Peptide Synthesis by Prokaryotic-Eukaryotic Hybrid Ribosomes

Abstract The small (40 S) ribosomal subunits of embryos of the brine shrimp Artemia salina and the large (50 S) subunits of Escherichia coli can engage in limited, coordinated hybrid function, exemplified by the synthesis of fMet-puromycin and N-acetyl-Phe-Phe. Larger peptides are not produced. At high Mg2+ concentrations these reactions occur non-enzymatically, i.e. in the absence of initiation and elongation factors. At low Mg2+ concentrations the A. salina supernatant initiation factor (eukaryotic initiation factor 1, EIF-1) is required for N-acylaminoacyl-tRNA binding to the 40 S subunit and subsequent puromycin reaction, and aminoacyl transfer factor is needed for Phe-tRNA binding to the hybrid ribosomes and ensuing peptide synthesis. Although these processes are mediated by the large subunit, in this case E. coli 50 S, N-acetyl-Phe-Phe synthesis takes place with eukaryotic elongation factor (EF-1) as well as with prokaryotic factor (EF-T). Glutaraldehyde fixation enables display of the hybrid couples by sucrose density gradient centrifugation.