First Observations

By 1940 Albert Claude had succeeded in isolating from animal cells cytoplasmic RNA-containing granules that were smaller than mitochondria. These granules varied from 50 to 200 m^ in diameter and later Claude began calling them microsomes. Chemical analyses indicated that Claude's microsomes were "phospholipid-ribonucleoprotein complexes".

On the other hand, cytochemical studies by Caspersson (1941) and Brachet (1942) demonstrated the preferentially cytoplasmic localization of RNA and the existence of a correlation between the amount of RNA in the cytoplasm and the intensity of protein synthesis. Later, a number of scientists reported on the isolation of RNA-containing particles, which were much smaller than microsomes, from the cytoplasm of animal and plant cells as well as from bacteria. Electron microscopy and sedimentation analysis in the ultracentrifuge indicated that these particles were compact; had a more or less spherical shape; were homogeneous in size, with a diameter of 100 to 200 A; and exhibited sharp sedimentation boundaries corresponding to sedimentation coefficients of from 30S to 100S. The first unambiguous evidence that such particles from bacteria are ribonucleoproteins was probably obtained by Schachman, Pardee, and Stanier in 1952.

Improved techniques of microtomy and electron microscopy of ultrathin sections of animal cells resulted in the detection of uniform dense granules, with a diameter of about 150 A, directly in the cell. Palade's electron microscopic studies (1955) demonstrated that small dense granules are abundant in animal cell cytoplasm. These granules were seen either attached to the membrane of the endoplasmic reticulum or freely dispersed throughout the cytoplasm. Claude's microsomes were identified as fragments of the endoplasmic reticulum with these granules attached. It became clear that the Palade granules were ribonucleoprotein particles and that they accounted for most of the cytoplasmic RNA involved in protein synthesis (Palade & Siekevitz, 1956).

Purified preparations of ribonucleoprotein particles were isolated and studied in several laboratories between 1956 and 1958; these investigations included isolating 80S particles from yeast, accomplished by Chao and Schachman (1956); from plants, by Ts'o, Bonner, and Vinograd (1956); and from animals, by Petermann and Hamilton (1957); and isolating 70S particles from bacteria (E. coli), by Tissieres and Watson (1958). In 1958 the first symposium devoted to these particles and their participation in protein biosynthesis took place (First Symposium of the Biophysical Society at the Massachusetts Institute of Technology, Cambridge, Mass., February 5, 6, and 8, 1958); during this symposium it was suggested that ribonucleoprotein particles be called ribosomes (see Roberts, 1958).

Studies of the functional role played by ribosomes proceeded hand-in-hand with their structural description. The experiments of Zamecnik and co-workers provided the first convincing demonstration that ribonucleoprotein particles of microsomes are responsible for the incorporation of amino acids into newly synthesized proteins (Littlefield et al., 1955). This was followed by other experiments conducted at the same laboratory which demonstrated that the free ribosomes unattached to the endoplasmic reticulum membranes also incorporate amino acids and synthesize the protein released into the soluble phase (Littlefield & Keller, 1957). The functions of bacterial ribosomes were the subject of intense studies conducted by Roberts' group; the 1959 publication of McQuillen, Roberts, and Britten finally established that proteins are synthesized on ribosomes and then distributed throughout the bacterial cell.

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