An internal ribosome entry segment in the 5' untranslated region of the mnt gene.

Mnt is a transcriptional repressor related to the Myc/Mad family of transcription factors. It is expressed in proliferating, resting and differentiating cells and is believed to antagonize the function of Myc. Here we have characterized the major transcription initiation site of the mnt gene. In doing so we noted a remarkable level of sequence conservation between the murine and human 5' untranslated regions. Our experiments revealed that this sequence contains an internal ribosome entry segment (IRES). In addition, we show that sequences at both the 5' and 3' end of the IRES are essential for its function. These findings indicate that mnt can be translated by internal initiation. Such a mechanism may allow efficient Mnt synthesis when cap-dependent translation initiation is reduced.