Recognition of initiation codons in modified f2 RNA by Escherichia coli ribosomes

f2 phage RNA treated with O-methylhydroxylamine under denaturing conditions loses its ordered structure with consequent exposure of the normally hidden initiation codons. In the presence of Escherichia coli ribosomes and crude initiation factors modified f2 RNA binds about 50 times more f-[3H]Met-tRNA than native f2 RNA. The interaction of native f2[14C]RNA with ribosomes requires initiation factors. The binding of O-methylhydroxylamine-modified f2 [14C]RNA to E. coli 70-S or 20-S ribosomes does not depend on the presence of initiation factors. A significant number of ribosomes deficient in initiation factors interact with a molecule of modified f2 [14C]RNA. Treatment of the resultant polysomal complex with pancreatic RNase yields ribosomes with f2 RNA fragments protected against RNase. Almost all AUG/GUG codons in the f2 RNA are located on the RNase-insensitive ribosome-bound fragments, constituting only 25% of the entire molecule. Addition of crude initiation factors to such ribosomes with fragments of modified f2 RNA promotes binding of f-[3H]Met-tRNA. The resultant complex is fully reactive with puromycin. No binding of Ac-Phe-tRNA takes place under similar conditions.