Control of translation initiation is important in a wide range of normal biological processes, from development to learning and memory to cell death. Improper execution and control of translation initiation are involved in the etiology of a variety of human diseases, including viral infection, neurodegeneration and cancer. In order to understand how translation initiation is controlled, and how this control can break down, we must first understand the underlying molecular mechanics of the normal process. The experiments proposed in this application will deepen our understanding of the molecular basis of translation initiation in eukaryotes. The work will employ a fully reconstituted S. cerevisiae translation initiation system and will involve close collaboration with the laboratory of Alan Hinnebusch at the NIH/NICHD. The studies proposed will address two major gaps in our understanding of eukaryotic translation initiation. The first aim focuses on the mechanism through which the start codon in an mRNA is recognized by the ribosomal pre-initiation complex. Recognition of the start codon is one of the most important readings of the genetic code because it sets the frame for decoding of the message. Improper start site selection results in production of a miscoded, potentially toxic, protein. The proposed studies will elucidate the pathways of information transfer within the pre-initiation complex, from initial formation of three base pairs between the start codon and initiator tRNA anticodon to the downstream irreversible events that commit the complex to finishing initiation at the selected point on the message. Our studies will focus on the roles that the initiator tRNA itself and the initiation factor eIF5 play in this process. A significant amount of data indicates that these two components are missing links in our understanding of the molecular basis of start codon recognition. The second major aim is to dissect the molecular mechanics of mRNA recruitment to the pre-initiation complex, which is a critical event in protein synthesis and a key target of regulation. In this grant cycle, studies of mRNA recruitment will focus on the mechanisms of action of the large, heteromultimeric factor eIF3 and the ssRNA binding protein eIF4B. Preliminary work indicates central roles for both factors in this process, yet currently little is known about how they function.