Disassembly of African cassava mosaic virus.

The plant-infecting geminiviruses encapsidate their single-stranded DNA genome in characteristic twinned particles that are unique among viruses. These particles are formed by joining two incomplete T=1 icosahedra. African cassava mosaic virions were purified by density-gradient centrifugation from infected Nicotiana benthamiana plants and analysed for their stability with respect to pH changes and heat treatment by using electron microscopy. Negative staining and rotary shadowing revealed stable virions as well as isolated capsomeres between pH 4.0 and 8.5. At pH 9.0 and above, particles disintegrated, whereas they mainly aggregated at a pH below 6.0. Heating the preparations to 55 degrees C and above resulted in the complete loss of any discernible structure. A low proportion (approx. 10 %) of particles ejected their DNA within the pH range of 6.0-8.5. Most virions released their DNA at the top (15.9 %) or the shoulder (71.4 %) of the twin particles and only 12.7 % at the waist. Compared with the expected numbers of pentameric capsomeres at the top (9 %), the shoulder (45.5 %) or the waist (45.5 %), the results revealed a preferential DNA release from the top and shoulder of the geminate particle.