title: The role of the codon and the initiation factor IF-2 in the selection of N-blocked aminoacyl-tRNA for initiation.
abstract: Poly(uridylic acid) [poly(U)] and poly(xanthidylic acid) [poly(X)] strongly stimulate the IF-2-dependent binding of fMET-tRNA to 30-S ribosomal subunits from Escherichia coli [Van der Laken et al. (1979) FEBS Lett. 100, 230-234]. The N-formylmethionine moiety is incorporated into poly(phenylalanine) upon subsequent addition of other components required for protein synthesis when poly(U) is used as template. This paper shows that N-acetylated Phe-tRNAPhe (AcPhe-tRNA), but not Phe-tRNAPhe or tRNAPhe, competes with fMET-tRNA for binding to poly(U)-programmed 30-S ribosomal subunits. The two species of N-blocked aminoacyl-tRNA are bound to poly(U)-programmed and poly(X)-programmed 30-S subunits in a ratio that is linearly dependent on the ratio of the two species added. With poly(U) as template there is no apparent preference for either fMET-tRNA or AcPhe-tRNA, whereas with poly(X) there is a 2-3-fold preference for fMET-tRNA. The initiation factor IF-2, which is strictly required for the binding of N-blocked aminoacyl-tRNAs, has a higher affinity for fMET-tRNA than for AcPhe-tRNA. It is concluded that (a) interaction of the 30-S ribosomal subunit with poly(U) or poly(X) leads to IF-2-dependent binding of N-blocked aminoacyl-tRNA; (b) the initiation factor IF-2-discriminates in favour of fMET-TRNA; (c) the presence of the cognate codon discriminates in favour of the corresponding N-blocked aminoacyl-tRNA.
mesh_terms: D003062:Codon; D004926:Escherichia coli / Q000378:metabolism*; D015852:Eukaryotic Initiation Factor-2; D007700:Kinetics; D010448:Peptide Initiation Factors / Q000378:metabolism*; D011131:Polyribonucleotides / Q000378:metabolism; D011506:Proteins / Q000378:metabolism*; D012333:RNA, Messenger; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism*; D012270:Ribosomes / Q000378:metabolism; D013329:Structure-Activity Relationship
chemical_list: D003062:Codon; D015852:Eukaryotic Initiation Factor-2; D010448:Peptide Initiation Factors; D011131:Polyribonucleotides; D011506:Proteins; D012333:RNA, Messenger; D012346:RNA, Transfer, Amino Acyl