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J. Virol., Jan 1996, 148-158, Vol 70, No. 1
BM Orozco and L Hanley-Bowdoin
The genome of the geminivirus tomato golden mosaic virus (TGMV) consists of
two single-stranded circular DNAs, A and B, that replicate through a
rolling-circle mechanism in nuclei of infected plant cells. The TGMV origin
of replication is located in a conserved 5' intergenic region and includes
at least two functional elements: the origin recognition site of the
essential viral replication protein, AL1, and a sequence motif with the
potential to form a hairpin or cruciform structure. To address the role of
the hairpin motif during TGMV replication, we constructed a series of
B-component mutants that resolved sequence changes from structural
alterations of the motif. Only those mutant B DNAs that retained the
capacity to form the hairpin structure replicated to wild-type levels in
tobacco protoplasts when the viral replication proteins were provided in
trans from a plant expression cassette. In contrast, the same B DNAs
replicated to significantly lower levels in transient assays that included
replicating, wild-type TGMV A DNA. These data established that the hairpin
structure is essential for TGMV replication, whereas its sequence affects
the efficiency of replication. We also showed that TGMV AL1 functions as a
site-specific endonuclease in vitro and mapped the cleavage site to the
loop of the hairpin. In vitro cleavage analysis of two TGMV B mutants with
different replication phenotypes indicated that there is a correlation
between the two assays for origin activity. These results suggest that the
in vivo replication results may reflect structural and sequence
requirements for DNA cleavage during initiation of rolling-circle
replication.
Copyright © 1996, American Society for Microbiology
A DNA structure is required for geminivirus replication origin function
Department of Biochemistry, North Carolina State University, Raleigh 27695-7622, USA.
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