MODIFICATION OF THE CASCADE MODEL FOR REGULATION OF VACCINIA VIRUS GENE-EXPRESSION - PURIFICATION OF A PREREPLICATIVE, LATE-STAGE-SPECIFIC TRANSCRIPTION FACTOR

In vivo and in vitro studies have provided evidence that vaccinia viru s late gene transcription factors are intermediate gene products synth esized exclusively after DNA replication. Here, we describe an additio nal transcription factor (P3 factor) that stimulates late gene transcr iption between 10- and 40-fold but is made in the absence of viral DNA replication. P3 factor activity was not detected either in uninfected cells or in purified virions. A >1,500-fold purification of P3 factor was achieved by column chromatography of cytoplasmic extracts prepare d from cells infected with vaccinia virus in the presence of a DNA rep lication inhibitor. P3 factor was stage specific, since it could not s ubstitute for early or intermediate transcription factors. Evidence th at late stage-specific transcription factors are made both before and after DNA replication necessitates a modification of the cascade model for vaccinia virus gene regulation.