Molecular Plant Pathology - Pathogen Profiles
Turnip yellow mosaic virus: transfer RNA mimicry,
chloroplasts and a C-rich genome
Theo W. Dreher
Department of Microbiology and Center for Gene Research and
Biotechnology, Oregon State University, Corvallis, OR 97331, USA
||Turnip yellow mosaic virus
(TYMV) is the type species of the genus Tymovirus, family Tymoviridae.
TYMV is a positive strand RNA virus of the alphavirus-like supergroup.
||Virions are non-enveloped 28-nm T = 3
icosahedrons composed of a single 20-kDa coat protein that is clustered in
20 hexameric and 12 pentameric subunits. Infectious particles and empty
capsids coexist in infected tissue. The genomic RNA is 6.3 kb long,
with a 5'm7GpppG cap and a 3' untranslated region ending in a
tRNA-like structure to which valine can be covalently added. The genome
has a distinctive skewed C-rich, G-poor composition (39% C, 17% G).
||Two proteins, whose open reading
frames extensively overlap, are translated from the genomic RNA. p206,
which contains sequences indicative of RNA capping, NTPase/helicase and
polymerase activities, is the only viral protein that is necessary for
genome replication in single cells. It is produced as a polyprotein and
self-cleaved to yield 141- and 66-kDa proteins. p69 is required for virus
movement within the plant and is also a suppressor of gene silencing. The
coat protein is expressed from the single subgenomic RNA.
|Hosts and symptoms:
||TYMV has a narrow host range almost
completely restricted to the Cruciferae. Experimental host species
are Brassica pekinensis (Chinese cabbage) or B. rapa
(turnip), in which diffuse chlorotic local lesions and systemic yellow
mosaic symptoms appear. Arabidopsis thaliana can also be used.
Clumping of chloroplasts and the accumulation of vesicular invaginations
of the chloroplast outer membranes are distinctive cytopathological
symptoms. High yields of virus are produced in all leaf tissues, and the
virus is readily transmissible by mechanical inoculation. Localized
transmission by flea beetles may occur in the field
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