Initiation of protein synthesis. Binding of messenger RNA

Complexes between 30 S ribosomal subunits and fMet tRNA are formed during incubation of 30 S subunits with fMet tRNA and all other components for initiation of protein synthesis, except R17 bacteriophage RNA. That these complexes serve as intermediates in the binding of messenger RNA is demonstrated directly by the finding that upon addition of R17 RNA, fMet tRNA in preformed fMet RNA-30 S complexes preferentially enters fMet tRNA-30 S-R17 RNA complexes. On the other hand, incubation of 30 S ribosomal subunits with R17 RNA and all other components for initiation except fMet tRNA does not yield 30 S-R17 RNA complexes that can act subsequently as functional intermediates in the binding of fMet tRNA: formation of fMet tRNA 30 S-R17 RNA complexes does not occur when fMet tRNA is added and further binding of R17 RNA to 30 S subunits is prevented by specific inhibitors. These experiments lead to an unambiguous order of events in the sequence of initiation, in which binding of fMet tRNA to the small ribosomal subunit must occur before messenger RNA can be bound and phased correctly. Complexes between fMet tRNA and 30 S subunits are in rapid equilibrium with the free components, and have a half life of less than 2 min at 37°. This explains why such complexes are not detected in sucrose gradient, unless they are first fixed with glutaraldehyde. Attachment of R17 RNA, however, results in formation of an fMet tRNA-30 S-R17 RNA complex that is stabilized greatly; fMet tRNA in this complex exchanges only very slowly with free fMet tRNA. Initiation factor IF 3 has two functions in initiation. The first is to direct the binding of messenger RNA to the 30 S fMet tRNA complex. This function is not needed when initiation complex formation occurs on ApUpG triplets, in which case the second function of IF 3 is detected, that of providing free 30 S subunits for initiation. The ability of IF 3 to bind directly to R17 RNA may be related to its requirement in messenger RNA recognition. However, since IF 3 exhibits a greater affinity for the 30 S subunit than for R17 RNA, it appears that the recognition function of IF 3 is expressed while IF 3 is associated with the 30 S subunit.