The Saccharomyces cerevisiae DNA repair gene RAD6 encodes a ubiquitin-conjugating enzyme which polyubiquitinates histones in vitro. Here we show that mutations in rad6 increase the frequency of transposition of the retrotransposon Ty into the CANI and URA3 loci. Using isogenic RAD6 and rad6 strains, we measured a more than 100-fold increase in the spontaneous rate of retrotransposition due to rad6, although there was no increase in the Ty message level. This is the first time that a mutation in a host gene has been shown to result in an increased rate of retrotransposition.Retrotransposons such as Ty are common in eucaryotes. While they are not infectious, they otherwise resemble retroviruses in their structural organization and replication in viruslike particles via an RNA intermediate (for a review, see reference 1). The finding that two-to fourfold increases in the transcription of Ty message caused disproportionately large increases in Ty transposition led to speculation that the cell may exert controls on transposition at a posttranscriptional level (7). Possibly the extra Ty mRNA leads to a complete saturation of this hypothetical posttranscriptional regulatory system. Other evidence that the rate of Ty transposition is not simply proportional to the Ty message level comes from the observation that growth at a low temperature (17 versus 30°C) causes a 50-to 100-fold increase in the rate of Ty transposition without changing the level of Ty mRNA (24,25). One explanation of this observation is that the reverse transcriptase activity is optimal at low temperatures (10).Mutations in the Saccharomyces cerevisiae RAD6 gene may result in several phenotypes, including extreme sensitivity to DNA-damaging agents, lack of induced mutagenesis, defective sporulation, increased mitotic recombination, and an increased rate of spontaneous mutation (for a review, see reference 13). RAD6 encodes an E22ok enzyme which polyubiquitinates histones H2A and H2B in vitro (16, 35) and which may influence chromatin structure (16,29,35). Here we show that among the spontaneous mutations stimulated by rad6 are mutations due to the transposition of Ty elements. Since the yeast RAD6 protein may be analogous to a mammalian 20-kilodalton E2 ubiquitin-conjugating enzyme (16,26), these studies may be relevant to studies of the cellular mechanisms used to control the movements of retrotransposons and retroviruses in other eucaryotes. LP2752-4B and LP2752-4Brad6A and plasmid pR67 were kindly provided by Louise Prakash. N-152, also from Louise Prakash, is a rad6-3 mutant (28) and was derived from B-635, a cycl-115 mutant derived from D311-3A (a Iys2 hisi trp2), both kindly provided by Fred Sherman.Mutant isolation. A large number of independent colonies from each strain were used to inoculate individual slants of complete medium, YPD (32), which were grown at 30°C. Cells suspended from these slants were plated either to synthetic complete medium lacking arginine and containing 60 ,ug of canavanine per ml (-Arg +Can) (32) or to medium containin...

Changes in the dosage of genes encoding elongation factor EF-la were shown to cause parallel changes in the misreading of nonsense codons. Higher amounts of EF-la were correlated with increased nonsense suppression, suggesting that the level of EF-la is critically involved in translational fidelity.To understand how translational accuracy is controlled, it is important to identify which genes are involved and to determine how they exert their effects. The best-studied mutations that alter translational fidelity in yeast cells are the codon-specific suppressors caused by altered tRNA anticodons (for reviews, see references 32 and 40). Among other genes that affect translational accuracy are the omnipotent suppressors, e.g., sup35 and sup45, which cause non-codonspecific misreading (3,9,11,17, 42), and the recessive antisuppressor, asu9, which increases translational fidelity in sup35 and sup45 strains (19,20). While attempting to clone asu9, we found that an extra copy of one of two redundant genes coding for the translation elongation factor EF-la increased suppression efficiency in a sup45-2 asu9 strain.EF-la is an important component of the translational apparatus. It promotes the GTP-dependent binding of aminoacyl-tRNA to the ribosome and participates in the proofreading of the codon-anticodon match (16,23,38). Yeast EF-la is encoded by two unlinked genes, TEF1 and TEF2 (5,25,26,31). The two genes are efficiently transcribed at about the same level, and the presence of either gene is sufficient for cell viability and normal growth rates (4). Mutations in the yeast EF-la and its Escherichia coli analog EF-Tu have been found to affect translational fidelity (13,14,30,37,41). In yeast cells, single mutational changes in TEF2 were shown to cause dominant suppression of both nonsense and frameshift mutations (30). In this paper, we show that the level of EF-la also affects translational fidelity.(Preliminary accounts of some of these results have been reported elsewhere [J. M. Song and S. W. Liebman, Yeast 2:S364, 1986; S. W. Liebman, J. M. Song, J. All-Robyn, E. Griffin, and D. Kelley-Geraghty, NATO ASI Ser. H14: [403][404][405][406][407][408][409][410][411][412][413][414] 1988].)A DNA clone containing the TEF1 gene complements asu9. Saccharomyces cerevisiae strain SL680-4B (a asu9-1 sup45-2 leu2-1 met8-1 trpl-1 ade3-26 hisS-2 lys2-1 canl-132 ura3-52) was chosen as the host for selection of asu9 complementing clones. In this strain, the asu9-1 mutation prevented growth on medium containing trichodermin (0.5 ,ug/ml; kindly provided by Leo Pharmaceutical Products), and the asu9-1 and sup45-2 mutations together permitted only a low * Corresponding author. and replated on medium with uracil, leucine, and methionine omitted and containing 0.5 jxg of trichodermin per ml to select for asu9 complementing clones. Plasmid pJS7, isolated from one of the fastest-growing transformants on this medium, caused both a slight reduction in the trichodermin sensitivity and enhanced suppression of leu2-1 and met8-1 upon retransformat...

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The Saccharomyces cerevisiae DNA repair gene RAD6 encodes a ubiquitin-conjugating enzyme which polyubiquitinates histones in vitro. Here we show that mutations in rad6 increase the frequency of transposition of the retrotransposon Ty into the CAN1 and URA3 loci. Using isogenic RAD6 and rad6 strains, we measured a more than 100-fold increase in the spontaneous rate of retrotransposition due to rad6, although there was no increase in the Ty message level. This is the first time that a mutation in a host gene has been shown to result in an increased rate of retrotransposition.

We examined Ty-mediated genomic rearrangements in three related mitotically dividing haploid yeast strains having the same configuration of Ty elements in the CYC1-sup4 interval of chromosome X. Surprisingly, quite different types and frequencies of rearrangements were found in the three strains. In one strain we found only Ty-mediated deletions, which occurred with a frequency of about 1 X 10(-6). Another strain yielded similar deletions, but approximately one-third of these were accompanied by adjacent Ty-mediated inversions. A third strain was found to have an extremely high rate of inversion/reinversion between two of the three Ty elements. This rate was conservatively estimated to be 1.4 +/- 0.2 X 10(-2) per cell per generation, which is at least 2 orders of magnitude higher than previously reported values for Ty-mediated rearrangements. These data provide evidence that local regions of the genome can, in some cases, be much more fluid than had been previously believed.