Molecular biology
VIRION
Non-enveloped, round, icosahedral symmetry (T=1), about 18-19 nm in diameter.
GENOME
6 circular genomic (+)ssDNA about 1kb in size. Each ssDNA have a common stem-loop region and are encapsidated in a separate particle.
In addition to genomic DNA, up to 4 satellite-like DNAs are found in some isolates, usually encoding for accessory Rep proteins (para-Rep). These satellite-rep proteins are only able to initiate replication of their genomic DNA, unlike genomic encoded Master-Rep (M-Rep) which promotes replication of all 6 viral genomic ssDNAs (trans-replication).
GENE EXPRESSION
Each genome ssDNA are monocistronic, except for BBTV DNA-R which contains a second small ORF U5 internal to the Rep-encoding ORF. U3 and U5 are absent in ABTV.
REPLICATION
NUCLEAR
- Virus penetrates into the host cell.
- Uncoating, and release of the viral genomic ssDNA into the nucleus.
- The ssDNA is converted into dsDNA with the participation of cellular factors.
- Transcription from the intergenic region (IR) promoter produces viral mRNAs and translation of viral proteins.
- Replication is initiated by cleavage of the(+)strand in the IR by Rep, and occurs by rolling circle producing ssDNA genomes.
- 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 that will be released in the extracellular space.