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Molecular and Cellular Biology, February 1999, p. 1595-1604, Vol. 19, No. 2
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Transcription Termination and 3'-End Processing of the Spliced Leader RNA in Kinetoplastids

Nancy R. Sturm, Michael C. Yu, and David A. Campbell*

Department of Microbiology and Immunology, University of California Los Angeles School of Medicine, Los Angeles, California 90095-1747

Received 16 July 1998/Returned for modification 14 October 1998/Accepted 6 November 1998

Addition of a 39-nucleotide (nt) spliced leader (SL) by trans splicing is a basic requirement for all trypanosome nuclear mRNAs. The SL RNA in Leishmania tarentolae is a 96-nt precursor transcript synthesized by a polymerase that resembles polymerase II most closely. To analyze SL RNA genesis, we mutated SL RNA intron structures and sequence elements: stem-loops II and III, the Sm-binding site, and the downstream T tract. Using an exon-tagged SL RNA gene, we examined the phenotypes produced by a second-site 10-bp linker scan mutagenic series and directed mutagenesis. Here we report that transcription is terminated by the T tract, which is common to the 3' end of all kinetoplastid SL RNA genes, and that more than six T's are required for efficient termination in vivo. We describe mutants whose SL RNAs end in the T tract or appear to lack efficient termination but can generate wild-type 3' ends. Transcriptionally active nuclear extracts show staggered products in the T tract, directed by eight or more T's. The in vivo and in vitro data suggest that SL RNA transcription termination is staggered in the T tract and is followed by nucleolytic processing to generate the mature 3' end. We show that the Sm-binding site and stem-loop III structures are necessary for correct 3'-end formation. Thus, we have defined the transcription termination element for the SL RNA gene. The termination mechanism differs from that of vertebrate small nuclear RNA genes and the SL RNA homologue in Ascaris.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, UCLA School of Medicine, 10833 Le Conte Ave., Los Angeles, CA 90095. Phone: (310) 825-4195. Fax: (310) 206-3865. E-mail: dc{at}ucla.edu.

Molecular and Cellular Biology, February 1999, p. 1595-1604, Vol. 19, No. 2
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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