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Mol. Cell. Biol., Jan 1998, 343-352, Vol 18, No. 1
LL Elrick, MB Humphrey, TA Cooper and SM Berget
Purine-rich enhancers are exon sequences that promote inclusion of
alternative exons, usually via activation of weak upstream 3' splice sites.
A recently described purine-rich enhancer from the caldesmon gene has an
additional activity by which it directs selection of competing 5' splice
sites within an alternative exon. In this study, we have compared the
caldesmon enhancer with another purine-rich enhancer from the chicken
cardiac troponin T (cTNT) gene for the ability to regulate flanking splice
sites. Although similar in sequence and length, the two enhancers
demonstrated strikingly different specificities towards 5' splice site
choice when placed between competing 5' splice sites in an internal exon.
The 32-nucleotide caldesmon enhancer caused effective usage of the
exon-internal 5' splice site, whereas the 30-nucleotide cTNT enhancer
caused effective usage of the exon-terminal 5' splice site. Both
enhancer-mediated splicing pathways represented modulation of the default
pathway in which both 5' splice sites were utilized. Each enhancer is
multipartite, consisting of two purine-rich sequences of a simple (GAR)n
repeat interdigitated with two enhancer-specific sequences. The entire
enhancer was necessary for maximal splice site selectivity; however, a 5-
to 7-nucleotide region from the 3' end of each enhancer dictated splice
site selectivity. Mutations that interchanged this short region of the two
enhancers switched specificity. The portion of the cTNT enhancer
determinative for 5' splice site selectivity was different than that shown
to be maximally important for activation of a 3' splice site, suggesting
that enhancer environment can have a major impact on activity. These
results are the first indication that individual purine-rich enhancers can
differentiate between flanking splice sites. Furthermore, localization of
the specificity of splice site choice to a short region within both
enhancers indicates that subtle differences in enhancer sequence can have
profound effects on the splicing pathway.
Copyright © 1998, American Society for Microbiology
A short sequence within two purine-rich enhancers determines 5' splice site specificity
Verna and Marrs McLean Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77030, USA.
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