title: Possible role of flanking nucleotides in recognition of the AUG initiator codon by eukaryotic ribosomes.
abstract: Sequences flanking the initiator codon in eukaryotic mRNAs are not random. Out of 153 messages examined, 151 have either a purine in position -3, or a G in position +4, or both. Thus, [A/G]XXAUGG emerges as the favored sequence for eukaryotic initiation sites. Nucleotides flanking nonfunctional AUG triplets, which occur in the 5'-noncoding region of a few eukaryotic messages, are different from those found at most functional sites. Whereas most authentic initiator codons are preceded by a purine (usually A) in position -3, most nonfunctional AUGs have a pyrimidine in that position. The observed asymmetry suggests that purines in positions -3 and +4 might facilitate recognition of the AUG condon during formation of initiation complexes. To test this idea, in vitro binding studies were carried out with 32P-labeled oligonucleotides. Binding of AUG-containing oligonucleotides to wheat germ ribosomes was significantly enhanced by placing a purine in position -3 or +4. The scanning model, which postulates that 40S ribosomal subunits attach at the 5'-end of a message and migrate down to the AUG codon, is discussed in light of these new observations. A modified version of the scanning mechanism is proposed.
mesh_terms: D001483:Base Sequence; D003062:Codon / Q000235:genetics*; D005815:Genetic Code; D009843:Oligoribonucleotides / Q000032:analysis; D010944:Plants / Q000378:metabolism; D014176:Protein Biosynthesis; D012333:RNA, Messenger / Q000235:genetics*; D012270:Ribosomes / Q000378:metabolism*; D014908:Triticum / Q000378:metabolism
chemical_list: D003062:Codon; D009843:Oligoribonucleotides; D012333:RNA, Messenger