Parahepadnavirus (taxid:2733291)

VIRION

Enveloped, spherical. Diameter from about 42nm. Icosahedric capsid with a T=4 symmetry.

GENOME

Partially dsDNA circular genome, about 3.2 kb in size. Encodes for 3 to 5 proteins.

GENE EXPRESSION

The minichromosome is transcribed by cellular RNA polymerase II. The pre-genomic RNA is alternatively spliced. The unspliced form is exported from the nucleus through a PRE motif possibly by capsid protein. The Polymerase and short S proteins are expressed by leaky scanning from the pg mRNA and the S mRNA respectively.

ENZYMES

• Reverse transcriptase
  • RNA-dependent DNA polymerase [P]
  • DNA-dependent DNA polymerase [P]
  • Protein-primed terminal transferase [P] Q69028
• RNAse H [P]

REPLICATION

NUCLEO-CYTOPLASMIC

1. Virus attaches to host receptors through major surface antigen and enters the cell by an unknown mechanism.
2. Relaxed circular DNA (RC-DNA) and capsid are transported via microtubules to the nucleus where DNA is released through the nuclear pore, and repaired to form covalently closed circular DNA (cccDNA).
3. Transcription by RNA polymerase II of the viral mRNAs and the pregenomic RNA (pgRNA).
4. The unspliced pregenomic RNA leaves the nucleus by nuclear pore export.
5. In the cytoplasm pgRNA is encapsidated, together with the P protein, and reverse-transcribed inside the nucleocapsid in new RC-DNA.
6. Transport to the nucleus of new RC-DNA leads to cccDNA amplification; alternatively, the RC-DNA containing nucleocapsids are enveloped at the ER, and new virions are released by exocytosis.