Satellite virus: infectious only if host cell is co-infected with Adenovirus or Herpesvirus

Molecular biology

VIRION

Non-enveloped, round, icosahedral symmetry (T=1), 18-26 nm in diameter. The capsid consists of 60 copies of CP protein.

GENOME

Linear, single-stranded DNA of about 4.7kb in size.
The genome is replicated through rolling-hairpin mechanism. All dependoviruses except Duck parvovirus and Goose parvovirus depend on helper Adenovirus or Herpesvirus to replicate efficiently.

GENE EXPRESSION

Host proteins transcribe the genomes into mRNAs. Transcription is regulated by the three promoters P5, P19 and P40: P5 transcripts expressed first, followed by those from P19, then those from P40. Alternative mRNAs splicing allows expression of three different mRNAs for each promoter. mRNA9 is translated into VP2 or VP3 by alternative initiation.

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 by cellular proteins.
4. viral mRNAs are transcribed when host cell enters S phase and translated to produce viral proteins.
5. Replication occurs through rolling-hairpin mechanism, whith NS1 nickase binding covalently to the 5’ genomic end.
6. Individual ssDNA genomes are excised from replication concatemers by a process called junction resolution.
7. These newly synthesized ssDNA can either
   a) be converted to dsDNA and serve as a template for transcription/replication
   b) be encapsidated to form new virions that can budd out of the host cell.