The size of encapsidated single-stranded DNA determines the multiplicity of African cassava mosaic virus particles.

Transgenic Nicotiana benthamiana plants harbouring a defective interfering (DI) DNA of African cassava mosaic virus (ACMV) and control plants were inoculated with ACMV. Virus particles were purified from infected plants, separated in sucrose gradients and fractions were analysed by Southern blotting. Transgenic plant-derived virus particles taken from the top fractions of sucrose gradients contained DI DNA, middle fractions contained a mixture of genomic and DI DNA and bottom fractions contained a mixture of multimeric, genomic and DI DNA. Virus particles from selected top, middle and bottom fractions were analysed by electron microscopy. In fractions containing only DI DNA, isometric particles of 18-20 nm were detected. In fractions containing DI DNA as well as genomic size DNA, isometric and geminate particles were found. Fractions containing multimeric size DNA were found to comprise particles consisting of three subunits adjacent to geminate particles. From these data, it is concluded that the size of encapsidated DNA determines the multiplicity of ACMV particles.