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Biomedical subjects

R K Mortimer

Publications and source records attributed to R K Mortimer.

At least 55 records · Page 3Linked to original sources
PubMed1979

Replication in Saccharomyces cerevisiae of plasmid pBR313 carrying DNA from the yeast trpl region.

Plasmid pBR313 carrying a 1.4 kb EcoRI fragment from the yeast TRP1 region (designated pLC544) is capable of transforming yeast trp1 mutants to Trp+ at high frequency (10(3)--10(4) transformants/micrograms DNA). Transformation can be achieved either by using purified plasmid DNA or by fusion of yeast spheroplasts with partially lysed Escherichia coli [pLC544] protoplast preparations. The Trp+ yeast transformants are highly unstable, segregating Trp- cells at frequencies of 0.18 per cell per generation (haploids) and 0.056 per cell per generation (diploids) in media containing tryptophan. Plasmid pLC544 replicates autonomously in the nucleus of yeast cells and segregation of Trp-cells is associated with the complete loss of plasmid sequences. In genetic crosses, pLC544 is randomly assorted during meiosis and is carried unchanged through the mating process into haploid recombinants.

Cell Nucleus↗
PubMed1975

Two mutations which confer temperature-sensitive radiation sensitivity in the yeast Saccharomyces cerevisiae.

X-ray survival curves for two mutations, rad54 and rad55, in the yeast Saccharomyces cerevisiae are presented. These mutations confer temperature sensitive X-ray sensitivity; that is rad54 and rad55 strains display a wild type X-ray survival response at permissive temperatures and a radiosensitive X-ray survival response at restrictive temperatures. The survival response of cells which were shifted from a permissive to a restrictive temperature or vice versa at various post-irradiation times indicates that repair and fixation of X-ray induced lesions is largely complete three hours after X-irradiation. Experiments to determine the utilization sequence of the rad54 and rad55 gene products in the repair of X-ray induced damage suggest that the two products are required in an interdependent manner.

DNA Repair↗
PubMed1972

Conversion-associated recombination in yeast (hybrids-meiosis-tetrads-marker loci-models).

Gene conversion and conversion-associated reciprocal recombination have been studied in various Saccharomyces cerevisiae hybrids. In a sample of 11,023 unselected meiotic tetrads, 907 conversions were observed at the arg4, thr3, his1, and SUP6 loci. Of these conversions, 445 (or 49.1%) were associated with reciprocal recombination of bracketing markers no more than 20-centimorgans apart. For conversions of two other loci, his2 and thr1, for which the bracketing markers were more than 20-centimorgans apart, recombination frequencies were significantly greater than 50%. These findings are discussed in terms of current models of genetic recombination. It is suggested that all meiotic crossing-over is characterized by the recombination events that are associated with conversion.

Alleles↗
PubMed1970

L-asparaginase-deficient mutants of yeast.

Yeast L-asparaginase is a multimeric enzyme for which only a single structural gene has been found. Fourteen mutants deficient in L-asparaginase have been isolated, and they have been located at one site on the genetic map of Saccharomyces cerevisiae. The L-asparaginase gene (aspl) is located about 18 centimorgans from a gene governing tryptophan synthesis (trp4) on fragment 2 of the map.

Asparaginase↗
PubMed1970

Isolation of monosomics in yeast.

Tetraploid cultures of Saccharomyces cerevisiae triplex (A/A/A/a) at several loci were sporulated. All the diploid spores are expected to be homozygous (A/A) or heterozygous (A/a) and, hence, to have the dominant phenotype. Cells lacking one of the chromosomes (monosomics) may show the recessive phenotype (a/-). Therefore, spores that grew on complete medium but failed to grow on the relevant synthetic single-omission media were presumed to be monosomic. These isolates were further characterized by sporulation, and several stable monosomics were established.

Chromosome Aberrations↗
PubMed1969

Informational transfer in meiotic gene conversion.

Aberrant meiotic segregations attributable to intragenic events have been analyzed in an unselected sample of 1611 tetrads from three heteroallelic diploids of Saccharomyces cerevisiae. Reciprocal recombination between alleles accounts for only a minor fraction of the total aberrant tetrads, while the majority component is represented by single- and double-site conversions. The frequency of double-site conversion is inversely related to the physical length of the interallelic interval. Since double-site conversions do not yield prototrophs, their occurrence leads to biased estimates of intragenic distances. Conversion is viewed as a process of informational transfer distinct from conventional crossing-over. The implications of the findings for genetic fine structure mapping and evolutionary theory are discussed briefly.

Alleles↗