Cap-independent translation conferred by the 5′-untranslated region of human thymidine kinase mRNA

Translational control is one of the mechanisms that regulate thymidine kinase (TK) expression in the cell cycle. Evidence for the TK mRNA sequence that is involved in its own translation has been lacking. In this report, we show that TK-deficient mouse fibroblasts transfected with pFLAG-TK express a TK mRNA containing the 5′-untranslated region (5′UTR) and produce two polypeptides, FLAG-TK and TK, resulting from an alternative initiation of translation. Most interestingly, the 5′UTR of TK allowed the translation of FLAG-TK mRNA to become cap-independent in an in vitro translation system. Furthermore, this 5′UTR sequence decreased significantly the efficiency of translation from the AUG codon of FLAG when the concentration of FLAG-TK RNA was low. Here, we also show that in normal human IMR-90 fibroblasts the induction of TK polypeptide by serum stimulation is insensitive to rapamycin treatment, which is known to inhibit the translations of transcripts of some growth-controlled genes by affecting the cap-binding efficiency. Taken together, we propose that the 5′UTR in TK mRNA might actually confer a secondary structure to regulate ribosome binding during translation in a cap-independent manner.