Mastrevirus (taxid:10812)

VIRION

Non-enveloped, about 38 nm in length and 22 nm in diameter (for MSV), twinned (geminate) incomplete T=1 icosahedral symmetry capsid that contains 22 pentameric capsomers made of 110 capsid proteins (CP). Each geminate particle contains only a single circular ssDNA.

GENOME

Monopartite, circular, ssDNA genome (+) genome of about 2.6-2.8 kb. 3' terminus has no poly(A) tract. There are coding regions in both the virion (positive) and complementary (negative) sense strands.
The genome is replicated through double-stranded intermediates. The replication (Rep) protein initiates and terminates rolling circle replication, the host DNA polymerase being used for DNA replication itself. There is a potential stem-loop structure in the long intergenic region (LIR) that includes a conserved nona-nucleotide sequence (TAATATTAC) where ssDNA synthesis is initiated.
The short intergenic region (SIR) contains bidirectional polyadenylation signals.

GENE EXPRESSION

Transcription is bidirectional from the long intergenic region (LIR) which contains two divergent promoters: a V-(virion) and a C-(complementary) sense. The resulting C- and V-transcripts encode four proteins. On the V-sense: the movement and CP. On the C-sense: RepA and Rep (which is expressed as a splice product of RepA and RepB ORFs).

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 capsid protein and form virions released by cell lysis
   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

Mastrevirus protein RepA 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. .