High frequency excision of Ty elements during transformation of yeast

The role of recombination and RAD52 in mutation of chromosomal DNA transformed into yeast

While transformation is a prominent tool for genetic analysis and genome manipulation in many organisms, transforming DNA has often been found to be unstable relative to established molecules. We determined the potential for transformation-associated mutations in a 360 kb yeast chromosome III composed primarily of unique DNA. Wild-type and rad52 Saccharomyces cerevisiae strains were transformed with either a homologous chromosome III or a diverged chromosome III from S. carlsbergensis. The host strain chromosome III had a conditional centromere allowing it to be lost on galactose medium so that recessive mutations in the transformed chromosome could be identified. Following transformation of a RAD+ strain with the homologous chromosome, there were frequent changes in the incoming chromosome, including large deletions and mutations that do not lead to detectable changes in chromosome size. Based on results with the diverged chromosome, interchromosomal recombinational interactions were the source of many of the changes. Even though rad52 exhibits elevated mitotic mutation rates, the percentage of transformed diverged chromosomes incapable of substituting for the resident chromosome was not increased in rad52 compared to the wild-type strain, indicating that the mutator phenotype does not extend to transforming chromosomal DNA. Based on these results and our previous observation that the incidence of large mutations is reduced during the cloning of mammalian DNA into a rad52 as compared to a RAD+ strain, a rad52 host is well-suited for cloning DNA segments in which gene function must be maintained.

Transformation-associated recombination between diverged and homologous DNA repeats is induced by strand breaks

Rearrangements within plasmid DNA are commonly observed during transformation of eukaryotic cells. One possible cause of rearrangements may be recombination between repeated sequences induced by some lesions in the plasmid. We have examined the mechanisms of transformation-associated recombination in the yeast Saccharomyces cerevisiae using a plasmid system which allowed the effects of physical state and/or extent of homology on recombination to be studied. The plasmids contain homologous or diverged (19%) repeats of the URA3 genes (from S. cerevisiae or S. carlsbergensis) separated by the genetically detectable ADE2 colour marker. Recombination during transformation for covalently closed circular plasmids was over 100-fold more frequent than during mitotic growth. The frequency of recombination is partly dependent on the method of transformation in that procedures involving lithium acetate or spheroplasting yield higher frequencies than electroporation. When present in the repeats, unique single-strand breaks that are ligatable, as well as double-strand breaks, lead to high levels of recombination between diverged and identical repeats. The transformation-associated recombination between repeat DNAs is under the influence of the RAD52 and RAD1 genes.

Gene isolation by complementation in Candida albicans and applications to physical and genetic mapping

We have isolated three genes, ARG57, SER57, and LYS1, on the basis of their function in Candida albicans. A C. albicans transformation vector containing the C. albicans URA3 gene, a Candida ARS sequence, and a portion of the Saccharomyces cerevisiae 2 microns circle containing the replication origin was constructed. Clones from genomic libraries in this vector were isolated by direct complementation of the auxotrophies in strain 1006 (arg57 ser57 lys1 ura3 MPA1). Transformants typically contain two to four plasmids in a mixed tandem multimer. A scheme to resolve mixed multimers into monomers in vivo by transformation of S. cerevisiae with Candida transformant DNA selecting Ura+ transformants was devised. Monomeric plasmids were then isolated by transformation of Escherichia coli with the S. cerevisiae transformant DNA. These were retested by transformation of strain 1006 to identify the specific plasmid that complemented the auxotrophy. The chromosomal locations of the genes were determined by hybridization to C. albicans chromosomes separated on contour-clamped homogenous electric field gels. We used these locations to assess the stability of individual C. albicans chromosomes in parasexual genetic analysis. The Lys(+)-complementing clone was shown to be LYS1 by complementation of S. cerevisiae lys1 mutants. These cloned genes help to align the Candida physical and genetic maps and provide additional markers for the transformation system.

Integration of heterologous genes into the chromosome of Saccharomyces cerevisiae using a delta sequence of yeast retrotransposon Ty

Distribution of a delta (delta) sequence of the Ty element on a chromosome of the yeast Saccharomyces cerevisiae was analysed by pulsed-field gel electrophoresis. More than 100 copies of the delta sequence were nonrandomly distributed on the chromosome. Using the delta sequence as a recombination site, mouse alpha-amylase and human beta-endorphin genes were introduced into the chromosomal DNA. The integration occurred on a particular chromosome in each case and the copy number was estimated as three to five. It was also found that single- or multi-copy integration occurred at a single or multiple sites on the particular chromosome. The integrants secreted alpha-amylase and beta-endorphin by three-to fivefold compared with single-copy integrants. This type of integration was mitotically stable over a period of 50 generations under non-selective conditions.

