Ribosomal Chamber Music: Toward an Understanding of IRES Mechanisms

Internal initiation is a 50-end-independent mode of translation initiation engaged by many virus-and putatively some cell-encoded templates. Internal initiation is facilitated by specific RNA tertiary folds, called internal ribosomal entry sites (IRESs), in the 50 untranslated region (UTR) of the respective transcripts. In this review we discuss recent structural insight into how established IRESs first capture and then manipulate the eukaryotic translation machinery through non-canonical interactions and by guiding the intrinsic conformational flexibility of the eukaryotic ribosome. Because IRESs operate with reduced complexity and constitute minimal systems of initiation, comparison with canonical initiation may allow common mechanistic principles of the ribosome to be delineated.