Mastreviruses largely infect grasses, though there are two which
infect dicots (Briddon and Markham, 1995; Liu et al., 1997). The viruses are obligately
transmitted by leafhoppers (Homoptera: Cicadellidae), and are largely phloem-limited.
The genus includes:
See the tree, and Rybicki (1994), Padidam et al. (1995), Briddon and Markham (1995), and Palmer and Rybicki (1998) for details of relationships. See also our database for details as to authors, sequences, etc.
Mastreviruses, like all other Geminiviridae, have geminate particles, apparently consisting of two incomplete T=1 icosahedra joined together so as to produce a structure with 22 pentameric capsomers, and 110 identical protein subunits (Hatta and Francki, 1979) [The fine structure of chloris striate mosaic virus Virology 92: 428-435]).
A representation of this is shown alongside the text. For this and other fine electron micrographs, we are indebted to the Rothamsted Experiment Station site: (Copyright) 1994 Rothamsted Experimental Station.
Mastreviruses generally have the genome arrangement depicted below (linearised from the end of the long intergenic region) (Palmer and Rybicki, 1998):
thus, they are bidirectionally transcribed from a "control region" - the long intergenic region or LIR - which contains a virion-sense origin of replication as well as both V(irion) and C(omplementary) sense promoters. The LIR contains an invariant nonanucleotide sequence (TAATATTAC) found in all Geminiviridae: this is in the loop region of a potential stem-loop structure also found in all Geminiviridae; the origin of replication for plus trand synthesis is the last T residue. Arrows labelled V and C represent open reading frames (ORFs), now known to be genes for the proteins mentioned.
The short intergenic region or SIR contains bidirectional polyadenylation signals. It also appears to be the origin of replication for complementary strand synthesis: nearly all Mastreviruses characterised to date have been shown to have a short complementary strand oligonucleotide primer annealed to virion sense sscDNA in the region of the SIR.
Mastreviruses have three genes that are known to be necessary for infectious virion synthesis / infections of plants: these are the:
The MP appears to localise to cell walls in infected cells, and is presumed to attach specifically to plasmodesmata. The MP gene is the most variable sequence - apart from the LIR - in the Mastrevirus genome (Rybicki, 1994; Padidam et al., 1995). Like other movement proteins, it is presumed to have a role in moving viral nucleic acid and/or nucleoprotein complexes through plasmodesmata to adjacent cells.
The CP is the single capsid protein, Mr +/- 28 kDa, and is known to be involved in specifying vector transmission as well as being involved in long-distance movement (coat protein minus virus mutants are non-infectious, although they can replicate). The CP is relatively strongly conserved among Mastreviruses, and is one of the two protein sequences usable to show relationships among Geminiviridae (Howarth and Vandemark, 1989; Rybicki, 1994; Padidam et al., 1995) (see also the CP tree).
There is some debate as to whether RepA may be synthesised and have a function as a separate protein; the Rep of Begomo- and Curtoviruses is encoded in a single ORF which resembles the splice product of RepA and RepB ORFs of Mastreviruses. Rep (or Reps) is necessary for replication in that the protein nicks double-stranded replicative form DNA (dsRF-DNA) between the last T and A in the invariant nonanucleotide sequence TAATATT|AC; the protein then binds via a conserved tyrosine residue to the 5'-A, while cellular polymerases (presumably repair enzymes) elongate the new 3'-T on the complementary strand as template in a classic "rolling circle" replication pathway (Palmer and Rybicki, 1998). The 5'-bound Rep then circularises the nascent ssDNA after this is cleaved in the new TAATATTAC sequence. Rep - and especially RepB - is the most conserved among Mastrevirus proteins, and is the other of the two proteins used to show Geminiviridae relationships ( (Howarth and Vandemark, 1989; Rybicki, 1994; Padidam et al., 1995) (see also the Rep tree). It is possible to amplify via PCR essentially the whole range of genomic diversity of cereal Mastreviruses at least, by means of a pair of degenerate 17-mer oligonucleotide primers derived from a 254 base sequence in the RepB ORF (Rybicki and Hughes, 1990).
Copyright Ed Rybicki, November 1997, April 1999
(unless otherwise stated)