factor eRF1 toward a stop signal in the ribosomal decoding site, we applied photoactivatable mRNA analogs, derivatives of oligoribonucleotides. The human eRF1 peptides cross-linked to these short mRNAs were identified. Cross-linkers on the guanines at the second, third, and fourth stop signal positions modified fragment 31-33, and to lesser extent amino acids within region 121-131 (the "YxCxxxF loop") in the N domain. Hence, both regions are involved in the recognition of the purines. A cross-linker at the first uridine of the stop codon modifies Val66 near the NIKS loop (positions 61-64), and this region is important for recognition of the first uridine of stop codons. Since the N domain distinct regions of eRF1 are involved in a stop-codon decoding, the eRF1 decoding site is discontinuous and is not of "protein anticodon" type. By molecular modeling, the eRF1 molecule can be fitted to the A site proximal to the P-site-bound tRNA and to a stop codon in mRNA via a large conformational change to one of its three domains. In the simulated eRF1 conformation, the YxCxxxF motif and positions 31-33 are very close to a stop codon, which becomes also proximal to several parts of the C domain. Thus, in the A-site-bound state, the eRF1 conformation significantly differs from those in crystals and solution. The model suggested for eRF1 conformation in the ribosomal A site and cross-linking data are compatible.