CHEMICAL AND PHYSICAL CHARACTERIZATION OF “NUCLEAR CAPS” ISOLATED FROM BLASTOCLADIELLA ZOOSPORES

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Abstract
Lovett, James S. (Purdue University, West Lafayette, Ind.). Chemical and physical characterization of “nuclear caps” isolated from Blastocladiella zoospores. J. Bacteriol. 85: 1235–1246. 1963.—Electron micrographs of Blastocladiella zoospores have shown the nuclear cap to contain essentially all of the small (250 to 300 A) electron-dense particles of the cell. Preparations of clean, whole nuclear caps were isolated to study the composition of the intact organelles and their particulate contents. The cap is strongly basophilic, and is composed of 60% protein and 40% ribonucleic acid (RNA). It represents 18% of the dry weight, and contains 69% of the total RNA, of the spore. The amino acid composition of cap proteins is similar to the ribosomal protein of other organisms. The nuclear cap contents have been extracted and isolated by high-speed centrifugation. More than 95% of the material has a sedimentation coefficient of 83S in 0.005 m Mg. The 83S particles form aggregates at higher Mg concentrations and dissociate to yield 63S and 41S peaks at low Mg concentrations. Purified cap particles contain 37% protein and 63% RNA. The RNA has a nucleotide composition (in moles per cent) of 18.5% cytidylic, 26.2% adenylic, 31.8% guanylic, and 23.5% uridylic acid. The particles contain a latent ribonuclease, which can be activated by urea, and are susceptible to degradation by added pancreatic ribonuclease. The available evidence supports a concept of the zoospore nuclear cap as an unusual intracellular “packet” of ribosomes.