A soluble factor partially purified from calf liver increases transcription by RNA polymerase I in isolated nuclei. Addition of the factor to reactions which have reached a plateau owing to the inability to reinitiate on the endogenous chromatin template restores the initial rate of synthesis and stimulates an increased accumulation of RNA product. The RNA synthesis stimulated by factor addition is identical with that initiated in vivo in that it is resistant to heparin disruption.
