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Molecular and Cellular Biology, September 1998, p. 5425-5434, Vol. 18, No. 9
Verna and Marrs McLean Department of
Biochemistry, Program in Cell and Molecular Biology, Baylor College
of Medicine, Houston, Texas 77030,1 and
Département de Biologie Cellulaire, Sciences III,
Université de Genève, CH-1211 Geneva 4, Switzerland2
Received 5 February 1998/Returned for modification 24 March
1998/Accepted 25 May 1998
One of the earliest steps in pre-mRNA recognition involves binding
of the splicing factor U2 snRNP auxiliary factor (U2AF or MUD2 in
Saccharomyces cerevisiae) to the 3' splice site region. U2AF interacts with a number of other proteins, including members of
the serine/arginine (SR) family of splicing factors as well as splicing
factor 1 (SF1 or branch point bridging protein in S. cerevisiae), thereby participating in bridging either exons or
introns. In vertebrates, the binding site for U2AF is the
pyrimidine tract located between the branch point and 3'
splice site. Many small introns, especially those in nonvertebrates,
lack a classical 3' pyrimidine tract. Here we show that a
59-nucleotide Drosophila melanogaster intron contains
C-rich pyrimidine tracts between the 5' splice site and
branch point that are needed for maximal binding of both U1 snRNPs and
U2 snRNPs to the 5' and 3' splice site, respectively, suggesting that
the tracts are the binding site for an intron bridging factor. The
tracts are shown to bind both U2AF and the SR protein SRp54 but not
SF1. Addition of a strong 3' pyrimidine tract downstream of
the branch point increases binding of SF1, but in this context, the
upstream pyrimidine tracts are inhibitory. We suggest that
U2AF- and/or SRp54-mediated intron bridging may be an alternative early
recognition mode to SF1-directed bridging for small introns, suggesting
gene-specific early spliceosome assembly.
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
A Role for SRp54 during Intron Bridging of Small
Introns with Pyrimidine Tracts Upstream of the Branch Point
*
Corresponding author. Mailing address: Verna and Marrs
McLean Department of Biochemistry, 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.
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