Mapping of the minimal internal ribosome entry site element in the human embryonic stem cell gene OCT4B mRNA.

The OCT4 gene is an important regulator of self-renewal in embryonic stem cells and can generate three spliced variants, OCT4A, OCT4B, and OCT4B1. In OCT4B, the single mRNA can generate at least three protein isoforms, OCT4B-164, OCT4B-190, and OCT4B-265, using alternative translation initiation. OCT4B-164 and OCT4B-190 can be translated by an internal ribosome entry site (IRES)-mediated mechanism. Our work previously demonstrated that nucleotides (nt) 102-326 contained an IRES. We have mapped a 30-nt sequence (nt 201-231), which is sufficient to promote internal initiation of translation of OCT4B mRNA. The minimal element contains a sequence unique to OCT4B as well as a sequence common to OCT4A and OCT4B, and the two are essential for IRES activity. Like other cellular IRESs, the IRES activity of the minimal element shows significant variation in different cell lines. The minimal element is also functional under oxidative stress.