A UV-responsive internal ribosome entry site enhances serine hydroxymethyltransferase 1 expression for DNA damage repair.

Thymidine nucleotides are required for faithful DNA synthesis and repair, and their de novo biosynthesis is regulated by serine hydroxymethyltransferase 1 (SHMT1). The SHMT1 transcript contains a heavy chain ferritin, heterogeneous nuclear ribonucleoprotein H2, and CUG-binding protein 1-responsive internal ribosome entry site (IRES) that regulates SHMT1 translation. In this study a non-lethal dose of UVC is shown to increase SHMT1 IRES activity and protein levels in four different cell lines. The mechanism for the UV-induced activation of the SHMT1 IRES involves an increase in heavy chain ferritin and heterogeneous nuclear ribonucleoprotein H2 expression and the translocation of CUG-binding protein 1 from the nucleus to the cytoplasm. The UV-induced increase in SHMT1 translation is accompanied by an increase in the small ubiquitin-like modifier-dependent nuclear localization of the de novo thymidylate biosynthesis pathway and a decrease in DNA strand breaks, indicating a role for SHMT1 and nuclear folate metabolism in DNA repair.