The initiation of protein synthesis: joining of the 50S ribosomal subunit to the initiation complex.

In a previous paper,1 we have shown that RNA from the RNA phage f2 directs the binding of N-formylmethionyl-tRNAF (F-met-tRNA)2' to 30S ribosomal sub- units (30S particles)2 in the absence of 50S subunits.50S particles inhibit this specific binding of F-met-tRNA, although they stimulate the binding of other aminoacyl-tRNA's directed by the appropriate synthetic polynucleotides.From these results, it has been proposed that the first step in protein synthesis is the forma- tion of a complex (the initiation complex) consisting of the 30S particle, mRNA, and F-met-tRNA.Since peptide bond formation takes place on 70S particles,4 that is, on the 30S-50S complex, the obvious next step is the joining of the 50S particle to the initiation complex.Such a joining reaction has now been demonstrated by adding 50S particles to the preformed initiation complex.We have also observed that in the presence of the "initiation factor," phage f2 RNA stimulates the binding of F-met- tRNA to "native" 70S particles, but that this binding is inhibited by the addition of 50S particles.From these results, we conclude that the mRNA-directed binding of F-met-tRNA to the 70S particle (the 30S-50S complex) involves formation of the initiation complex (30S particle-mRNA-F-met-tRNA complex) as an obligatory intermediate.Materials and Methods.-Fractionatedmet-tRNAF was used in some experiments (Table 1).This was a gift from Dr. H. G.