Vaccinia virus is the prototypical member of the poxvirus family that includes several important human and animal pathogens. The pattern of vaccinia virus gene expression can be divided into an early and a late phase of development. Early proteins are encoded by messenger RNA synthesized by enzymes encapsidated within the virus particle. The mechanism of vaccinia early messenger RNA synthesis will be studied in vitro using proteins extracted from virion. Individual proteins participating in the transcription process will be identified, purified, and characterized with the aim of developing a defined in vitro transcription system. The previously identified vaccinia early transcription factor will be characterized as to its interaction with transcription initiation sites on viral DNA, its interaction with the viral RNA polymerase, and its associated adenosine triphosphatase activity. Finally, the viral genes for proteins functioning in RNA synthesis and modification will be identified to permit genetic analysis of the vaccinia transcription system.