The control of protein synthesis is one of the basic mechanisms involved in regulating the growth of cells. Newly discovered components of the translational apparatus must be fully investigated to establish their function and importance in this process. The 4.5S RNA of E. coli, one such newly discovered component, is involved in the initiation phase of protein synthesis. This project is the first detailed study of the function of this molecule on a molecular level. Through genetic construction, a bacterial strain has been made in which synthesis of 4.5S RNA can be regulated. When synthesis is turned off, normal cellular physiology is disrupted, eventually leading to death of the cell. Initiation of protein synthesis is the first observable metabolic process to be affected. This project will investigate in detail the defect in initiation, and determine the exact reaction which requires 4.5S RNA. In addition, one specific model which has been proposed for 4.5S RNA function will be directly tested.