Cytoplasmic extracts of HeLa cells contain particles which, after fixation, equilibrate in CsCl gradients at densities of 1.40 to 1.44 g/ml (Perry & Kelley, 1966; Miller, 1972) and it has been suggested that they may contain messenger RNA. A possibly analogous particle bearing virus RNA was found in cells infected with poliovirus (Miller, 1972). Isopycnic sedimentation of ribonucleoprotein particles in CsCl solution relies on their prior fixation with formaldehyde (Spirin, Belitsina & Lerman, 1965; Perry & Kelley, 1966) or glutaraldehyde (Baltimore & Huang, 1968): omission of this step results in the dissociation of the complexes into protein and RNA. On the other hand, virus particles are often stable without fixation in CsCl gradients and it is possible, therefore, to examine the effect of fixation on their buoyant density. The buoyant density of newly matured poliovirus particles increases sharply on fixation with formaldehyde (Fenwick & Wall, 1972), and further details of this effect are given here.


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