Notes on Family: Betaflexiviridae

flexuous filaments
flexuous filaments


General Description

The family Flexiviridae was originally created to contain plant-infecting viruses with:
  1. Flexuous filamentous virions 12-13 nm in diameter (giving the family its name)
  2. Monopartite, positive sense, ssRNA genomes with a 3'-polyA tail
  3. Translation of at least some ORFs from subgenomic mRNAs
  4. Up to 6 open reading frames ordered from 5' to 3':
    • An alpha-like replication protein (150-250 kDa) containing conserved methyl transferase, helicase and RNA-dependent RNA polymerase (RdRp) motifs
    • One or more movement proteins
    • A single coat protein (CP) of 22-44 kDa
    • A sixth ORF in some viruses, which may partially overlap the 3'-end of the CP gene and is thought to have nucleotide-binding properties.

Recently, the family has been divided

  1. to reflect the very different lineages of replication protein (potex-like and carla-like)
  2. to include some related viruses of fungi
  3. to support the inclusion of these viruses with members of the family Tymoviridae in the order Tymovirales.

The family Betaflexiviridae includes the plant viruses with a carla-like polymerase while those viruses with a smaller, potex-like, polymerase are now classified within the family Alphaflexiviridae.


Virions filamentous, not enveloped, 600-1000 nm or more long and 12-13 nm in diameter.


Monopartite, linear, polyadenylated ssRNA of 6.5-9.0 kb.

Genera in the Family

The genera are distinguished by the type of movement protein (triple gene block, TGB or a single protein of the '30K' superfamily), the number of ORFs and the sizes of the virion, the replication protein and the coat protein. Phylogenetic analysis also supports these distinctions:
GenusVirion length
No of
Type of
movement protein
Replication protein
Coat protein
Capillovirus640-700 2 or 3'30K' 210-24525-27
Carlavirus610-700 6TGB215-225 32-36
Citrivirus960 3'30K'227 41
Foveavirusc. 800+ 5TGB230-250 28-44
Tepovirusc.640 3'30K' c.18524
Trichovirus640-760 3 or 4'30K'215-220 21-24
Vitivirus725-785 5'30K' 190-20018-22