CADMIUM MODIFICATION OF NUCLEOLAR FUNCTION AND STRUCTURE IN PHYSARUM-POLYCEPHALUM

Exposure of Physarum to CdSO4, 1.5 × 10-3 m for 2-3 hr in early S phase (the onset of DNA synthesis and nucleolar reconstruction) resulted in the appearance of ring-shaped nucleoli. We have investigated the effects of cadmium on nucleolar (ribosomal) RNA synthesis during this interval with acrylamide-agarose gel electrophoresis. Cadmium significantly depressed [3H]uridine incorporation into rRNA, with the extent of inhibition increasing with time. The effect of cadmium on RNA synthesis was quantitative, not qualitative: Cadmium depressed the amplitude of individual RNA peaks but not their distribution in RNA profiles. Cadmium exposure did not result in the appearance of new (or altered) rRNA peaks, induce detectable degradation/turnover of RNA, or alter rRNA processing. Surprisingly, no significant difference in rRNA content and distribution was observed in cells treated with cadmium which induced rings and shorter exposures which did not, suggesting that the inhibition of rRNA synthesis and the formation of ring-shaped nucleoli are coincident manifestations of cadmium toxicity at the cellular level but are not causally related. © 1979.