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Mol Cell. 2001 May;7(5):1103-9.

Base-pairing between untranslated regions facilitates translation of uncapped, nonpolyadenylated viral RNA.

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Interdepartmental Genetics, Plant Pathology Department, 351 Bessey Hall, Iowa State University, Ames, IA 50011, USA.


Translationally competent mRNAs form a closed loop via interaction of initiation factors with the 5' cap and poly(A) tail. However, many viral mRNAs lack a cap and/or a poly(A) tail. We show that an uncapped, nonpolyadenylated plant viral mRNA forms a closed loop by direct base-pairing (kissing) of a stem loop in the 3' untranslated region (UTR) with a stem loop in the 5' UTR. This allows a sequence in the 3' UTR to confer translation initiation at the 5'-proximal AUG. This base-pairing is also required for replication. Unlike other cap-independent translation mechanisms, the ribosome enters at the 5' end of the mRNA. This remarkably long-distance base-pairing reveals a novel mechanism of cap-independent translation and means by which mRNA UTRs can communicate.

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