The genus Tremovirus consists of two species, Tremovirus A (formerly named Avian encephalomyelitis virus; previously it was a tentative member of the genus Hepatovirus) and Tremovirus B.

The following arguments were used to support the re-classification of AEV in a new genus.

i) AEV possesses a HCV-like IRES (type IV; Hellen and de Breyne, 2007) distinct from that of HAV. HAV has an IRES which is unique amongst the picornaviruses (Brown et al., 1994).

ii) AEV possesses a 2A distinct from HAV. The 2A protein of AEV is a member of the H-rev107 family of proteins involved in the control of cell proliferation (Hughes and Stanway, 2000). Similar 2A proteins are found in members of the genera Parechovirus and Kobuvirus (Hughes and Stanway, 2000) and a newly sequenced unassigned picornavirus, duck hepatitis virus (Kim et al., 2006; Tseng et al., 2007), but not in hepatitis A virus. The 2A of HAV is thought to inhibit cap-dependent gene expression by an unknown mechanism (Maltese et al., 2000).

iii) AEV has a 2B protein which is very different and possibly non-homologous to that of HAV.

iv) AEV and HAV have 3A polypeptides which share little primary sequence identity; however, both (in common with all picornaviruses) have a predicted transmembrane alpha-helix.

v) The percentage amino acid identities between AEV and HAV in the P1, P2 and P3 regions are 49.2, 28.0 and 37.3, respectively (Marvil et al., 1999). Only the P1 region exceeds the values in the new Picornaviridae Study Group guidelines.

The complete genome sequences of three AEV strains have now been determined: Calnek (AJ225173; Marvil et al., 1999), L2Z-China (AY275539) and Van Roekel (AY517471); all are closely related and confirm the novel characteristics of the AEV genome.

The relationship of AEV to the other picornaviruses is shown in Fig. 1.

References

Brown, E.A., Zajac, A.J. and Lemon, S.M. (1994). In vitro characterization of an internal ribosomal entry site (IRES) present within the 5' nontranslated region of hepatitis A virus RNA: comparison with the IRES of encephalomyocarditis virus. J. Virol. 68: 1066-1074.

Hellen, C.U. and de Breyne, S. (2007). A distinct group of hepacivirus/pestivirus-like internal ribosomal entry sites in members of diverse picornavirus genera: evidence for modular exchange of functional noncoding RNA elements by recombination. J. Virol. 81: 5850-5863.

Hughes, P.J. and Stanway, G. (2000). The 2A proteins of three diverse picornaviruses are related to each other and to the H-rev107 family of proteins involved in the control of cell proliferation. J. Gen. Virol. 81: 201-207.

Kim, M.C., Kwon, Y.K., Joh, S.J., Lindberg, A.M., Kwon, J.H., Kim, J.H. and Kim, S.J. (2006). Molecular analysis of duck hepatitis virus type 1 reveals a novel lineage close to the genus Parechovirus in the family Picornaviridae. J Gen Virol. 87: 3307-3316.

Maltese, E., Bucci, M., Macchia, S., Latorre, P., Pagnotti, P., Pierangeli, A. and Pérez Bercoff, R. (2000). Inhibition of cap-dependent gene expression induced by protein 2A of hepatitis A virus. J. Gen. Virol. 81: 1373-1381.

Marvil, P., Knowles, N.J., Mockett, A.P.A., Britton, P., Brown, T.D.K. and Cavanagh, D. (1999). Avian encephalomyelitis virus is a picornavirus and is most closely related to hepatitis A virus. J. Gen. Virol. 80: 653-662.

Tseng, C.H., Knowles, N.J. and Tsai, H.J. (2007). Molecular analysis of duck hepatitis virus type 1 indicates that it should be assigned to a new genus. Virus Res. 123: 190-203. Epub 2006 Oct 25.