In homogenates of HeLa cells 10 to 20% of the ribosomes are bound to membranous structures (Attardi, Cravioto & Attardi, 1969; Rosbash & Penman, 1971; Roumiantzeff, Maizel & Summers, 1971). After treatment with sodium deoxycholate (DOC) to dissolve the membranes, some of these ribosomes equilibrate in density gradients of CsCl at the same position as free ribosomes but others are less dense, having a higher ratio of protein to RNA (Rosbash & Penman, 1971). Large polysomes bearing virus RNA appear in cells infected with poliovirus in the presence of actinomycin D (Penman 1963). These polio polysomes are attached to membranes until treated with DOC (Penman, Becker & Darnell, 1964; Caliguiri & Tamm, 1970; Roumiantzeff 1971). We report here studies of their density in CsCl gradients.

HeLa cells grown in suspension (Harris & Watts, 1962) were resuspended in phosphate-buffered saline containing Ca and Mg (Oxoid Ltd.) at 10 cells/ml and kept at 0 °C for 30 min after adding about 50 p.f.u./cell of poliovirus type 1, strain ts (Cooper, Johnson & Garwes, 1966).


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