The termination of protein synthesis in ribosomes is governed by termination (stop) codons in messenger RNAs and by polypeptide chain release factors (RFs). Although the primary structure of prokaryotic RFs and yeast mitochrondrial RF is established, that of the only known eukaryotic RF (eRF) remains obscure. Here we report the assignment of a family of tightly related proteins (designated eRF1) from lower and higher eukaryotes which are structurally and functionally similar to rabbit eRF. Two of these proteins, one from human and the other from Xenopus laevis, have been expressed in yeast and Escherichia coli, respectively, purified and shown to be active in the in vitro RF assay. The other protein of this family, sup45 (sup1) of Saccharomyces cerevisiae, is involved in omnipotent suppression during translation. The amino-acid sequence of the eRF1 family is highly conserved. We conclude that the eRF1 proteins are directly implicated in the termination of translation in eukaryotes.

The cdc28-srm mutation in Saccharomyces cerevisiae decreases spontaneous and induced mitochondrial rho- mutability and the mitotic stability of native chromosomes and recombinant circular minichromosomes. The effects of cdc28-srm on the genetic stability of cells support the hypothesis that links cell cycle regulation in yeast to changes in chromatin organization dependent on the start gene CDC28 (Hayles and Nurse, 1986).

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CDC28‐srm, a non‐temperature‐sensitive (ts) mutation in the CDC28 gene of Saccharomyces cerevisiae that affects fidelity of mitotic transmission of both mitochondrial and nuclear genetic structures (Devin et al., 1990), also affected cell growth and sensitivity to lethal effects of ionizing radiation. At 30°C CDC28‐13, a ts mutation, was without appreciable effects on spontaneous mitochondrial rho−‐mutagenesis, cell growth and radiation sensitivity, whereas all three cell characteristics mentioned were affected (although to a lesser degree than by CDC28‐srm) by CDC28‐1, another ts mutation. CDC28‐srm was without any significant effect on the rates of spontaneous nuclear gene mutations and γ‐ray‐induced mitotic recombination. An analysis of double mutants as regards their radiation sensitivity has revealed additive or even synergistic interactions between the CDC28‐srm mutation and every one of the rad6‐1 and rad52‐1 mutations. The rad9Δ allele was found to be epistatic to CDC28‐srm. These data suggest that the p34CDC28 protein is involved in the RAD9‐dependent feedback control of DNA integrity operating at the cell cycle checkpoints. © 1998 John Wiley & Sons, Ltd.