The picornaviruses have a wide range of buoyant densities in caesium chloride. Whereas the density of the pH 3-stable viruses is 1.34 g./ml. (Mattern, 1962; Schaffer & Frommhagen, 1965), the acid-sensitive rhinoviruses and foot-and-mouth disease viruses have densities of 1.38–1.41 g./ml. (Dans, Forsyth & Chanock, 1966; Chapple & Harris, 1966; McGregor, Phillips & Mayor, 1966; Gerin 1968) and 1.43 g./ml. (Trautman & Breese, 1962; Wild & Brown, 1967). Although the reason for this difference in density is not understood, it seems likely that the higher values obtained with the acid-labile group are due to reaction of the caesium ions with the more accessible RNA of these viruses (McGregor , 1966). Recently, however, McGregor & Mayor (1968) suggested, on the basis of comparative measurements of the strand lengths of the ribonucleoproteins isolated from strains of poliovirus and rhinovirus, that the higher buoyant density of the rhinovirus was due to the high molecular weight (4 × 10) of the virus RNA.


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