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Molecular and Cellular Biology, September 1998, p. 5404-5413, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

The Exon Splicing Silencer in Human Immunodeficiency Virus Type 1 Tat Exon 3 Is Bipartite and Acts Early in Spliceosome Assembly

Zhi-hai Si, Dan Rauch, and C. Martin Stoltzfus*

Department of Microbiology, University of Iowa, Iowa City, Iowa 52242

Received 10 February 1998/Returned for modification 24 March 1998/Accepted 8 June 1998

Inefficient splicing of human immunodeficiency virus type 1 (HIV-1) RNA is necessary to preserve unspliced and singly spliced viral RNAs for transport to the cytoplasm by the Rev-dependent pathway. Signals within the HIV-1 genome that control the rate of splicing include weak 3' splice sites, exon splicing enhancers (ESE), and exon splicing silencers (ESS). We have previously shown that an ESS present within tat exon 2 (ESS2) and a suboptimal 3' splice site together act to inhibit splicing at the 3' splice site flanking tat exon 2. This occurs at an early step in spliceosome assembly. Splicing at the 3' splice site flanking tat exon 3 is regulated by a bipartite element composed of an ESE and an ESS (ESS3). Here we show that ESS3 is composed of two smaller elements (AGAUCC and UUAG) that can inhibit splicing independently. We also show that ESS3 is more active in the context of a heterologous suboptimal splice site than of an optimal 3' splice site. ESS3 inhibits splicing by blocking the formation of a functional spliceosome at an early step, since A complexes are not detected in the presence of ESS3. Competitor RNAs containing either ESS2 or ESS3 relieve inhibition of splicing of substrates containing ESS3 or ESS2. This suggests that a common cellular factor(s) may be required for the inhibition of tat mRNA splicing mediated by ESS2 and ESS3.

* Corresponding author. Mailing address: Department of Microbiology, University of Iowa, 3770 Bowen Science Building, Iowa City, IA 52242. Phone: (319) 335-7793. Fax: (319) 335-9006. E-mail: marty-stoltzfus{at}uiowa.edu.

Molecular and Cellular Biology, September 1998, p. 5404-5413, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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