The envelope spikes of Sindbis and Semliki Forest virus are arranged in a T=4 icosahedral surface lattice and, by deduction, it has been suggested that the nucleocapsid proteins are similarly arranged. After treatment of the virions with a non-ionic detergent the released nucleocapsids sediment in sucrose gradients at about 160S and 150S and have densities in CsCl of 1.42 g/ml and 1.425 g/ml, respectively, for Sindbis and Semliki Forest virus. At pH 6.0 Sindbis nucleocapsids do not contract like those of Semliki Forest virus. Nucleocapsids of both viruses are sensitive to the action of ribonuclease but only those of Semliki Forest virus undergo a drastic structural rearrangement due to the treatment. EDTA treatment in hypotonic conditions results in a decrease in the S-value for both particles. Electron micrographs show that the SFV nucleocapsids are partly ‘unfolded’ while those of Sindbis appear slightly contracted after exposure to EDTA.


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