Molecular biology

VIRION

Non-enveloped, about 38 nm in length and 22 nm in diameter (for MSV), twinned (geminate) incomplete icosahedral(T=1) capsid that contains 22 pentameric capsomers made of 110 capsid proteins (CP). Each geminate particle contains only a single circular ssDNA.

GENOME

Monopartite, circular, ssDNA(+) genome of 2.86 kb. 3’ terminus has no poly(A) tract. There are coding regions in both the virion (positive) and complementary (negative) sense strands.
The genome is replicated through double-stranded intermediates. The replication (Rep) protein initiates and terminates rolling-circle replication, the host DNA polymerase being used for DNA replication itself. There is a potential stem-loop structure in the intergenic region that includes a conserved nonanucleotide sequence (TAATATTAC) where ssDNA synthesis is initiated.

GENE EXPRESSION

Transcription is bidirectional from the common region (CR), which corresponds to the non-coding region common for topocu- and begomoviruses. 6 proteins are produced. In the V-sense: V2 and CP. In the C-sense: Rep, C2, C3 and C4.

REPLICATION

NUCLEAR

1. Virus penetrates into the host cell.
2. Uncoating, and release of the viral genomic ssDNA into the nucleus.
3. The ssDNA is converted into dsDNA with the participation of cellular factors.
4. Bidirectional transcription from the IR promoter produces viral mRNAs and translation of viral proteins.
5. Replication is initiated by cleavage of the(+)strand by Rep, and occurs by rolling circle producing ssDNA genomes
6. These newly synthesized ssDNA can either
   a) be converted to dsDNA and serve as a template for transcription/replication
   b) be encapsidated by CP and form virions released from the cell by budding
   c) be transported outside the nucleus, to a neighbouring cell through plasmodesmata (cell-cell movement)with the help of viral movement protein(s).