Notes on Genus: Influenzavirus B

enveloped spheres or pleomorphic
enveloped spheres or pleomorphic

Type member: Influenza B virus


General Description

The genus Influenzavirus B is one of the genera in the family Orthomyxoviridae. It is named from the type (and only) member (derived from the latin influentia = epidemic). Genome organisation is very similar to that for the genus Influenzavirus A but there are large serological differences and a different organisation of RNA segment 6.


Virions spherical or pleomorphic, enveloped, 80-120 nm in diameter, with projections 4-6 nm in diameter and 10-14 nm long.


Single-stranded negative sense RNA in 8 segments totalling about 14.6 kb. The segments do not have a genome-linked protein (VPg) at their 5'-terminus nor a 3'-polyA tail.

Genus Genomic Organization

There are 8 RNA segments. The larger genome segments each encode a single protein while the smaller segments often encode additional proteins from spliced or bicistronic mRNAs.

Type Member Genomic Organization

There are 8 RNA segments:
  • RNA1 (2368 nts) encodes a basic polymerase (PB1) of 84.4 kDa
  • RNA2 (2396 nts) encodes a basic polymerase (PB2) of 87.9 kDa
  • RNA3 (2308 nts) encodes an acid polymerase (PA) of 83.2 kDa
  • RNA4 (1882 nts) encodes a hemagglutinin (HA) of 63.3 kDa
  • RNA5 (1841 nts) encodes the coat protein (Nucleoprotein, NP) of 61.8 kDa
  • RNA6 (1557 nts) encodes a neuraminidase (NA) of 51.4 kDa and a membrane protein (NB) of 11.0 kDa
  • RNA7 (1191 nts) encodes two matrix proteins (M1 and M2) of 27.5 and 12.5 kDa
  • RNA8 (1096 nts) encodes two non-structural proteins (NS1 and NS2) of 32.1 and 14.2 kDa