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Molecular and Cellular Biology, February 2000, p. 883-891, Vol. 20, No. 3
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Trypanosoma brucei Guide RNA Poly(U) Tail Formation Is Stabilized by Cognate mRNA

Michael T. McManus,1 Brian K. Adler,2 Victoria W. Pollard,3 and Stephen L. Hajduk1,*

Department of Biochemistry and Molecular Genetics1 and Department of Medicine, School of Medicine,2 University of Alabama at Birmingham, Birmingham, Alabama 35294, and Department of Microbiology, School of Medicine, University of Pennsylvania, Philadelphia, Pennsylvania 19104-61483

Received 10 August 1999/Returned for modification 11 October 1999/Accepted 5 November 1999

Guide RNAs (gRNAs) are small RNAs that provide specificity for uridine addition and deletion during mRNA editing in trypanosomes. Terminal uridylyl transferase (TUTase) adds uridines to pre-mRNAs during RNA editing and adds a poly(U) tail to the 3' end of gRNAs. The poly(U) tail may stabilize the association of gRNAs with cognate mRNA during editing. Both TUTase and gRNAs associate with two ribonucleoprotein complexes, I (19S) and II (35S to 40S). Complex II is believed to be the fully assembled active editing complex, since it contains pre-edited mRNA and enzymes thought necessary for editing. Purification of TUTase from mitochondrial extracts resulted in the identification of two chromatographically distinct TUTase activities. Stable single-uridine addition to different substrate RNAs is performed by the 19S complex, despite the presence of a uridine-specific 3' exonuclease within this complex. Multiple uridines are added to substrate RNAs by a 10S particle that may be an unstable subunit of complex I lacking the uridine-specific 3' exonuclease. Multiple uridines could be stably added onto gRNAs by complex I when the cognate mRNA is present. We propose a model in which the purine-rich region of the cognate mRNA protects the uridine tail from a uridine exonuclease activity that is present within the complex. To test this model, we have mutated the purine-rich region of the pre-mRNA to abolish base-pairing interaction with the poly(U) tail of the gRNA. This RNA fails to protect the uridine tail of the gRNA from exoribonucleolytic trimming and is consistent with a role for the purine-rich region of the mRNA in gRNA maturation.

* Corresponding author. Mailing address: Department of Biochemistry & Molecular Genetics, University of Alabama at Birmingham, Birmingham, AL 35294. Phone: (205) 934-6033. Fax: (205) 975-2547. E-mail: shajduk{at}bmg.bhs.uab.edu.

Molecular and Cellular Biology, February 2000, p. 883-891, Vol. 20, No. 3
0270-7306/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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