Notes on Genus: Closterovirus

flexuous filaments

Type member: Beet yellows virus

General Description

This is one of three genera in the family Closteroviridae, which are characterised by their very long (950-2000 nm) and very flexuous particles. Members of the genus Closterovirus (from the Greek for "thread") are the largest of all positive sense RNA plant viruses. Members previously classified as closteroviruses but which have a bipartite genome are now classified in the genus Crinivirus. Members previously classified as closteroviruses but which are transmitted by mealybugs are now classified in the genus Ampelovirus. Infected plant cells contain massive particle aggregates, often organized into banded inclusions.

Morphology

Virions filamentous, not enveloped, very flexuous, 950-2000 nm long and 10-13 nm in diameter. There is a distinct virion tail. Symmetry helical, with pitch of 3.4-3.7 nm.

Virions composed of 5% nucleic acid.

Genome

Monopartite, linear, single-stranded, positive sense RNA 15,000 to 20,000 nucleotides long. There are sequences with potential hairpin structure near the 3'-end. 3' terminus has no poly(A) tract and the 5' terminus probably has a methylated nucleotide cap.

Genus Genomic Organization

The RNA is translated from a large number of reading frames (9-12 in the better known members). There is a large replicase near the 5'-end and the remaining ORFs are translated from a set of nested mRNAs. There are two coat proteins towards (but not at) the 3'-end.

Type Member Genomic Organization

The RNA of about 15,500 nucleotides is translated from nine open reading frames:

1a	295 kDa	contains methyltransferase and RNA helicase motifs. A leader protease self-cleaves from the N-terminus
1b	53 kDa	polymerase expressed as a 1a/1b fusion protein of 348 kDa by +1 frameshift
2	6 kDa	small hydrophobic membrane protein involved with products of ORFs 3 and 4 in cell-to-cell movement
3	65 kDa	similar to the HSP70 heat-shock proteins which are molecular chaperones
4	64 kDa	probably involved with products of ORFs 2 and 3 in cell-to-cell movement
5	24 kDa	Minor coat protein incorporated into the virion tail together with the products of ORFs3 and 4
6	22 kDa	Coat protein; assembled into particles as the major virion protein
7	20 kDa	long distance transport protein
8	21 kDa	involved in RNA replication