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Mol Cell. 2000 Apr;5(4):607-16.

A cell cycle-dependent internal ribosome entry site.

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Department of Biochemistry and McGill Cancer Center, McGill University, Montréal, Qúebec, Canada.


The eukaryotic mRNA 5' cap structure facilitates translation. However, cap-dependent translation is impaired at mitosis, suggesting a cap-independent mechanism for mRNAs translated during mitosis. Translation of ornithine decarboxylase (ODC), the rate-limiting enzyme in the biosynthesis of polyamines, peaks twice during the cell cycle, at the G1/S transition and at G2/M. Here, we describe a cap-independent internal ribosome entry site (IRES) in the ODC mRNA that functions exclusively at G2/M. This ensures elevated levels of polyamines, which are implicated in mitotic spindle formation and chromatin condensation. c-myc mRNA also contains an IRES that functions during mitosis. Thus, IRES-dependent translation is likely to be a general mechanism to synthesize short-lived proteins even at mitosis, when cap-dependent translation is interdicted.

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