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J. Biol. Chem., Vol. 279, Issue 21, 22166-22175, May 21, 2004

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Polypyrimidine Tract-binding Protein Is Involved in Vivo in Repression of a Composite Internal/3' -Terminal Exon of the Xenopus -Tropomyosin Pre-mRNA^*

Sandra Hamon, Caroline Le Sommer, Agnès Mereau, Marie-Rose Allo, and Serge Hardy

From the UMR 6061 CNRS, Université de Rennes 1, Faculté de Médecine, 35043 Rennes cedex, France

The Xenopus _fast-tropomyosin gene contains, at its 3' -end, a composite internal/3' -terminal exon (exon 9A9'), which is subjected to three different patterns of splicing according to the cell type. Exon 9A9' is included as a terminal exon in the myotome and as an internal exon in adult striated muscles, whereas it is skipped in nonmuscle cells. We have developed an in vivo model based on transient expression of minigenes encompassing the regulated exon 9A9' in Xenopus oocytes and embryos. We first show that the different -tropomyosin minigenes recapitulate the splicing pattern of the endogenous gene and constitute valuable tools to seek regulatory sequences involved in exon 9A9' usage. A mutational analysis led to the identification of an intronic element that is involved in the repression of exon 9A9' in nonmuscle cells. This element harbors four polypyrimidine track-binding protein (PTB) binding sites that are essential for the repression of exon 9A9'. We show using UV cross-linking and immunoprecipitation experiments that Xenopus PTB (XPTB) interacts with these PTB binding sites. Finally, we show that depletion of endogenous XPTB in Xenopus embryos using a morpholinobased translational inhibition strategy resulted in exon 9A9' inclusion in embryonic epidermal cells. These results demonstrate that XPTB is required in vivo to repress the terminal exon 9A9' and suggest that PTB could be a major actor in the repression of regulated 3' -terminal exon.

Received for publication, December 17, 2003 , and in revised form, March 5, 2004.

The nucleotide sequence(s) reported in this paper has been submitted to the GenBank^TM/EBI Data Bank with accession number(s) AY536368.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

The on-line version of this article (available at http://www.jbc.org)contains three supplemental figures.

To whom correspondence should be addressed. Tel.: 33-2-23-23-44-66; Fax: 33-2-23-23-44-78; E-mail: serge.hardy{at}univ-rennes1.fr.

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