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Molecular and Cellular Biology, June 2000, p. 3988-3995, Vol. 20, No. 11
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
A 5' Splice Site-Proximal Enhancer Binds SF1 and
Activates Exon Bridging of a Microexon
Troy
Carlo,1,2,
Rebecca
Sierra,1 and
Susan M.
Berget1,2,*
Verna and Marrs McLean Department of
Biochemistry1 and Program in Cell and
Molecular Biology,2 Baylor College of
Medicine, Houston, Texas 77030
Received 14 July 1999/Returned for modification 25 August
1999/Accepted 15 March 2000
Internal exon size in vertebrates occurs over a narrow size range.
Experimentally, exons shorter than 50 nucleotides are poorly included
in mRNA unless accompanied by strengthened splice sites or accessory
sequences that act as splicing enhancers, suggesting steric
interference between snRNPs and other splicing factors binding
simultaneously to the 3' and 5' splice sites of microexons. Despite
these problems, very small naturally occurring exons exist. Here we
studied the factors and mechanism involved in recognizing a
constitutively included six-nucleotide exon from the cardiac troponin T
gene. Inclusion of this exon is dependent on an enhancer located
downstream of the 5' splice site. This enhancer contains six copies of
the simple sequence GGGGCUG. The enhancer activates heterologous microexons and will work when located either upstream or
downstream of the target exon, suggesting an ability to bind factors
that bridge splicing units. A single copy of this sequence is
sufficient for in vivo exon inclusion and is the binding site for the
known bridging mammalian splicing factor 1 (SF1). The enhancer and its
bound SF1 act to increase recognition of the upstream exon during exon
definition, such that competition of in vitro reactions with RNAs
containing the GGGGCUG repeated sequence depress splicing of
the upstream intron, assembly of the spliceosome on the 3' splice site
of the exon, and cross-linking of SF1. These results suggest a model in
which SF1 bridges the small exon during initial assembly, thereby
effectively extending the domain of the exon.
*
Corresponding author. Mailing address: Verna and Marrs
McLean Department of Biochemistry and Program in Cell and Molecular Biology, Baylor College of Medicine, One Baylor Plaza, Houston, TX
77030. Phone: (713) 798-5758. Fax: (713) 795-5487. E-mail: sberget{at}bcm.tmc.edu.

Present address: Department of Biology, Brandeis University,
Waltham, MA 02254-9110.
Molecular and Cellular Biology, June 2000, p. 3988-3995, Vol. 20, No. 11
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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