Yeast, Saccharomyces cerevisiae, cell-free translation: the inhibition of translation by high temperature is reversible

A conditional-lethal translation termination defect in a sup45 mutant of the yeast Saccharomyces cerevisiae

Genetic studies have indicated that the product of the yeast SUP45 gene encodes a component of the translational-termination machinery. In higher eukaryotes, genes similar to SUP45 encode eukaryote release factor 1 (eRF1), which has a stop-codon-dependent peptidyl-release activity. Using a conditional-lethal mutant allele of SUP45 (sup45-2) and a combination of in vivo and in vitro approaches, we demonstrate that the product of the SUP45 gene (Sup45p or eRF1) is a factor required for translation termination in yeast. A homologous in vitro assay based on suppressor-tRNA-mediated readthrough of stop codons is used to show that a translating lysate from a sup45-2 mutant strain exhibits a termination defect when heated for short periods to greater than the non-permissive temperature (37 degrees C). This defect can be complemented with a purified preparation of Sup45p (eRF1) expressed in Eschericha coli. The termination defect in this strain appears to be due to an inability of the Sup45p protein to bind the ribosome, resulting in vivo in a reduced ability of Sup45p to release nascent polypeptides from the ribosome at the non-permissive temperature. Cell-free translation lysates from the sup45-2 strain do not show a defect in sense-codon translation at the non-permissive temperature. These data demonstrate that yeast eRF1 plays a role in translation termination and is functionally equivalent to its higher eukaryotic homologues.

Translation initiation factor-dependent extracts from yeast Saccharomyces cerevisiae

Translation initiation factor-dependent extracts are prepared from Saccharomyces cerevisiae strains that have no or reduced activity of a translation initiation factor. Elimination of factor activity can be achieved by deletion of the gene encoding the factor if it is not essential for the survival of the strain. If the gene is essential it is placed under the control of the regulatable GAL1 promoter and its expression is shut off in vivo. Alternatively, a temperature-sensitive mutation can be introduced into the gene and the activity of the gene product eliminated in vitro by preincubation of the extract at the nonpermissive temperature. Factor-dependent extracts can be complemented in vitro with purified initiation factor preparations isolated from S. cerevisiae or from Escherichia coli cells expressing them from plasmid-encoded constructs. To simplify the purification of the factors they may be expressed as fusion proteins with N- or C-terminal tags. Initiation factor-dependent extracts can be used to study initiation factor structure-function relationships and initiation factor requirements for specific mRNA translation.

Isolation of a protein complex containing translation initiation factor Prt1 from Saccharomyces cerevisiae

Translation initiation factor Prt1 was purified from a ribosomal salt wash fraction of Saccharomyces cerevisiae cells by ammonium sulfate precipitation, DEAE chromatography, phosphocellulose chromatography, sucrose density gradient centrifugation, and non-denaturing polyacrylamide gel electrophoresis. Prt1 protein cofractionates with four other polypeptides during all steps of purification suggesting that it is part of a protein complex containing polypeptide subunits with apparent molecular masses of 130, 80, 75 (Prt1), 40, and 32 kDa. Deletion of the first AUG codon in the published sequence of the PRT1 gene results in the synthesis of functional Prt1 protein indicating that the actual molecular mass of the Prt1 subunit is 82.7 kDa. This is in agreement with results from primer extension experiments reported earlier by Keierleber et al. (Keierleber, C., Wittekind, M., Qin, S., and McLaughlin, C. S. (1986) Mol. Cell. Biol. 6, 4419-4424). The Prt1-containing protein complex is an active translation factor as shown by its ability to restore translation in a cell-free system derived from a temperature-sensitive prt1 mutant strain in which endogenous Prt1 activity is inactivated by heating the extract to 37 degrees C. The question of whether the Prt1-containing protein complex represents the yeast homologue of mammalian translation initiation factor eIF-3 is discussed.

Translation in Saccharomyces cerevisiae: initiation factor 4E-dependent cell-free system

The gene encoding translation initiation factor 4E (eIF-4E) from Saccharomyces cerevisiae was randomly mutagenized in vitro. The mutagenized gene was reintroduced on a plasmid into S. cerevisiae cells having their only wild-type eIF-4E gene on a plasmid under the control of the regulatable GAL1 promoter. Transcription from the GAL1 promoter (and consequently the production of wild-type eIF-4E) was then shut off by plating these cells on glucose-containing medium. Under these conditions, the phenotype conferred upon the cells by the mutated eIF-4E gene became apparent. Temperature-sensitive S. cerevisiae strains were identified by replica plating. The properties of one strain, 4-2, were further analyzed. Strain 4-2 has two point mutations in the eIF-4E gene. Upon incubation at 37 degrees C, incorporation of [35S]methionine was reduced to 15% of the wild-type level. Cell-free translation systems derived from strain 4-2 were dependent on exogenous eIF-4E for efficient translation of certain mRNAs, and this dependence was enhanced by preincubation of the extract at 37 degrees C. Not all mRNAs tested required exogenous eIF-4E for translation.

Translation in Saccharomyces cerevisiae: initiation factor 4E-dependent cell-free system

The gene encoding translation initiation factor 4E (eIF-4E) from Saccharomyces cerevisiae was randomly mutagenized in vitro. The mutagenized gene was reintroduced on a plasmid into S. cerevisiae cells having their only wild-type eIF-4E gene on a plasmid under the control of the regulatable GAL1 promoter. Transcription from the GAL1 promoter (and consequently the production of wild-type eIF-4E) was then shut off by plating these cells on glucose-containing medium. Under these conditions, the phenotype conferred upon the cells by the mutated eIF-4E gene became apparent. Temperature-sensitive S. cerevisiae strains were identified by replica plating. The properties of one strain, 4-2, were further analyzed. Strain 4-2 has two point mutations in the eIF-4E gene. Upon incubation at 37 degrees C, incorporation of [35S]methionine was reduced to 15% of the wild-type level. Cell-free translation systems derived from strain 4-2 were dependent on exogenous eIF-4E for efficient translation of certain mRNAs, and this dependence was enhanced by preincubation of the extract at 37 degrees C. Not all mRNAs tested required exogenous eIF-4E for translation.