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Journal of Virology, November 1999, p. 9080-9088, Vol. 73, No. 11
Department of Biochemistry, University of
California, Riverside, California 92521-0129
Received 12 May 1999/Accepted 20 July 1999
Translation in plants is highly cap dependent, and the only plant
mRNAs known to naturally lack a cap structure (m7GpppN) are
viral in origin. The genomic RNA of tobacco etch virus (TEV), a
potyvirus that belongs to the picornavirus superfamily, is a
polyadenylated mRNA that is naturally uncapped and yet is a highly
competitive mRNA during translation. The 143-nucleotide 5' leader
is responsible for conferring cap-independent translation even on
reporter mRNAs. We have carried out a deletion analysis of the TEV
5' leader to identify the elements responsible for its regulatory
function and have identified two centrally located cap-independent
regulatory elements (CIREs) that promote cap-independent translation.
The introduction of a stable stem-loop structure upstream of each
element demonstrated that CIRE-1 is less 5' end dependent in function
than CIRE-2. In a dicistronic mRNA, the presence of the TEV 5'
leader sequence in the intercistronic region increased expression of
the second cistron, suggesting that the viral sequence can function in
a 5'-distal position. Interestingly, the introduction of a stable
stem-loop upstream of the TEV leader sequence or upstream of either
CIRE in dicistronic constructs markedly increased their regulatory
function. These data suggest that the TEV 5' leader contains two
elements that together promote internal initiation but that the
function of one element, in particular, is facilitated by proximity to
the 5' end.
0022-538X/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Identification and Characterization of the
Functional Elements within the Tobacco Etch Virus 5' Leader Required
for Cap-Independent Translation
*
Corresponding author. Mailing address: Department of
Biochemistry, University of California, Riverside, CA 92521-0129. Phone: (909) 787-7298. Fax: (909) 787-3590. E-mail:
drgallie{at}citrus.ucr.edu.
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