In 15 to 60% sucrose density gradients the distribution of standard preparations of influenza viruses measured by haemagglutination or infectivity was always bimodal. Approximately 17% of virus particles sedimented in a diffuse leading peak (fraction 1), while the rest was confined to a second homogeneous peak (fraction 2). Virus particles comprising fraction 1 were less dense (1.250 g./cm.) than those of fraction 2 (1.257 g./cm.).

RNA labelled with P was extracted by phenol + sodium dodecyl sulphate from four strains of influenza A virus ( 8, , 2//1/57 and fowl plague) and from one strain of influenza B virus (). The sedimentation characteristics of each preparation of RNA on sucrose density gradients varied under different conditions of ionic strength. In the presence of 0.1 -NaCl + 0.001 -EDTA, RNA from fraction 2 could be resolved into at least two species. The predominant species had a sedimentation coefficient of 18 to 21 and (with the exception of the 2 RNA) about 10 to 15% of the total RNA sedimented as a diffuse band with a sedimentation coefficient of 32 to 42 . The and strains contained a third species of RNA, with a sedimentation coefficient of 7 to 9 . In 0.005 -EDTA without NaCl the RNA from fraction 2 ( 8) sedimented principally as a 14 component, and the 32 to 42 component was not found. Fraction 1 virus particles also contained several RNA components when analysed under conditions of high ionic strength. A 7 to 9 RNA species was found in amounts equal to, or greater than, the 18 to 21 RNA component, together with a small amount of ‘38’ RNA. In the presence of 0.005 -EDTA the 38 component was not found, but three components with sedimentation coefficients of 16 , 9 and 5 were present in approximately equivalent amounts. From these findings it is concluded that the RNA of influenza viruses consists largely of 18 molecules, under defined conditions of ionic strength, and that the 32 to 42 component is an artifact caused by aggregation. It is further concluded that sucrose gradients containing 0.1 -NaCl are unsatisfactory for studying the sedimentation characteristics of influenza RNA.


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