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Vascular endothelial growth factor (VEGF) is a hypoxia-inducible angiogenic growth factor that promotes compensatory angiogenesis in circumstances of oxygen shortage. The requirement for translational regulation of VEGF is imposed by the cumbersome structure of the 5' untranslated region (5'UTR), which is incompatible with efficient translation by ribosomal scanning, and by the physiologic requirement for maximal VEGF production under conditions of hypoxia, where overall protein synthesis is compromised. Using bicistronic reporter gene constructs, we show that the 1,014-bp 5'UTR of VEGF contains a functional internal ribosome entry site (IRES). Efficient cap-independent translation is maintained under hypoxia, thereby securing efficient production of VEGF even under unfavorable stress conditions. To identify sequences within the 5'UTR required for maximal IRES activity, deletion mutants were analyzed. Elimination of the majority (851 nucleotides) of internal 5'UTR sequences not only maintained full IRES activity but also generated a significantly more potent IRES. Activity of the 163-bp long "improved" IRES element was abrogated, however, following substitution of a few bases near the 5' terminus as well as substitutions close to the translation start codon. Both the full-length 5'UTR and its truncated version function as translational enhancers in the context of a monocistronic mRNA.
Stein et al. (Mon,) studied this question.