Family: Pseudoviridae

Genus: Sirevirus

Distinguishing features

Based on the sequence of putative primer binding sites, members of most species in this genus likely use the acceptor stem of an initiator tRNA^Met as the primer for minus-strand DNA synthesis during reverse transcription, similar to members of the genus Pseudovirus. Clusters of conserved polypurine tracts (PPTs) are also present in the genomes of many members of different species, and highly conserved DNA sequence motifs of unknown function are present in the long terminal repeats (LTRs) and internal regions. A unique ORF and multiple gene expression mechanisms characterize members of this genus. For example, viruses such as Glycine max SIRE1 virus (GmaSIRV), Arabidopsis thaliana Endovir virus (AthEndV) and Lycopersicon esculentum ToRTL1 virus (LesToRV) have an extra ORF between pol and the 3′-LTR (Figure 2. Pseudoviridae). This ORF, referred to as env, encodes a putative envelope protein with conserved transmembrane domains which is believed to be required for extracellular spread of the virions. For GmaSIRV, expression of Env is regulated by stop codon suppression. However, stop codon suppression does not seem to be utilized by the other viruses (e.g., AthEndV and LesToRV), because in these viruses env is separated from pol by a non-coding sequence. In addition, based on ORF organization, two different Gag-Pol expression mechanisms are likely to be utilized by members of this genus. Some viruses, such as GmaSIRV and AthEndV, encode Gag and Pol proteins in a single ORF, whereas in other viruses, such as Zea Mays Opie-2 virus (ZmaOp2V) and Zea Mays Prem-2 virus (ZmaPr2V), the pol gene is in the +1 frame (Figure 2. Pseudoviridae). A unifying characteristic of viruses in the genus is that in all members, the gag gene is nearly twice the size of the gag gene of pseudoviruses and hemiviruses. Furthermore, some sireviruses carry a significant amount of non-coding DNA after the end of the pol gene (from few hundred bp to over 1 kb), while in members of the Pseudovirus and Hemivirus genera pol gene is separated from the 3′-LTR by a few dozens of nucleotides or less (Grandbastien 2008). 

Virion

See discussion under family description. 

Genome organization and replication

The genome of GmaSIRV is 9.8 kb, including two unusually long LTRs each of 1,148 nt. The internal region of this virus contains a primer binding site (PBS), complementary to the initiator tRNA^Met, as well as two ORFs and a PPT. The larger gag-pol gene encodes the typical Gag and Pol polyproteins, whereas the second ORF, separated by a single stop codon, encodes a putative Env proteins. 

Biology

Members of this genus have been identified only in plants. Their mode of transmission is unknown, although presumed to be through vertical inheritance. Although they are considered to have an extracellular phase similar to retroviruses, their infectivity is yet to be demonstrated.

Species demarcation criteria

See discussion under family description.

Related, unclassified viruses

Virus names and virus abbreviations are not official ICTV designations.