Some genetic and biochemical aspects of the regulatory program for slime mold development

Actinomycin D and the hormonal induction of amylase synthesis in barley aleurone layers

The induction of amylase synthesis in barley aleurone layers by gibberellic acid is most sensitive to Actinomycin D (AM) over a short interval late in the lag phase. The duration of the lag phase may be extended as much as 3 fold by lower temperatures over the range 30° to 15° C. At each temperature the AM sensitive period remains close to the end of the lag phase, the period we have previously determined as the stage less sensitive to temperature.Lack of sensitivity to the inhibitor at other periods is not due to failure to penetrate, or to degradation. AM has no effect on tissue respiration, leucine, uridine or uracil uptake, leucine incorporation, or leucine pool size. At all stages it inhibits uracil and uridine incorporation into RNA. Thus AM probably acts by inhibiting RNA synthesis.

Ultraviolet irradiation of the vegetative cells of Dictyostelium discoideum

Experiments on the effect of ultraviolet (UV) light on the survival of vegetative Dictyostelium discoideum cells indicate that this is a relatively UV-resistant organism. Several factors suggest the presence of some type of repair process. Experiments to test for liquid-holding recovery and simple photoreactivation yielded negative results. Acriflavine and caffeine were utilized to possibly interfere with dark repair. Acriflavine produced no UV sensitization, but caffeine did cause a concentration-dependent decrease in survival of irradiated cells. When UV-irradiated cells were illuminated with photoreactivating light while suspended in caffeine, the survival increased above that for cells treated with caffeine alone, suggesting an overlap between lesions repaired by photorepair and dark repair. Growth experiments showed that UV light induced a dose-dependent division delay, followed by a period of retarded growth characterized by the presence of a constant fraction of nonviable cells in the irradiated population. The delayed exposure of cells to caffeine after irradiation showed that the magnitude of the caffeine sensitization diminished throughout the division-delay period. An action spectrum indicated probable nucleoprotein involvement in the induction of division delay. UV light retarded ribonucleic acid and protein synthesis and temporarily blocked deoxyribonucleic acid synthesis. However, synthesis of all three accelerated prior to the end of the division-delay period and then closely paralleled the increase in cell number.