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Journal of Virology, July 2000, p. 5825-5835, Vol. 74, No. 13
Institut für Virologie (FB
Veterinärmedizin), Justus-Liebig-Universität Giessen,
D-35392 Giessen, Germany
Received 16 November 1999/Accepted 12 April 2000
Bovine viral diarrhea virus (BVDV), a
Pestivirus member of the Flaviviridae family,
has a positive-stranded RNA genome which consists of a single open
reading frame (ORF) and untranslated regions (UTRs) at the 5' and 3'
ends. The 5' UTR harbors extensive RNA structure motifs; most of them
were shown to contribute to an internal ribosomal entry site (IRES),
which mediates cap-independent translation of the ORF. The extreme
5'-terminal region of the BVDV genome had so far been believed not to
be required for IRES function. By structure probing techniques, we
initially verified the existence of a computer-predicted stem-loop
motif at the 5' end of the viral genome (hairpin Ia) as well as at the
3' end of the complementary negative-strand replication intermediate [termed hairpin Ia (
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
A Stem-Loop Motif Formed by the Immediate 5' Terminus of the
Bovine Viral Diarrhea Virus Genome Modulates Translation as well as
Replication of the Viral RNA
)]. While the stem of this structure is mainly constituted of nucleotides that are conserved among pestiviruses, the
loop region is predominantly composed of variable residues. Taking a
reverse genetics approach to a subgenomic BVDV replicon RNA (DI9c)
which could be equally employed in a translation as well as replication
assay system based on BHK-21 cells, we obtained the following results.
(i) Proper folding of the Ia stem was found to be crucial for efficient
translation. Thus, in the context of an authentic replication-competent
viral RNA, the 5'-terminal motif operates apparently as an integral
functional part of the ribosome entry. (ii) An intact loop structure
and a stretch of nucleotide residues that constitute a portion of the
stem of the Ia or the Ia (
) motif, respectively, were defined to
represent important determinants of the RNA replication pathway. (iii)
Formation of the stem structure of the Ia (
) motif was determined to
be not critical for RNA replication. In summary, our findings affirmed that the 5'-terminal region of the BVDV genome encodes a bifunctional secondary structure motif which may enable the viral RNA to switch from
the translation to the replicative cycle and vice versa.
*
Corresponding author. Mailing address: Institut
für Virologie (FB Veterinärmedizin),
Justus-Liebig-Universität Giessen, Frankfurter Str. 107, D-35392
Giessen, Germany. Phone: 496419938373. Fax: 496419938359. E-mail:
Sven-Erik.Behrens{at}vetmed.uni-giessen.de.
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