Nanovirus (taxid:104766)

VIRION

Non-enveloped, round, T=1 icosahedral symmetry, about 18-19 nm in diameter.

GENOME

Multipartite ssDNA genome (+) composed of 6 to 8 circular segments about 1kb in size. Each ssDNA segment 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 commonly found, 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 6-8 viral genomic ssDNAs (trans-replication).

GENE EXPRESSION

Each ssDNA genome segment is monocistronic.

ENZYMES

• Cress-type Endonuclease [Rep]

REPLICATION

NUCLEAR

1. Virus penetrates into the host cell.
2. Uncoating, the viral ssDNA genome penetrates into the nucleus.
3. The ssDNA is converted into dsDNA with the participation of cellular factors.
4. Bidirectional dsDNA 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 that will be released in the extracellular space. c) be transported outside the nucleus, to a neighboring cell through plasmodesmata (cell-cell movement) with the help of viral movement proteins.

Host-virus interaction

Cell-cycle modulation

Nanovirus Clink protein is responsible for inhibiting host retinoblastoma protein and inducing transition from the G1 to S phase in preparation for virus replication since the virus targets differentiated non-dividing cells. .