CHAIN INITIATION DURING POLYPEPTIDE SYNTHESIS IN CELL-FREE BACTERIAL SYSTEMS PROGRAMMED WITH PLANT VIRAL MESSENGERS .3. A COMPARISON WITH PHAGE RNA-DIRECTED SYSTEM

1. 1. The binding of phage RNA and plant viral RNA to Escherichia coli ribosomes has been studied in the presence and absence of ribosomal initiation factors. A procedure has been worked out to follow the binding of phage messenger (28-S) to 30-S ribosomes. Two differences have been found: (a) MS2-[3H]RNA binds primarily to 30-S particles, not to 70-S particles, and this binding is fully dependent on ribosomal factors; (b) alfalfa mosaic virus (AMV) [32P]RNA binds to 70-S ribosomes, and this binding does not require ribosomal protein factors. 2. 2. AMV-RNA and turnip yellow mosaic virus (TYMV) RNA promote the binding of fMet-tRNAfMet to the ribosomes as MS2-RNA does. Binding in all cases is only observed in the presence of ribosomal factors. The same holds true for formylmethionine incorporation into biosynthetic polypeptides formed under direction of the plant viral RNA's and MS2-RNA. 3. 3. Binding of aminoacyl-tRNA to the complex of ribosomes and plant viral RNA which does not require previous binding of fMet-tRNAfMet or free 30-S ribosomes has been observed. The AMV-RNA-ribosome complex, bearing aminoacyl-tRNA, sediments in the 70-S region. Transferase T, isolated from the soluble fraction of the cell, stimulates the binding of appropriate aminoacyl-tRNA's to the AMV-RNA-ribosome complex. 4. 4. The differences observed in the various binding reactions are discussed in connection with the specificity of chain initiation in the homologous and the heterologous systems presently studied. © 1969.