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Journal of Virology, July 2000, p. 5988-5996, Vol. 74, No. 13
Plant Pathology Department, Iowa State
University, Ames, Iowa 50011-1020
Received 29 December 1999/Accepted 16 April 2000
Numerous RNA viruses generate subgenomic mRNAs (sgRNAs) for
expression of their 3'-proximal genes. A major step in control of viral
gene expression is the regulation of sgRNA synthesis by specific
promoter elements. We used barley yellow dwarf virus (BYDV) as a model
system to study transcriptional control in a virus with multiple
sgRNAs. BYDV generates three sgRNAs during infection. The sgRNA1
promoter has been mapped previously to a 98-nucleotide (nt) region
which forms two stem-loop structures. It was determined that sgRNA1 is
not required for BYDV RNA replication in oat protoplasts. In this
study, we show that neither sgRNA2 nor sgRNA3 is required for BYDV RNA
replication. The promoters for sgRNA2 and sgRNA3 synthesis were mapped
by using deletion mutagenesis. The minimal sgRNA2 promoter is
approximately 143 nt long (nt 4810 to 4952) and is located immediately
downstream of the putative sgRNA2 start site (nt 4809). The minimal
sgRNA3 core promoter is 44 nt long (nt 5345 to 5388), with most of the sequence located downstream of sgRNA3 start site (nt 5348). For both
promoters, additional sequences upstream of the start site enhanced
sgRNA promoter activity. These promoters contrast to the sgRNA1
promoter, in which almost all of the promoter is located upstream of
the transcription initiation site. Comparison of RNA sequences and
computer-predicted secondary structures revealed little or no homology
between the three sgRNA promoter elements. Thus, a small RNA virus with
multiple sgRNAs can have very different subgenomic promoters, which
implies a complex system for promoter recognition and regulation of
subgenomic RNA synthesis.
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
A Positive-Strand RNA Virus with Three Very
Different Subgenomic RNA Promoters
and
*
Corresponding author. Mailing address: Plant Pathology
Department, 351 Bessey Hall, Iowa State University, Ames, IA
50011-1020. Phone: (515) 294-2436. Fax: (515) 294-9420. E-mail:
wamiller{at}iastate.edu.
This is paper no. J-18710 of the Iowa Agriculture and Home
Economics Experiment Station, project 3545.
Present address: Howard Hughes Medical Institute, Department of
Molecular Microbiology and Immunology, University of Southern California School of Medicine, Los Angeles, CA 90033-1054.
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