Association of nascent polypeptide and transfer ribonucleic acid with 30S ribosomal subunits.

1. Crude extracts of Escherichia coli programmed in protein synthesis by endogenous mRNA have incorporated amino acids into protein. Analysis of such extracts by sucrose-gradient centrifugation in low Mg(2+) concentration has revealed that 30S ribosomal subunits carry associated radioactive material of which a considerable proportion can be removed from ribosomes by treatment of pre-labelled extracts with puromycin. 2. Gradient analyses of incorporations carried out in the additional presence of added (32)P-labelled tRNA have indicated that tRNA sediments in the regions of the newly synthesized nascent protein and that both labels are associated with all ribosomal components detected on the gradients under the experimental conditions employed. 3. 30S ribosomal subunits carrying both (32)P and (14)C labels have been isolated, disrupted with sodium dodecyl sulphate, and analysed by chromatography on Sephadex G-200 columns. Both labels elute closely together and well away from a tRNA marker analysed under identical conditions. 4. It is proposed that 30S ribosomal subunits, isolated from extracts which have synthesized nascent peptides under the direction of endogenous mRNA, carry associated peptidyl-tRNA.