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Molecular and Cellular Biology, September 1998, p. 5272-5283, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

At Least One Intron Is Required for the Nonsense-Mediated Decay of Triosephosphate Isomerase mRNA: a Possible Link between Nuclear Splicing and Cytoplasmic Translation

Jing Zhang,dagger Xiaolei Sun, Yimei Qian,Dagger Jeffrey P. LaDuca, and Lynne E. Maquat*

Department of Cancer Genetics, Roswell Park Cancer Institute, New York State Department of Health, Buffalo, New York 14263

Received 19 November 1997/Returned for modification 21 January 1998/Accepted 1 June 1998

Mammalian cells have established mechanisms to reduce the abundance of mRNAs that harbor a nonsense codon and prematurely terminate translation. In the case of the human triosephosphate isomerase (TPI gene), nonsense codons located less than 50 to 55 bp upstream of intron 6, the 3'-most intron, fail to mediate mRNA decay. With the aim of understanding the feature(s) of TPI intron 6 that confer function in positioning the boundary between nonsense codons that do and do not mediate decay, the effects of deleting or duplicating introns have been assessed. The results demonstrate that TPI intron 6 functions to position the boundary because it is the 3'-most intron. Since decay takes place after pre-mRNA splicing, it is conceivable that removal of the 3'-most intron from pre-mRNA "marks" the 3'-most exon-exon junction of product mRNA so that only nonsense codons located more than 50 to 55 nucleotides upstream of the "mark" mediate mRNA decay. Decay may be elicited by the failure of translating ribosomes to translate sufficiently close to the mark or, more likely, the scanning or looping out of some component(s) of the translation termination complex to the mark. In support of scanning, a nonsense codon does not elicit decay if some of the introns that normally reside downstream of the nonsense codon are deleted so the nonsense codon is located (i) too far away from a downstream intron, suggesting that all exon-exon junctions may be marked, and (ii) too far away from a downstream failsafe sequence that appears to function on behalf of intron 6, i.e., when intron 6 fails to leave a mark. Notably, the proposed scanning complex may have a greater unwinding capability than the complex that scans for a translation initiation codon since a hairpin structure strong enough to block translation initiation when inserted into the 5' untranslated region does not block nonsense-mediated decay when inserted into exon 6 between a nonsense codon residing in exon 6 and intron 6.

* Corresponding author. Mailing address: Department of Cancer Genetics, Roswell Park Cancer Institute, Elm & Carlton Streets, Buffalo, NY 14263. Phone: (716) 845-3325. Fax: (716) 845-8449. E-mail: Maquat{at}sc3101.med.buffalo.edu.

dagger Present address: Millennium Pharmaceutical, Inc., Cambridge, MA 02139.

Dagger Present address: Albert Einstein College of Medicine, Department of Molecular Pharmacology, Bronx, NY 10461.

Molecular and Cellular Biology, September 1998, p. 5272-5283, Vol. 18, No. 9
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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