Notes on Genus: Tungrovirus

bacilliform
bacilliform

Type member: Rice tungro bacilliform virus

Contents

General Description

This is one of 7 genera in the family Caulimoviridae, which are distinctive amongst plant viruses in having double-stranded DNA. Members of this genus are those with bacilliform, rather than isometric, particles and with 4 open reading frames in the genome. The single member of the genus was previously classified in the Badnavirus genus, but this now only includes those viruses with 3 open reading frames. The name is derived from the type member, Rice tungro bacilliform virus.

Morphology

Virions bacilliform, not enveloped, 30-35 nm in diameter and 110-400 nm in length.

Genome

Monopartite, open circular, double stranded DNA of c.8000 base pairs with a single-strand discontinuity at one site in each strand.

Genus Genomic Organization

The genome has direct terminal repeats, reiterated internally in inverted form and is translated from 4 open reading frames (ORFs) on the same DNA strand. The largest of the ORFs is probably a polyprotein, subsequently processed into several functional products.

Type Member Genomic Organization

There are 4 open reading frames on the same DNA strand:
ORF1 26 kDa unknown function
ORF2 12 kDa DNA binding protein
ORF3 194 kDa polyprotein which is cleaved by aspartic proteinase to give a putative movement protein, a coat protein, a proteinase, and a replicase which has a reverse transcriptase activity, a DNA polymerase DNA dependant activity, and a RNAse H activity
ORF4 46 kDa similar to the caulimovirus transactivator factor inclusion body protein