Ribosomes were isolated from mycobacteria (BCG) by differential centrifugation by use of a modification of Nirenberg's procedure. Individual components of the ribosomal spectrum were resolved in linear sucrose density gradients. Sedimentation constants of the individual components were determined by analytical ultracentrifugation. Ribosomes from escherichia coli labeled with 14C-uracil were used as markers for an independent confirmation of the identity of individual peaks. The typical ribosomal spectrum of 70S units and 50S and 30S subunits was obtained in tris(hydroxymethyl)aminomethane buffer with 0.01 m Mg(CH3COO)2. This corresponds to the ribosomal spectrum obtained in E. coli under comparable conditions. In 0.0001 m Mg2+, the 70S units were dissociated to 50S and 30S particles. Pancreatic ribonuclease produced no significant changes in main ribosomal components. The isolation techniques used in this work precluded the recognition of polyribosomes
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