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Mol. Cell. Biol., 08 1995, 3979-3988, Vol 15, No. 8
MB Humphrey, J Bryan, TA Cooper and SM Berget
Large alternatively spliced internal exons are uncommon in vertebrate
genes, and the mechanisms governing their usage are unknown. In this
report, we examined alternative splicing of a 1-kb internal exon from the
human caldesmon gene containing two regulated 5' splice sites that are 687
nucleotides apart. In cell lines normally splicing caldesmon RNA via
utilization of the exon-internal 5' splice site, inclusion of the
differential exon required a long purine-rich sequence located between the
two competing 5' splice sites. This element consisted of four identical
32-nucleotide purine-rich repeats that resemble exon- splicing enhancers
(ESE) identified in other genes. One 32-nucleotide repeat supported exon
inclusion, repressed usage of the terminal 5' splice site, and functioned
in a heterologous exon dependent on exon enhancers for inclusion,
indicating that the caldesmon purine-rich sequence can be classified as an
ESE. The ESE was required for utilization of the internal 5' splice site
only in the presence of the competing 5' splice site and had no effect when
placed downstream of the terminal 5' splice site. In the absence of the
internal 5' splice site, the ESE activated a normally silent cryptic 5'
splice site near the natural internal 5' splice site, indicating that the
ESE stimulates upstream 5' splice site selection. We propose that the
caldesmon ESE functions to regulate competition between two 5' splice sites
within a differential internal exon.
Copyright © 1995, American Society for Microbiology
A 32-nucleotide exon-splicing enhancer regulates usage of competing 5' splice sites in a differential internal exon
Department of Cell Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
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