Notes on Genus: Betaretrovirus

enveloped spheres
enveloped spheres

Type member: Mouse mammary tumor virus


General Description

This is one of the genera in the family Retroviridae. Members are distinguished from those of the other genera by the presence of 5 ORFs, a genome of less than 10 kb and because virions assemble as intracytoplasmic particles, and not at the cell membrane.


Virions are spherical and enveloped, 80-100 nm in diameter, with suface projections of glycoprotein.


A dimer of linear, positive sense, ssRNA molecules, each 7.5-9 kb long and with a 3'-polyA tail.

Genus Genomic Organization

In addition to the standard 4 genes of the family (gag, pro, pol and env) an additional gene (sag) at the 3'-end of the genome encodes a superantigen. Frameshifting is used to generate the pro and the pol from the same mRNA as the gag. Separate spliced mRNAs are used to generate the env and sag genes.

Type Member Genomic Organization

The five major genes are
  1. gag: structural polyprotein of 66 kDa, consisting of MA (11 kDa), XX (11 kDa), p3 (4 kDa), p8 (3 kDa), CA (27 kDa) and NC (26 kDa)
  2. pro: protease of 29 kDa
  3. pol: polyprotein of 102 kDa
  4. env: envelope proteins, consisting of surface protein (SU, 42 kDa) and transmembrane protein (TM, 24 kDa)
  5. sag: superantigen of 37 kDa