The stage specific inhibition of Dictyostelium discoideum spore germination by the mutagen 4-nitroquinoline 1-oxide.

The drug 4-nitroquinoline 1-oxide (4NQO) is a potent inhibitor of Dictyostelium discoideum spore germination. This inexpensive, water soluble drug is active at a concentration of 5 micrograms/ml (26 microM) and permeates the spore at all stages in germination. Spores subjected to 4NQO treatment exhibit an irreversible blockage of myxamoebae emergence, but spore activation, post-activation lag, and swelling are not affected. Swollen 4NQO-treated spores lose the outer two spore walls but lack the ability to degrade the innermost wall. The drug does not affect oxygen uptake during post-activation lag or swelling, and only a stage specific depression in O2 uptake is observed when control spores begin to release myxamoebae. When added early in germination, 4NQO blocks the incorporation of [3H] uracil into a cold trichloroacetic acid (TCA) insoluble fraction by 98%. However, when the drug is added midway through germination and followed by a pulse labelling period of 1 h, only 65% inhibition of RNA synthesis is observed. This lack of complete inhibition may occur because the drug requires metabolic activation; thus, new rounds of RNA synthesis may have initiated before the drug became fully activated. 4NQO also blocks the de novo expression of beta-glucosidase activity when added early in germination. Additionally, we observe that vegetative cellular slime mold cells are 100 times more resistant than spores to 4NQO-induced damage. Taken together, our results support the observation that RNA synthesis is only required for the emergence stage of germination and that dormant D. discoideum spores may lack efficient excision repair mechanisms.