Nanoviridae

VIRION

image

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

image

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, satellite-like DNA are commonly found, usually encoding for Rep proteins. These satellite-Rep proteins are only able to initiate replication of their genomic DNA, unlike genomic Rep which promotes replication of all 6-8 viral genomic ssDNAs.

GENE EXPRESSION

Each ssDNA genome segment is monocistronic. In rare cases a gene encodes for two overlapping ORFs.

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

Nanoviruses Clink protein is probably involved in binding and inhibiting host retinoblastoma protein. Inhibition of retinoblastoma protein induces transition from the G1 to S phase for optimal replication conditions.