title: Formylmethionyl-tRNA binding to 30 S ribosomes programmed with homopolynucleotides and the effect of translational initiation factor 3.
abstract: Binding of the polynucleotides poly(U), poly(X) and poly(dT) to 30 S ribosomes of Escherichia coli triggers IF2-dependent binding of initiator-tRNA (fMet-tRNA) to these particles. Poly(A) and poly(C) are inactive. A minimum chain-length of approximately 100 residues in poly(U) is required for full activity in fMet-tRNA binding, although much shorter polymers bind tightly to 30 S particles and do stimulate the binding of acPhe-tRNA. The stimulation of fMet-tRNA binding to 30 S ribosomes is strongly reduced under conditions where the polynucleotides adopt secondary structure. Complexes containing fMet-tRNA and the non-cognate codon UUU or XXX are destabilized by IF3, whereas the formation of such a complex containing an AUG codon is slightly enhanced by the factor. Consistent with previous observations, it was found that all model initiation complexes containing acPhe-tRNA are strongly destabilized by IF3, even when the cognate codon (UUU) is present. Our results suggest that IF3 counteracts 'unnatural' initiation events in vitro and suggest a regulatory role for this factor in vivo.
mesh_terms: D000926:Anticodon / Q000378:metabolism; D003062:Codon / Q000378:metabolism; D004926:Escherichia coli / Q000378:metabolism; D009690:Nucleic Acid Conformation; D010448:Peptide Initiation Factors / Q000494:pharmacology*; D011071:Poly T / Q000378:metabolism; D011072:Poly U / Q000378:metabolism; D011119:Polynucleotides / Q000378:metabolism; D011131:Polyribonucleotides / Q000378:metabolism*; D039665:Prokaryotic Initiation Factor-3; D014176:Protein Biosynthesis; D012346:RNA, Transfer, Amino Acyl / Q000378:metabolism*; D012358:RNA, Transfer, Met; D012270:Ribosomes / Q000378:metabolism*
chemical_list: D000926:Anticodon; D003062:Codon; D010448:Peptide Initiation Factors; D011119:Polynucleotides; D011131:Polyribonucleotides; D039665:Prokaryotic Initiation Factor-3; D012346:RNA, Transfer, Amino Acyl; D012358:RNA, Transfer, Met; C024988:tRNA, formylmethionine-; D011071:Poly T; C010304:poly(xanthylic acid); D011072:Poly U