Two different E2F6 proteins generated by alternative splicing and internal translation initiation

E2F transcription factors play an important role in the regulation of cell cycle progression. E2F6, the most recently identified member of the E2F family, is a retinoblastoma-protein-independent transcriptional repressor that is required for developmental patterning of the axial skeleton. It has recently been shown that the E2f6 locus produces two different mRNAs, E2F6 and E2F6b. The E2F6b mRNA contains an additional exon that is inserted by alternative splicing. This exon contains an in-frame stop-codon and an in-frame translation initiation codon. However, whether a protein is translated from the E2F6b mRNA has not yet been addressed. We now show that internal translation initiation gives rise to E2F6b, an amino-terminal truncated E2F6 protein. We also show that E2F6 and E2F6b mRNAs are ubiquitously expressed in primary mouse tissues. During the cell cycle, the highest expression of both forms is found at the G1 to S transition. The 5' untranslated regions of E2F6 and E2F6b are unusually long, and they contain several upstream AUG codons followed by short reading frames. Our results suggest that translation of E2F6b is initiated by internal ribosome entry. We propose that regulated translation initiation can produce distinct E2F6 isoforms under different physiological conditions.