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Viruses assigned to the genus Tupavirus form a distinct monophyletic group based on well-supported Maximum Likelihood trees inferred from complete L sequences. Tupaviruses infect mammals and birds. The genome organisation features an alternative long ORF (Px or C) in the P gene and a gene encoding a small hydrophobic protein (U1 or SH) between the M gene and the G gene.
Both bullet-shaped virions and bacilliform virions (i.e., two rounded ends) have been reported for tupaviruses. For Klamath virus (KLAV) and Tupaia rhabdovirus (TUPV), enveloped bullet-shaped virions (125–220 nm x 68–80 nm) with helical cores have been observed in cell culture supernates or purified virus preparations under negative stain (Kurz et al., 1986, Murphy et al., 1972). Surface projections (8–11 nm) forming a regular reticular pattern were also reported. In ultra-thin sections of infected cells (KLAV, TUPV and Durham virus (DURV)), both bullet-shaped and bacilliform virions have been observed budding from intracytoplasmic membranes and at the cell surface (Kurz et al., 1986, Murphy et al., 1972, Allison et al., 2011).
Tupavirus genomes consist of a single molecule of negative-sense, single-stranded RNA and range from approximately 11.1–11.5 kb (Walker et al., 2015, Allison et al., 2011, Springfeld et al., 2005).
The N, P, M, G and L share sequence homology and/or structural characteristics with the cognate proteins of other rhabdoviruses. Other proteins are encoded in the tupavirus genome but have not yet been identified in infected cells. The Px (or C) ORFs, present in all tupaviruses, encode highly basic proteins of 136 to 221 amino acids (15.7–25.8 kDa) (Walker et al., 2015, Allison et al., 2011, Springfeld et al., 2005). The U1 (or SH) genes, also present in all tupaviruses, encode predicted small hydrophobic proteins of 77 to 93 amino acids (9.2–10.8 kDa). The KLAV U2 gene encodes encodes a very small basic protein (6.5 kDa) (Walker et al., 2015).
Tupavirus genomes include five genes (N, P, M, G and L) encoding the structural proteins and multiple additional long ORFs (Figure 1.Tupavirus) (Walker et al., 2015, Allison et al., 2011, Springfeld et al., 2005). The genomes of all tupaviruses feature an alternative long ORF (Px or C) in the P gene and an additional gene (U1 or SH) between the M gene and G gene. The KLAV genome also features an additional gene (U2) located between the G gene and L gene. The U1 and U2 genes are organised as independent transcriptional units including conserved transcription initiation and transcription termination/polyadenylation sequences, indicating that they are highly likely to be expressed in infected cells.
Figure 1.Tupavirus. Schematic representation of tupavirus genome organisations. N, P, M, G and L represent ORFs encoding the structural proteins. ORF Px (green) is in an alternative reading frame in the P gene; ORF U1 (red) and ORF U2 (blue) are in independent transcriptional units bounded by consensus transcription initiation and transcription termination/polyadenylation sequences. Other small ORFs (≥150 nt or 50 aa) occur in alternative reading frames (grey) in Tupaia rhabdovirus and Klamath virus but may or may not be expressed.
KLAV has been shown to cross-react in indirect immunofluorescence tests with TUPV (Calisher et al., 1989). However, immune serum to a wide range of other animal rhabdoviruses failed to cross-react with KLAV in complement-fixation (CF) tests and immune serum to KLAV failed to react in CF tests with DURV (Tesh et al., 1983, Allison et al., 2011).
Tupaviruses infect various birds and mammals. TUPV was isolated in Germany from liver tumour cells removed from a northern tree shrew (Tupaia belangeri) imported from Thailand (Kurz et al., 1986). DURV was isolated in the USA from an American coot (Fulica americana) with signs of neurological disease (Allison et al., 2011). KLAV has been isolated in the USA from a montane vole (Microtus montanus), northern red-backed voles (Clethrionomys rutilus) and tundra voles (Microtus oeconomus) (Allison et al., 2011, Johnson 1965) and neutralising antibodies to KLAV have been detected in deer of several species, American bison (Bison bison) and humans (Eldridge et al., 1987, Zarnke et al., 1983, Stansfield et al., 1988).
Viruses assigned to different species within the genus Tupavirus have several of the following characteristics: A) minimum amino acid sequence divergence of 5% in N proteins; B) minimum sequence divergence of 10% in L; C) minimum amino acid sequence divergence of 15% in G; D) significant differences in genome organisation as evidenced by numbers and locations of ORFs; E) can be distinguished in serological tests; and F) occupy different ecological niches as evidenced by differences in hosts and/or arthropod vectors.
Tupavirus: from the scientific name of the northern tree shrew (Tupaia belangeri) from which Tupaia rhabdovirus, a member of the type species Tupaia tupavirus, was first isolated.
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