J Virol, July 1998, p. 5589-5598, Vol. 72, No. 7
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
B and Sp1 Binding Elements
Institute for Clinical and Molecular Virology, University of Erlangen-Nuernberg, 91054 Erlangen, Germany,1 and New England Regional Primate Research Center, Harvard Medical School, Southborough, Massachusetts 017722
Received 5 September 1997/Accepted 23 March 1998
Large deletions of the upstream U3 sequences in the long terminal
repeats (LTRs) of human immunodeficiency virus and simian immunodeficiency virus (SIV) accumulate in vivo in the absence of an
intact nef gene. In the SIV U3 region, about 65 bp just upstream of the single NF-
B binding site always remained intact, and
some evidence for a novel enhancer element in this region exists. We
analyzed the transcriptional and replicative capacities of
SIVmac239 mutants containing deletions or mutations in these upstream
U3 sequences and/or the NF-
B and Sp1 binding sites. Even in the
absence of 400 bp of upstream U3 sequences, the NF-
B site and all
four Sp1 binding sites, the SIV promoter maintained about 15% of the
wild-type LTR activity and was fully responsive to Tat activation in
transient reporter assays. The effects of these deletions on virus
production after transfection of COS-1 cells with full-length proviral
constructs were much greater. Deletion of the upstream U3 sequences had
no significant influence on viral replication when either the single
NF-
B site or the Sp1 binding sites were intact. In contrast, the 26 bp of sequence located immediately upstream of the NF-
B site was
essential for efficient replication when all core enhancer elements
were deleted. A purine-rich site in this region binds specifically to
the transcription factor Elf-1, a member of the ets
proto-oncogene-encoded family. Our results indicate a high degree of
functional redundancy in the SIVmac U3 region. Furthermore, we defined
a novel regulatory element located immediately upstream of the NF-
B
binding site that allows efficient viral replication in the absence of
the entire core enhancer region.
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