Abstract
Nuceloli were isolated from the cellular slime mold, Dictyostelium discoideum, strain A-3 without treating the cells with detergent. Electron microscopy showed that the nucleoli were clean and free of contamination by other subcellular organelles. The nucleoli were not surrounded with nucleolus-associated chromatin, but were tightly attached to the nuclear envelopes. The nucleoli contained very little DNA (1.2 %), but contained considerable RNA (16.3 %). Analysis of the protein components on a SDS-polyacrylamide gel showed many protein bands in a range of molecular weight between. 10, 000 and 50, 000 daltons that are related to ribosomal proteins in the cytoplasm. The phosphorylation profile in vitro indicated that most of the nucleolar proteins were labeled to some extent with [32P]. [5-3H]UTP incorporation into the nucleoli in vitro was inhibited by actinomycin D, but was completely insensitive to α-amanitin; thus, the isolated nucleoli were morphologically intact, and were engaged in ribosomal RNA synthesis.
