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Molecular and Cellular Biology, September 1998, p. 5425-5434, Vol. 18, No. 9
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

Catharine F. Kennedy,1 Angela Krämer,2 and Susan M. Berget1,*

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.

* 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.

Molecular and Cellular Biology, September 1998, p. 5425-5434, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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