Abstract
Geminivirus particles, consisting of a pair of twinned isometric structures, have one of the most distinctive capsids in the virological world. Until recently, there was little information as to how these structures are generated. To address this, we developed a system to produce capsid structures following the delivery of geminivirus coat protein and replicating circular single-stranded DNA (cssDNA) by the infiltration of gene constructs into plant leaves. The transencapsidation of cssDNA of the Begomovirus genus by coat protein of different geminivirus genera was shown to occur with full-length but not half-length molecules. Double capsid structures, distinct from geminate capsid structures, were also generated in this expression system. By increasing the length of the encapsidated cssDNA, triple geminate capsid structures, consisting of straight, bent and condensed forms were generated. The straight geminate triple structures generated were similar in morphology to those recorded for a potato-infecting virus from Peru. These finding demonstrate that the length of encapsidated DNA controls both the size and stability of geminivirus particles.
Highlights
The family Geminiviridae, named because of the characteristic twinned morphology of their particles [1], includes nine genera [2]
Electron microscope examination of the preparations of particles produced with the ageratum yellow vein virus (AYVV) and African cassava mosaic virus (ACMV) coat proteins showed they contained both isometric and geminate particles a situation similar to that seen in Figure 3 of Hesketh et al [38]
Studies on the specificity of circular single-stranded DNA (cssDNA) encapsidation have been limited to indirect methods such as examining how swapping the coat protein between different geminivirus genera affects insect transmission [20], and how less-than-genome length DNAs can be transmitted from plant to plant [46]; the DNA sequences of half-sized Mastrevirus DNAs encapsidated within intact isometric particles isolated from infected leaves have been characterized [25]
Summary
The family Geminiviridae, named because of the characteristic twinned morphology of their particles [1], includes nine genera [2] These are widely distributed and are responsible for many diseases of food and commodity crops worldwide [3,4,5]. The genomes of the Begomovirus genus are usually bipartite, consisting of two molecules of cssDNA that are approximately the same size (2.6–2.8 kb), each possessing an identical DNA sequence of about 200 nucleotides. This consists of a bidirectional mRNA promoter and the sequences necessary for the initiation and termination of genomic DNA synthesis [7]. These Begomoviruses harbour similar sized cssDNAs of the geminivirus-associated alphasatellites (family Alphasatellitidae, subfamily Geminialphasatellitinae) that are capable of self-replication but are not essential for the progression of the viral disease [10,11]
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