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J. Virol., 12 1997, 9096-9107, Vol 71, No. 12
ZM Zheng, PJ He and CC Baker
Alternative splicing plays an important role in regulation of bovine
papillomavirus type 1 (BPV-1) gene expression. We have recently identified
in BPV-1 late pre-mRNAs two purine-rich exonic splicing enhancers (SE1 and
SE2) which also stimulate splicing of a Drosophila doublesex (dsx) pre-mRNA
containing a suboptimal 3' splice site. In vivo studies now demonstrate
that both SE1 and SE2 are required for preferential use of the BPV-1
nucleotide (nt) 3225 3' splice site in nonpermissive cells. Deletion or
mutation of either element in a BPV-1 late pre-mRNA switches splicing to
the late-specific alternative 3' splice site at nt 3605. To investigate the
sequence specificity of these exonic splicing enhancers, various mutant SE1
or SE2 elements were connected to dsx pre-mRNAs and tested for their
stimulatory effects on dsx pre-mRNA splicing in vitro. Substitution of U
residues for either A or G residues in and around potential ASF/SF2 binding
sites in SE1 or SE2 resulted in a significant reduction of splicing
enhancer activity. However, the G-to-U substitutions in both enhancers had
the largest effect, reducing splicing to near control levels. Further in
vitro analyses showed that splicing enhancement by SE2 could be competed
with excess unlabeled SE2 RNA, indicating that SE2 activity in HeLa nuclear
extracts is mediated by trans-acting factors. UV cross- linking plus
immunoprecipitation assays showed that both wild-type SE1 and SE2 RNAs
could bind directly to purified HeLa SR proteins SRp30a (ASF/SF2), SRp55,
and SRp75. UV cross-linking experiments also identified a 23-kDa protein
which binds to SE2 but not SE1. This protein is present in both HeLa
nuclear extracts and S100 extracts but absent from SR protein preparations,
suggesting that it is not a classical SR protein. Mutant SE elements
(containing G- to U-mutations) which had minimal splicing enhancer activity
also had very weak binding capacity for these proteins, strongly suggesting
that the binding of these proteins is required for splicing enhancer
function.
Copyright © 1997, American Society for Microbiology
Structural, functional, and protein binding analyses of bovine papillomavirus type 1 exonic splicing enhancers
Division of Basic Sciences, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-5055, USA. Zhengt@dce41.nci.nih.gov
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