Termination of messenger RNA translation in a temperature-sensitive mutant of Escherichia coli

Abstract

A temperature-sensitive mutant of Escherichia, coli K12 is defective in a previously uncharacterized component essential for protein synthesis. The lesion in the mutant is expressed in vivo at 36 °C as the appearance of the capacity to suppress UAA and UGA nonsense mutations, and in cell-free extracts as the temperature-sensitive capacity to release newly formed R17 phage coat protein from the ribosome. 70 s ribosomes from extracts of the mutant are unable to participate in any R17 RNA-directed protein synthesis whereas the native subunits have limited polymerization activity at 43 °C. A non-ribosomal component (designated Z factor) which rescues the protein-synthesizing activity of the mutant crude extract has been partially purified from a parental strain. Z factor is shown to be distinct from G and T translocation factors. Evidence is also presented which indicates that Z factor is not one of the initiation factors, aminoacyl-tRNA synthetases, the release factor R 2, or the S factor. The inference presented is that the mutant is defective in a previously uncharacterized activity which is essential for completion of messenger RNA translation during or subsequent to recognition of the signal for polypeptide chain termination.