Sensitization of rad mutants of Dictyostelium discoideum to ultraviolet light by postirradiation treatment with caffeine

Live imaging of theDictyosteliumcell cycle reveals widespread S phase during development, a G2 bias in spore differentiation and a premitotic checkpoint

The regulation of the Dictyostelium cell cycle has remained ambiguous owing to difficulties in long-term imaging of motile cells and a lack of markers for defining cell cycle phases. There is controversy over whether cells replicate their DNA during development, and whether spores are in G1 or G2 of the cell cycle. We have introduced a live-cell S-phase marker into Dictyostelium cells that allows us to precisely define cycle phase. We show that during multicellular development, a large proportion of cells undergo nuclear DNA synthesis. Germinating spores enter S phase only after their first mitosis, indicating that spores are in G2. In addition, we demonstrate that Dictyostelium heterochromatin is copied late in S phase and replicates via accumulation of replication factors, rather than recruitment of DNA to pre-existing factories. Analysis of variability in cycle times indicates that regulation of the cycle manifests at a single random transition in G2, and we present the first identified checkpoint in Dictyostelium, which operates at the G2-M transition in response to DNA damage.

Caffeine, an inhibitor of endocytosis in Dictyostelium discoideum amoebae

The effect of the trimethylxanthine, caffeine, was examined on the growth and endocytosis pathways of the vegetative amoebae of the cellular slime mold Dictyostelium discoideum. Caffeine at concentrations of 1.5-3 mM was found to inhibit axenic growth, fluid-phase pinocytosis, and secretion of lysosomal enzymes. Cell viability was unaffected by incubation for 16 hours with 5 mM caffeine but decreased markedly thereafter. Phagocytosis of the bacterium Escherichia coli by Dictyostelium amoebae was also inhibited by caffeine, although at concentrations twofold to threefold higher. Caffeine rapidly entered into amoebae to reach an equilibrium between extracellular and intracellular concentrations, and it was not appreciably metabolized by Dictyostelium. Inhibition of growth and endocytosis was reversible upon removal of the drug and was partially counteracted by 10 mM adenosine. As caffeine discharged intracellular calcium stores in Dictyostelium (Abe et al., 1988), its inhibitory effect on endocytosis could result from the perturbation of calcium homeostasis. In agreement with this hypothesis, the cation La3+ (10 microM), a Ca2(+)-transport inhibitor, also strongly reduced fluid-phase pinocytosis.