A cell cycle-dependent internal ribosome entry site

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 dec arboxylase (ODC), the rate-limiting enzyme in the biosynthesis of polyamine s, peaks twice during the cell cycle, at the G1/S transition and at G2/M. H ere, we describe a cap-independent internal ribosome entry site (IRES) in t he ODC mRNA that functions exclusively at G2/M. This ensures elevated level s of polyamines, which are implicated in mitotic spindle formation and chro matin condensation. c-myc mRNA also contains an IRES that functions during mitosis. Thus, IRES-dependent translation is likely to be a general mechani sm to synthesize short-lived proteins even at mitosis, when cap-dependent t ranslation is interdicted.