A genetic study of aggregation in the cellular slime mould Dictyostelium discoideum using complementation analysis

A series of aggregation-deficient (aggregateless) mutants were isolated in genetically marked haploid strains of the cellular slime mould Dictyostelium discoideum. Diploids were produced from pairs of such haploid mutants by a fusion system based on this organism's parasexual cycle. The diploids were isolated from the haploids by using complementation of non-allelic growth-temperature-sensitive mutations and selection at the restrictive temperature. Complementation between the aggregateless mutations uas then assessed in 419 diploids so formed. The non-complementing aggregateless mutations fell into five complementation groups (ago A, B, C, D, and E) and a dominant aggregation class that allowed little or no aggregation when present in a diploid with any of the other mutations tested or the parental wild type. Complicating factors, including partial dominance, multiple mutations, and possible interallelic conplementation, are discussed. Data on the linkage of the aggregateless mutations was obtained by using recessive drug resistance mutations on three linkage groups to segregate haploids from the diploids. Calculations from our results suggest a genetic complexity of about 50 genes that are specific and essential for aggregation.

Evidence for the existence of two types of cAMP binding sites in aggregating cells of Dictyostelium discoideum

Inhibition by dithiothreitol of the cell-bound phosphodiesterase has allowed the measurement of cAMP binding to aggregation-competent Dictyostelium discoidium amebae. Two classes of binding sites were demonstrated: Type 1, of low affinity (Kd approximately 160 nM) and high capacity (Ro approximately 1 X 10(5) sites per cell); and Type 2, of high affinity (Kd approximately 9 nM) but low capacity (Ro approximately 1.5 X 10(4) sites per cell). Both sites are developmentally regulated and are expressed during aggregation. The specificities of both sites are consistent with the specificity observed in vivo for the chemotactic response.

Linkage analysis in Dictyostelium discoideum using temperature-sensitive growth mutants selected with bromodeoxyuridine

Amoebae of the cellular slime mold Dictyostelium discoideum grown in the presence of bromodeoxyuridine are killed on exposure to near-ultraviolet light. By using this phenomenon, a method was devised by which mutants of D. discoideum that are temperature-sensitive for growth can be readily obtained. Three such mutants have been characterized genetically and each was found to be associated with a different linkage group. Two of these linkage groups have not previously been described.

The isolation and subfractionation of plasma membrane from the cellular slime mould Dictyostelium discoideum

A procedure for the isolation and separation of three different subfractions of plasma membrane from the cellular slime mould Dictyostelium discoideum is described. The cells were disrupted by freeze-thawing in liquid N(2) and plasma membranes were purified by equilibrium centrifugation in a sucrose gradient. The cell surface was labelled with radioactive iodide by using the lactoperoxidase iodination method. Alkaline phosphatase was identified as a plasma-membrane marker by its co-distribution with [(125)I]iodide. 5'-Nucleotidase, which has been widely described as a plasma-membrane marker enzyme in mammalian tissues, was not localized to any marked extent in D. discoideum plasma membrane. The isolated plasma membranes showed a 24-fold enrichment of alkaline phosphatase specific activity relative to the homogenate and a yield of 50% of the total plasma membranes. Determination of succinate dehydrogenase and NADPH-cytochrome c reductase activities indicated that the preparation contained 2% of the total mitochondria and 3% of the endoplasmic reticulum. When the plasma-membrane preparation was further disrupted in a tight-fitting homogenizer, three plasma-membrane subfractions of different densities were obtained by isopycnic centrifugation. The enrichment of alkaline phosphatase was greatest in the subfraction with the lowest density. This fraction was enriched 36-fold relative to the homogenate and contained 19% of the total alkaline phosphatase activity but only 0.08% of the succinate dehydrogenase activity and 0.34% of the NADPH-cytochrome c reductase activity. Electron microscopy of this fraction showed it to consist of smooth membrane vesicles with no recognizable contaminants.

Isolation of germination mutants of Dictyostelium discoideum

A simple method to separate spores from amoebae of Dictyostelium discoideum has been devized and used to isolate spore germination mutants. A subclass of these mutants is temperature sensitive for germination and growth.

