DISTINGUISHING FEATURES
NUCLEIC ACID
Curtovirus genomes consist of a single circular ssDNA component
(2.9-3.0 kb).
PROTEINS
Curtoviruses produce six to seven proteins in infected plants. Virion-sense
ORFs V2 and V3 produce proteins involved in regulation of genomic
ss/dsDNA levels and movement respectively; ORF V1 protein (CP)
is involved in encapsidation and movement. Complementary
sense ORF C1 encodes the Rep protein; C2 encodes a protein of
unknown function; C3 encodes a protein analogous to the replication
enhancer protein (REn) of begomoviruses; ORF C4 produces a protein
which can initiate cell division, and is an important symptom
determinant.
GENOME ORGANIZATION AND REPLICATION
The complete nucleotide sequences of infectious genomic
clones of BCTV (3), HrCTV and TPCTV have been determined. The
genomes of BCTV and TPCTV encode seven genes, three on the virion
strand and four on the complementary strand. Replication
is initiated by the Rep protein in the stemloop sequence.
Figure 3: Genomic organization of curtoviruses. ORFs
are denoted as either being encoded on the virion (V) or complementary
(C) strand. Gene designations are shown where these are
known. The C3 ORF is not present in HrCTV. The position
of the stemloop containing the conserved nonanucleotide sequence
(TAATATTAC) is shown. CP coat protein, Rep; replication
associated protein, REn = replication enhancer.
ANTIGENIC PROPERTIES
Serological tests show BCTV, TPCTV and TLRV to be relatively
closely related. Distant relationships between curtoviruses
and begomoviruses have been shown in serological tests.
BIOLOGICAL PROPERTIES
HOST RANGE
Type species BCTV has a very wide host range, over 300 species
in 44 plant families. TPCTV has a narrower host range.
TRANSMISSION
Transmitted in nature by leafhoppers (Homoptera: Cicadellidae),
with the exception of TPCTV, which is transmitted by a treehopper
(Homoptera: Membracidae, Micrutalis malleifera Fowler). Mechanism
of transmission is persistent (circulative, non-propagative). BCTV
may be transmitted with difficulty by mechanical inoculation. Experimentally
some species have been transmitted by Agrobacterium-mediated
transfer using recombinant DNA methods (BCTV, HrCTV, TPCTV).
LIST OF SPECIES DEMARCATION CRITERIA IN THE GENUS
The list of species demarcation criteria in the genus Curtovirus
are:
- No trans-replication of genomes by Rep proteins,
- Complete genome sequence identity <80%,
- Virions react differently with key antibodies,
- Different vector species,
- Host range / pathogenicity different in key species.
