Notes on Genus: Orthohepadnavirus
enveloped spheres or pleomorphic
Type member: Hepatitis B virus
This is one of two genera in the family Hepadnaviridae
and consists of those viruses that infect mammals, causing primary liver cancer.
Virions enveloped, spherical or pleomorphic, 40-45 nm in diameter.
Monopartite, circular DNA 3.0-3.3 kb long. The genome is partially double-stranded with a full-length negative sense strand and a positive strand of variable length. The circular configuration is achieved by base-pairing between the overlapping 5'-ends of the two strands.
Genus Genomic Organization
There are four major overlapping ORFs in the same orientation. The largest (P) encodes the polymerase. The env or surface gene (S) can be expressed in different lengths to form the 3 surface proteins. The precore/core (preC/C) gene yields two distinct products and the X gene encodes a transcriptional activator.
Type Member Genomic Organization
The genome of 3.2 kbp is translated from four major overlapping ORFs in the same orientation:
- Polymerase (P) of 94.6 kDa
- The env or surface gene (S) is expressed in different lengths to form the 3 surface proteins. The small protein (S) is 25.4 kDa, the medium protein (M, 31.1 kDa) also contains the pre-S2 region and the large protein (L, 43.7 kDa) contains both the pre-S1 and pre-S2 regions.
- The precore/core (preC/C) gene yields two distinct products. The core protein (21.1 kDa) forms the virion shell while the longer product (precore protein, 24.3 kDa) is targeted to the cell's secretory process and is expressed as the "e antigen".
- The X gene encodes a transcriptional activator of 16.5 kDa.