Release factor regulating termination of complete protein in a eukaryotic organism

Recent advances in peptide chain termination

Peptide chain termination occurs when a stop codon is decoded by a release factor. In Escherichia coli two codon-specific release factors (RF1 and RF2) direct the termination of protein synthesis, while in eukaryotes a single factor is required. The E. coli factors have been purified and their genes isolated. A combination of protein and DNA sequence data reveal that the RFs are structurally similar and that RF2 is encoded in two reading frames. Frame-shifting from one reading frame to the next occurs at a rate of 50%, is regulated by the RF2-specific stop codon UGA, and involves the direct interaction of the RF2 mRNA with the 3' end of the 16S rRNA. The RF genes are located in two separate operons, with the RF1 gene located at 26.7 min and the RF2 gene at 62.3 min on the chromosome map. Ribosomal binding studies place the RF-binding region at the interface between the ribosomal subunits. A possible mechanism of stop-codon recognition is reviewed.

Protein biosynthesis in a cell-free system from bovine retina

A cell-free extract active in the incorporation of [3H]leucine into protein has been prepared from bovine retina. This system may be useful in identifying early precursors and transported forms of the visual pigment apoprotein. The system completed a minimum of 50% of the preexisting nascent polypeptide chains. 20% of the labelled product accumulated in the soluble fraction of the extract. Gel electrophoresis revealed that this fraction contained six major labelled polypeptide species, each of which correspond to a major unlabelled soluble polypeptide. Some completed polypeptides were retained in the particulate fraction of the extract; several of these were resolved on gel electrophoresis and were of very high specific radioactivity. An antiserum to rod outer segment disc proteins precipitated several labelled species from each fraction. The most interesting of these is a polypeptide of molecular weight 41 000 which was found only in the particulate fraction.