Dennis L. Welker

Identification of an endogenous plasmid in Dictyostelium discoideum.

A plasmid has been discovered in a strain of the eukaryote, Dictyostelium discoideum, which has an unstable, non‐chromosomal, cobalt resistance phenotype. The plasmid, termed Ddp1, is ˜13.5 kbp in size and is found in the nucleus. It has an A‐T content typical of Dictyostelium DNA as judged by its restriction enzyme digestion pattern, and it is not related to either mitochondrial or ribosomal DNA. Similar or identical plasmids have been found in two original, cobalt‐sensitive, isolates, NC4 and V12, but no plasmid was detected in three other isolates (WS472, WS526, WS584). The plasmid codes for non‐essential functions since it is absent from the latter isolates, and it is lost from mutant strains which are capable of axenic growth.

Genetic Loci Associated with Altered Resistance to Microtubule Inhibitors and with Spore Shape in Dictyostelium discoideum

SUMMARY: Mutations at 18 loci which map to five linkage groups in Dictyostelium discoideum are shown to affect resistance to antimicrotubule agents (including coumarin). The resistance or sensitivity mutations are classified according to whether their effects are limited to antimicrotubule agents, or whether cross-resistance or sensitivity to other compounds (notably acriflavin and cycloheximide) is observed. The latter class is likely to include mutations affecting permeability and is probably not of interest in the search for mutations directly affecting the cytoskeleton. All four acriflavin-resistance loci (acrA to acrD), many coumarin-sensitivity loci (couB to couE, couI), and both arsenate-resistance loci (arsA, arsB) fall into this category. Many mutants isolated on the basis of resistance to antimicrotubule agents (e.g. benlate, thiabendazole) in fact map at acrA or acrC. Some mutations affecting resistance to microtubule inhibitors also affect spore shape. Analysis of mutations such as couG370 and couH361, which affect coumarin sensitivity, spore shape, temperature sensitivity and, in the case of couH361, thiabendazole sensitivity provides the possibility of an ultrastructural approach to the study of the cytoskeleton in D. discoideum.

Genetic Analysis and Phenotypic Characterization of Effects on the Cytoskeleton of Coumarin-sensitivity Mutations in Dictyostelium discoideum

Summary: Five coumarin-sensitivity mutations were assigned to loci on three linkage groups: couB352 to group I: couC356, couE353 and couF354 to group II; and couD355 to group III. Coumarin inhibited cell division, affected cell shape and induced EDTA-sensitive cell–cell adhesion in vegetative cells at lower concentrations in coumarin-sensitive mutants (1.0 to 2.5 mm) than in wild-type strains (3.0 mm). One mutant, HU609 (couB352), lysed in the presence of coumarin (1.6mm). Coumarin, thiabendazole, and cambendazole induced multiple tips in pseudoplasmodia. Coumarin, unlike thiabendazole and cambendazole, did not induce metaphase arrest or haploidization of diploids. The couA351 and couF354 coumarin-sensitivity mutations are linked to growth temperature-sensitivity mutations, tsgK21 and tsgT360 respectively, which may be pleiotropic effects of the coumarin-sensitivity mutations. Exposure of vegetative cells of strains carrying either the couA351 or the couF354 mutation to the restrictive temperature (28°C) induced cell rounding and cell–cell adhesion similar to that seen on treatment with coumarin. Wild-type strains, other coumarin-sensitive strains and other temperature-sensitive strains did not exhibit this phenotype at 28°C. Furthermore, temperature-sensitivity and coumarin-sensitivity co-reverted in temperature-resistant derivatives selected from strains carrying the couA351/tsgK21 mutation.

Temperature-sensitive DNA repair mutations in the cellular slime mould Dictyostelium discoideum

SummaryThe nonallelic radB13 and radG5 DNA repair mutations lower the plating efficiency at 26.5 °C of unirradiated D. discoideum cells. Revertants selected on the basis of resistance to gamma rays or UV from strains bearing either the radB13 or radG5 mutations are no longer temperature-sensitive at 26.5 °C. Unlike the wild-type and radiation-resistant revertants, survival following UV irradiation is lower at 24.5 °C than at 17.0 °C or 21.0 °C in strains carrying either the radB13 or radG5 mutation. We conclude that the gene products of the radB and radG loci probably affect normal cell growth by affecting DNA metabolism. Seven radiation-sensitive mutations that effect six loci other than radB and radG do not have temperature-sensitive phenotypes.