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In the course of a study on the arrangement of the ribonucleoprotein in the bacterial cell (12), structural features were observed in purified 50S particles isolated from Bacillus subtilis. A thorough understanding of isolated purified components of the ribonueleoprotein appears to be a prerequisite for an understanding of the organization of the cytoplasm. Although much work is being done 6n. the biochemical properties of the ribosomes, the architecture of these minute organelles is, as yet, little understood. The ribosomes of B. subtilis, like those of E. coli (21), are composed of 50S and 30S subunits (23). Few interpretations of the fine structure of the purified 50S component of bacterial ribosomes have been published. From electron mlcrographs of shadowed preparations, Hall and Slayter (7) interpreted the 50S subunit as being an almost spherical, slightly flattened particle measuring 170 x 140 A. Huxley and Zubay (11) presented similar results after negative staining with phosphotungstic acid. The 50S particles are described as dome-shaped, measuring approximately 160 x 160 x 130 A. Spirin et al. (18) proposed a model for the 50S subunit derived from a heterogeneous mixture of particles obtained from cells treated with chloromycetin (CM particles). These CM particles were found to contain 75% RNA and 25% protein (14), whereas intact 50S and 30S ribosomal subunits would contain 63% RNA and 37% protein (21). The CM particles have sedimentation coefficients of 25S and 18S (17), and are supposed to be the protein-deficient precursors of the 50S and 30S components, respectively (19). The 25S CM particle would consist of five large subunits arranged in a pentagon, and this structure would be basic to the normal 50S particle (18, 13). The 50S subunit is supposed to be constructed from one folded ribonucleoprotein strand (18). In a hypothetical model the 50S particle resembles a pentagonal prism (18). Hart (8, 9), using a highly purified frozen-dried 50S preparation, arrived at a demonstration of surface features by means of the shadow-casting technique. These features are interpreted as arising from an approximately 35-A-thick, coiled ribonucleoprotein filament similar to that visualized by Spirin et al. (18). Hart 's point of view is illustrated by a tentative wire model showing a rounded particle (9) instead of Spirin's pentagonal prism. To the best of our knowledge, no fresh proposals for the "compact" ribosomal fine structure have been published since the suggestions made by Hart. The term "compact" has been used (18) to stress the difference between normal isolated ribosomes and their so called unfolded forms (18, 22, 5, 6).

FINE STRUCTURE OBSERVED IN 50S RIBOSOMAL SUBUNITS OF BACILLUS SUBTILIS