Ty RNA levels determine the spectrum of retrotransposition events that activate gene expression in Saccharomyces cerevisiae

To learn more about the variety of Ty elements capable of activating gene expression, we characterized 206 spontaneous Ty transpositions that activate the promoterless gene his3 delta 4. Most of the Ty elements appear to be full-length, although a few deleted elements were recovered. Over 95% of the insertions belong to the Ty1 family, and the rest are Ty2 elements. The excessive number of Ty1 transpositions was unexpected because there are only 2-fold more Ty1 than Ty2 elements in the yeast strains used in the selection. However, there is 20-fold more Ty1 than Ty2 RNA present in these yeast strains. This difference in RNA level explains the greater number of Ty1 verses Ty2 transpositions at his3 delta 4, because Ty elements transpose through an RNA intermediate. A similar association between the Ty transcript level and transpositional activation of his3 delta 4 is obtained in cells expressing GAL1-promoted Ty2-H556 or Ty2-917 elements, but only if the element does not contain a marker. Genetically marked Ty2-H556NEO and -917NEO elements transpose into and activate his3 delta 4 with the same efficiency as the previously characterized Ty1-H3NEO element, but are underrepresented relative to the levels of TyNEO transcript. We also found that chromosomal Ty transcripts are even more abundant than previously estimated and comprise about 1% of total cellular RNA.

Introduction of nonselectible 2 mu plasmid into [cir(o)] cells of the yeast S. cerevisiae by DNA transformation and in vivo site-specific resolution

The 2 mu DNA plasmid of the yeast Saccharomyces cerevisiae does not confer any known selectable phenotype to the host cell carrying it. Selection of cells transformed with purified 2 mu DNA therefore cannot be achieved, and the intracellular presence of 2 mu can only be assessed by molecular analysis of the DNA complement. In addition, 2 mu alone does not replicate in bacterial hosts, thus rendering its amplification by conventional methods impossible. We have isolated a shuttle plasmid, pBH-2L, generated by in vivo site-specific recombination between the endogenous 2 mu DNA plasmid and pRL, a pBR322 derivative containing the yeast LEU2 gene and one 2 mu repeat sequence associated with the origin of replication. This new shuttle plasmid has the property, when transformed into yeast, of undergoing site-specific recombinational resolution between its two direct repeat sequences. This releases 2 mu plasmid and pRL as individual molecules. The latter can undergo progressive mitotic loss during growth in nonselective medium, ultimately leaving leucine auxotrophic transformants that contain only 2 mu DNA plasmid. This system can be utilized to introduce 2 mu DNA alone into cells lacking it, thereby providing a novel means to study the biology and the molecular genetics of the plasmid and its potential practical applications as a vector.

The Saccharomyces cerevisiae genome contains functional and nonfunctional copies of transposon Ty1

Saccharomyces cerevisiae Ty elements are transposons closely related to retroviruses. The DNA sequence of a functional Ty element (TyH3) is presented. The long terminal repeat sequences are different, suggesting that TyH3 is a recombinant Ty element. A chromosomal Ty element near the LYS2 gene, Ty173, was found to be nonfunctional, even though it has no detectable insertions or deletions. The defect in Ty173 transposition is caused by a missense mutation giving rise to a Leu-to-Ile substitution in the TYB (pol) open reading frame. Several chromosomal Ty elements carry this lesion in their DNA, indicating that nonfunctional Ty elements are common in the yeast genome.

The Saccharomyces cerevisiae genome contains functional and nonfunctional copies of transposon Ty1

Saccharomyces cerevisiae Ty elements are transposons closely related to retroviruses. The DNA sequence of a functional Ty element (TyH3) is presented. The long terminal repeat sequences are different, suggesting that TyH3 is a recombinant Ty element. A chromosomal Ty element near the LYS2 gene, Ty173, was found to be nonfunctional, even though it has no detectable insertions or deletions. The defect in Ty173 transposition is caused by a missense mutation giving rise to a Leu-to-Ile substitution in the TYB (pol) open reading frame. Several chromosomal Ty elements carry this lesion in their DNA, indicating that nonfunctional Ty elements are common in the yeast genome.

Saccharomyces cerevisiae mutants that tolerate centromere plasmids at high copy number

Two yeast (Saccharomyces cerevisiae) mutants that tolerate centromere (CEN) plasmids at high copy number have been isolated. The mutations relieve the restraint normally imposed on plasmid copy number by a cloned CEN sequence. Our CEN plasmids specify resistance to G418 and are high copy plasmids only when the mutant host cells are grown on medium containing this antibiotic. The high copy number of the plasmids is independent of the specific cloned CEN sequence and recovered plasmids show no alteration in structure or function of the CEN DNA. The efficiency with which CEN plasmids go to high copy number is increased if the mutant cell is cotransformed by another CEN plasmid. The genomic mutation responsible for the high copy number (COP) is dominant and stable, and it segregates in a Mendelian manner. Homozygous COP/COP a/alpha diploids do not tolerate CEN plasmids at high copy number, suggesting that the mutation is regulated by mating type. The genomic DNA from both mutant cells contains an altered transposon (Ty) restriction fragment that cosegregates with the COP phenotype in crosses of mutant and wild-type strains. The mutations may be transposon-mediated events that identify a gene involved in centromere or mitotic spindle function.

Yeasts in molecular biology. Spheroplast preparation with Candida utilis, Schizosaccharomyces pombe and Saccharomyces cerevisiae

Efficient preparation of spheroplasts from Candida utilis, Saccharomyces cerevisiae, and Schizosaccharomyces pombe, using a purified mixture of enzymes from Trichoderma harzianum, is described. Limitations of other methods, and differences between yeasts are demonstrated.