Studies on the composition and structure of plant viruses can only be attempted after their purification from infected tissue homogenates. Owing to their intrinsic differences, however, the ease with which different viruses can be purified varies considerably. A few relatively stable viruses, e.g. tobacco mosaic virus or turnip yellow mosaic virus, can be treated with high concentrations of salt and precipitated by acid or alcohol without inactivation. With less stable viruses such methods are usually unsuccessful. The particles of such viruses, however, can be sedimented by ultracentrifugation and procedures developed frequently involve two treatments: () differential centrifugation and concentration after initial clarification of buffered homogenates such as with organic solvents (Steere, 1956; Tomlinson, Shepherd & Walker, 1959; Wetter, 1960), () further fractionation by rate or equilibrium density gradient centrifugation as developed by Brakke (1960). Because of the small capacity of the swinging-bucket rotor generally employed only relatively small volumes of the partially purified preparations can be fractionated in the gradients.


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