Alternative developmental pathways determined by environmental conditions in the cellular slime mold Dictyostelium discoideum

The cellular slime mold Dictyostelium discoideum grows in the soil as a population of independent, uninucleate amoebae. Upon entrance to the stationary phase, the amoebae collect in multicellular aggregates to form organized fruiting bodies composed of spores and stalk cells. Depending upon environmental conditions, the developing aggregate either constructs the fruiting body at the site of aggregation or transforms into a structure that can migrate to a more favorable location. Environmental conditions that favor migration are (i) the accumulation of metabolite(s) produced by the aggregate and (ii) a low ionic strength in the substratum. Conditions that prevent migration or that stop a migrating slug are (i) the presence of buffer and (ii) illumination by overhead light.

Biosynthesis of the pyrimidine moiety of thiamine. A new route of pyrimidine biosynthesis involving purine intermediates

1. The pattern of distribution on the purine pathway of mutants of Salmonella typhimurium LT2 that had the double growth requirement for a purine plus the pyrimidine moiety of thiamine (ath mutants) indicated that purines and the pyrimidine moiety of thiamine share the early part of their biosynthetic pathways, and that 4-aminoimidazole ribonucleotide (AIR) is the last common intermediate. Two mutants that at first appeared anomalous were further investigated and found not to affect this deduction. 2. The ribonucleoside form of AIR (AIR(s)) satisfied the requirements both for a purine and for the pyrimidine moiety of thiamine of an ath mutant. 3. Methionine was required for the conversion of AIR into the pyrimidine moiety. 4. Radioactive AIR(s) was converted into radioactive pyrimidine moiety by an ath mutant without significant dilution of specific radioactivity. 5. Possible mechanisms for pyrimidine-moiety biosynthesis from AIR are discussed.

Precursors of the pyrimidine moiety of thiamine

1. A method was devised for obtaining the pyrimidine moiety of thiamine in a pure form after its excretion into the medium by de-repressed washed-cell suspensions of mutants of Salmonella typhimurium LT2. 2. By using amino acid-requiring mutants, this excretion of pyrimidine moiety was shown to be dependent on the presence of both methionine and glycine. 3. In the presence of either [Me-(14)C]methionine or [G-(14)C]methionine, methionine-requiring mutants did not incorporate radioactivity into the pyrimidine moiety. 4. In contrast, both [1-(14)C]glycine and [2-(14)C]glycine were incorporated into the pyrimidine moiety excreted by glycine-requiring mutants, and this occurred with little or no dilution of specific radioactivity. 5. The possible requirement for methionine as a cofactor and the significance of the incorporation of both carbon atoms of glycine are discussed.

The de-repression of thiamine biosynthesis by adenosine a tool for investigating this biosynthetic pathway

1. Growth of Salmonella typhimurium LT2 in the presence of adenosine was shown to cause enormous synthesis of thiamine in washed-cell suspensions. 2. Evidence that this was due to de-repression and not an accumulation of precursors was obtained by using a mutant blocked in the biosynthesis of the thiazole moiety, which showed a similarly large synthesis of the pyrimidine of thiamine. 3. The specific requirements for a source of energy, nitrogen and sulphur were investigated, and indicated new synthesis in this system.

The control mechanism of thiamine biosynthesis a model for the study of control of converging pathways

1. Thiamine or the pyrimidine moiety of thiamine added in excess to a growing culture of Salmonella typhimurium LT2 repressed subsequent thiamine synthesis in non-growing organisms. 2. A mutant unable to convert added pyrimidine moiety into thiamine was not repressible by the pyrimidine, showing that thiamine, not the pyrimidine, was the repressor. 3. Thiamine repression occurred at 40mmug. of thiamine/mg. dry wt. or above and de-repression occurred at 30mmug. of thiamine/mg. dry wt. or below. 4. Thiamine controlled the pyrimidine and thiazole pathways at the same concentration and to the same extent. 5. Biosynthesis of the thiazole moiety had, in contrast with biosynthesis of the pyrimidine moiety, an additional feedback inhibition control that allowed utilization of the exogenous thiazole. 6. The enzymes joining the pyrimidine and thiazole moieties were repressible by high concentrations of thiamine. 7. Thiamine was rapidly converted into thiamine pyrophosphate and this appeared to be the active repressor. 8. Theoretical aspects of control of converging pathways are discussed.