Dr. Timothy C. Hall - Director
Plant viruses are pathogens of major crops. Because major groups of viruses contain animal, insect and
plant pathogens, basic studies on plant viruses provide important insight to general strategies for virus
replication and pathogenesis. Our studies have focused on brome mosaic virus (BMV), a pathogen of
grasses and cereals. BMV has a total genome of 8 kb, divided among three single-stranded, (+)-sense
RNAs. RNA-1 (3234 nt) encodes a 109,208 kD protein that contributes helicase, methylation and
other properties that, together with the core replicase encoded by RNA-2 (2865 nt) and currently
unidentified host factors, are essential to virus replication. The other genomic component, RNA-3
(2117 nt), encodes a protein thought to participate in cell-to cell movement and the capsid protein that
is expressed from subgenomic RNA-4 (876 nt).
We found that the 3' end of each RNA component of BMV has a tRNA-like structure that is capable of accepting tyrosine with
similar efficacy to that of tRNATyr (Hall et al., 1972).
This structure is multifunctional, serving also as the recognition site for (-) strand replicase and nucleotidyl transferase
(Bujarski et al., 1986).
We published many studies characterizing the promoters for (-) strand and subgenomic (+) strand synthesis and developed a novel
hypothesis concerning (+) strand promoter activity which invokes the participation of host pol III factors in replication, based on
the similarity of viral sequence motifs to internal control regions (ICRs) 1 and 2 of tRNA gene promoters (see reviews by
(Duggal et al., 1994; Rao et al., 1994)). We initiated studies on RNA-protein binding and a search for host factors involved in the
viral replication complex (Duggal and Hall, 1995).
We have used the natural plant host barley (Hordeum vulgare) for many studies in vivo that employed infectious transcripts of
cDNA clones containing a wide range of mutations introduced by site-directed mutagenesis and other approaches. We have also used
Chenopodium hybridum, a local lesion host, in novel approaches for understanding RNA recombination events that are intrinsic to
viral evolution and pathogenesis (Rao and Hall, 1993).
Recent work was directed towards elucidating the mechanism involved in gene-specific silencing of RNA-2 replication
(Iyer and Hall, 2000).
We made transgenic Nicotiana benthamiana plants that express the p2 protein encoded by BMV RNA-2. Protoplasts from leaves of these
plants support replication of RNAs -1 and -3. However, when these protoplasts are transfected with virion RNA that contains RNA-2 as
well as RNAs-1 and -3, no replication of RNA-2 occurs. By inoculating protoplasts with RNAs -2 and -3, we have shown that there is no
preferential degradation of RNA-2 in the transgenic (p2) protoplasts and, hence, induction of an RNA-2-specific nuclease is unlikely
to be involved. We have also inserted segments of RNA-2 (that collectively cover the entire genome) into RNA-3; the fact that no
silencing of the chimeric RNA-3 was detected further suggests that the silencing seen in this system is protein rather than
RNA-mediated. It is possible that competitive interaction of excess p2 present in the transgenic protoplasts with the replicase
complex displaces nascent RNA-2 during replication.
(Click here for a complete listing of our lab publications)
Bujarski, J.J., Ahlquist, P., Hall, T.C., Dreher, T.W. and Kaesberg, P. 1986. Modulation of
replication, aminoacylation, and adenylation in vitro and infectivity in vivo of BMV RNAs
containing deletions within the multifunctional 3' end. EMBO J. 5: 1769-1774.
Duggal, R. and Hall, T.C. 1995. Interaction of host proteins with the plus-strand promoter of brome
mosaic virus RNA-2. Virology 214: 638-641.
Duggal, R., Lahser, F.C., Rao, A.L.N. and Hall, T.C. 1994. Cis-acting sequences in the replication of
plant viruses with (+)-sense RNA genomes. In: R.J. Cook (ed) Ann. Rev. Phytopathol., pp.
287-309. Annual Reviews, Inc., Palo Alto.
Hall, T.C., Shih, D.S. and Kaesberg, P. 1972. Enzyme-mediated binding of tyrosine to brome-mosaic-virus ribonucleic acid. Biochem. J. 129: 969-976.
Iyer, L.M. and Hall, T.C. 2000. Virus recovery is induced in brome mosaic virus p2 transgenic plants
showing synchronous complementation and RNA-2-specific silencing. Mol. Plant Microbe
Interact. 13: 247-258.
Rao, A.L. and Hall, T.C. 1993. Recombination and polymerase error facilitate restoration of infectivity
in brome mosaic virus. J Virol 67: 969-79.
Rao, A.L.N., Duggal, R., Lahser, F.C. and Hall, T.C. 1994. Analysis of RNA replication in plant
viruses. In: K.W. Adolph (Eds.) Methods in Molecular Genetics: Molecular Virology,
Academic Press, Inc., Orlando, FL., pp. 216-236.