Most eukaryotic translation initiation is thought to be dependent on the 5'-cap structure of the mRNA. It is becoming apparent, however, that the mRNAs of many genes contain IRESs (internal ribosome entry segments) within the 5'-UTR (5'-untranslated region) that allow ribosomes to initiate translation independently of the 5'-cap. IRESs can enable the expression of these genes under conditions (such as viral infection, cellular stress and apoptosis) when cap-dependent translation initiation is compromised, and also provide a target for regulation of gene expression. Recent results from our laboratory and others suggest that 10% of mRNAs (approximately 4000 genes) use this mechanism to initiate translation. One of the central goals of those working in the field of translation is to identify the sequence motif(s) and proteins that are required for internal ribosome entry. We have identified recently a unique PTB (polypyrimidine tract-binding protein) motif (CCU)n that is present in a large subset of cellular IRESs, and the results suggest that PTB itself is involved either directly or indirectly in ribosome recruitment. Here, we describe further investigations of PTB with artificial sequences that harbour this motif.
