Mutations in ADE3 reduce the efficiency of the omnipotent suppressor sup45-2

Polypeptide chain termination and stop codon readthrough on eukaryotic ribosomes

During protein translation, a variety of quality control checks ensure that the resulting polypeptides deviate minimally from their genetic encoding template. Translational fidelity is central in order to preserve the function and integrity of each cell. Correct termination is an important aspect of translational fidelity, and a multitude of mechanisms and players participate in this exquisitely regulated process. This review explores our current understanding of eukaryotic termination by highlighting the roles of the different ribosomal components as well as termination factors and ribosome-associated proteins, such as chaperones.

A new family of yeast vectors and S288C-derived strains for the systematic analysis of gene function

The yeast genome has been shown to contain a significant number of gene families with more than three members. In order to study these families it is often necessary to generate strains carrying deletions of all members of the family, which can require a wide range of auxotrophic markers. To facilitate such studies, we have generated yeast strains containing deletions of a selection of nutritional marker genes (ade2, ade4, ade8, met3 and met14). We have also cloned the corresponding cognate genes, allowing their use in PCR-based gene disruptions. Two new pRS family Saccharomyces cerevisiae-Escherichia coli shuttle vectors containing ADE8 (one low-copy, pRS4110, and one high-copy, pRS4210) have been produced for use in conjunction with the new strains. A system for easier synthetic lethal screening using one of these new markers is also presented. The ADE8 and HIS3 genes have been cloned together on a high-copy vector (pRS4213), providing a plasmid for red-white colour screening in the ade2 Delta 0 ade8 Delta 0 strains we have generated. In contrast to some conventional systems, this plasmid allows for screening using gene libraries constructed in URA3 plasmids.

The N-terminal, dehydrogenase/cyclohydrolase domain of yeast cytoplasmic trifunctional C1-tetrahydrofolate synthase requires the C-terminal, synthetase domain for the catalytic activity in vitro

The yeast ADE3(1-333) gene which encodes a truncated protein containing the N-terminal 5,10-methylene-tetrahydrofolate (THF) dehydrogenase (D)/5,10-methyl-THF cyclohydrolase (C) domain of cytoplasmic trifunctional C1-THF synthase is able to complement all the phenotypes associated with ade3 mutations in vivo. However, expression of the ADE3(1-333) gene in an ade3 strain does not retain any D activity in vitro. Expression in a yeast ade3 strain of the ADE3(1-333) fused to the Escherichia coli lacZ gene or to the yeast SER2 gene allows detection of D and C activities in vitro. These results indicate that the N-terminal D/C domain of C1-THF synthase requires the C-terminal 10-formyl-THF synthetase domain for stable catalytic activity in vitro.

Nonsense suppression in thymine-requiring strains of Escherichia coli is a consequence of altered folate metabolism

Thymine-requiring strains of Escherichia coli suppress nonsense and frameshift mutants of T4 phage. We proposed that these mutants make errors during translation because of an imbalance in folate metabolism. A thymine-requiring strain grown under suppressing conditions has elevated levels of reduced folates. We tested the effect of either mutational blocks or the inhibition of various steps in folate biosynthesis on suppression. Conditions which prevent the accumulation of 5-methyl tetrahydrofolate inhibit suppression, suggesting that elevated levels of this folate are required for suppression. Furthermore, conditions that result in an accumulation in dihydrofolate inhibit suppression.

Function of yeast cytoplasmic C1-tetrahydrofolate synthase

The protein product of the ADE3 gene of the yeast Saccharomyces cerevisiae has been identified as the cytoplasmic trifunctional C1-tetrahydrofolate (THF) synthase, which possesses 10-formyl-THF synthetase (EC 6.3.4.3), 5,10-methenyl-THF cyclohydrolase (EC 3.5.4.9), and 5,10-methylene-THF dehydrogenase (EC 1.5.1.5) activities. However, it has been suggested that the ADE3-encoded C1-THF synthase does not play a role in providing the enzymes involved in the generation of one-carbon intermediates in the biosynthesis of the purine bases but functions in maintaining the structural integrity of the enzyme complex involved in purine biosynthesis [Barlowe, C. K. & Appling, D. A. (1990) Mol. Cell. Biol. 10, 5679-5687]. This hypothesis is based on their finding that the presence of the full-length ADE3 C1-THF synthase, whether catalytically active or not, is correlated with the Ade+ phenotype. In contrast to their results, our deletion analysis of the ADE3 gene indicates that the presence of either the synthetase or dehydrogenase/cyclohydrolase domains of C1-THF synthase is enough to complement the adenine requirement in ade3 strains. These results are also consistent with those obtained in heterologous expression of spinach and Clostridium acidiurici monofunctional synthetases in ade3 strains. Heterologous expression studies show that the high synthetase activity may be correlated with the increased growth in medium lacking adenine. These results suggest that the catalytic activity of the C1-THF synthase is involved in purine biosynthesis.

Conservative system for dosage-dependent modulation of translational fidelity in eukaryotes

Variations in dosage of some genes can alter the level of translational fidelity. The Saccharomyces cerevisiae genes that act as dosage-dependent suppressors and/or modulators of suppression, are the following: some tRNA genes (for example, tRNA(Gln)) inducing readthrough by mispairing; genes coding for either translational elongation factor or other proteins taking part in translation; and some genes of unknown function. We suggest that the SUP35 protein is a factor which may play a major role in balance-dependent regulation of translational fidelity. Homologues of this genes have been identified in other yeast genera (Pichia), green algae (Chlamydomonas) and various animals including man. No homologies have been found in the polychaeta (Nereis) or in insects (Drosophila). Rates of evolution differ for two separate parts of the genes; the N-terminal part, which is important for ambiguous translation in Saccharomyces, is markedly variable in the organisms tested. However, the C-terminal part which is required for yeast viability has a common origin but a separate evolution from that of the EF-Tu protein family.

Modification of the suppressor phenotype of thymine requiring strains of Escherichia coli

Thymine requiring strains of Escherichia coli suppress nonsense and frameshift mutations during translation. Strains with different genetic backgrounds exhibited different nonsense suppression spectra and showed differences in the apparent suppression efficiency. Part of this strain difference is due to a presumably novel gene (tsmA) mapping near 39 min. This gene affects the spectrum and apparent efficiency of suppression, and appears to affect the utilization of thymidine.