UNR translation can be driven by an IRES element that is negatively regulated by polypyrimidine tract binding protein

Nucleic Acids Res. 2005 May 31;33(10):3095-108. doi: 10.1093/nar/gki611. Print 2005.


Upstream of N-ras (Unr) has been described as an internal initiation trans-acting factor (ITAF) in the cap-independent translation of some particular viral and cellular mRNAs. Two factors led us to hypothesize that the UNR 5'-untranslated region (5'-UTR) may contain an internal ribosome entry site (IRES). The first was the requirement for persisting Unr expression under conditions that correlate with cap-independent translation. The other was the observation that the primary UNR transcript contains a 447 nt long 5'-UTR including two upstream AUGs that may restrict translation initiation via cap-dependent ribosome scanning. Here we report that the UNR 5'-UTR allows IRES-dependent translation, as revealed by a dicistronic reporter assay. Various controls ruled out the contribution of leaky scanning, cryptic promoter sequences or RNA processing events to the ability of the UNR 5'-UTR to mediate internal initiation of translation. Ultraviolet cross-linking analysis and RNA affinity chromatography revealed the binding of polypyrimidine tract binding protein (PTB) to the UNR IRES, requiring a pyrimidine-rich region (nucleotides 335-355). Whereas overexpression of PTB in several cell lines inhibited UNR IRES activity and UNR protein expression, depletion of endogenous PTB using RNAi increased UNR IRES activity. Moreover, a mutant version of the UNR IRES lacking the PTB binding site was more efficient at directing IRES-mediated translation. In conclusion, our results demonstrate that translation of the ITAF Unr can itself be regulated by an IRES that is downregulated by PTB.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • 5' Untranslated Regions / chemistry*
  • 5' Untranslated Regions / metabolism
  • Animals
  • Base Sequence
  • Cell Line
  • Cricetinae
  • DNA-Binding Proteins / biosynthesis
  • DNA-Binding Proteins / genetics*
  • Down-Regulation
  • Gene Expression Regulation*
  • Humans
  • Molecular Sequence Data
  • Peptide Chain Initiation, Translational*
  • Polypyrimidine Tract-Binding Protein / physiology*
  • RNA-Binding Proteins / biosynthesis
  • RNA-Binding Proteins / genetics*
  • Ribosomes / metabolism
  • Sequence Deletion
  • Sequence Homology, Nucleic Acid


  • 5' Untranslated Regions
  • CSDE1 protein, human
  • DNA-Binding Proteins
  • RNA-Binding Proteins
  • Polypyrimidine Tract-Binding Protein