The research proposal is aimed at the understanding of the functions and regulatory mechanisms of all the gene products coded on the phi X-174 genome. Our strategy to study these regulatory mechanisms is to compare in vivo and in vitro system. Recently we have proposed a model describing the functions of these gene products during single-stranded DNA synthesis and phage morphogenesis based mainly on physiological and genetic evidence. We are developing an in vitro system that synthesizes circular viral DNA of phi X-174. Information on these processes accumulated from in vivo studies will help us avoid in vitro artifacts and will provide the basis for testing the validity of results and interpretations obtained in vitro studies. In turn, the in vitro system will provide information which would be difficult to obtain from in vivo studies because of technical problems inherent in in vivo systems.