Transcription of DNA and chromatin with calf thymus RNA polymerase B in vitro

The interaction of calf thymus RNA polymerase B with DNA and chromatin has been studied under conditions which permit direct measurement of binding sites capable of supporting chain initiation. The number of such sites is determined by counting the number of growing RNA chains, using either sucrose gradient ultracentrifugation or incorporation of [γ-³²P]ATP and [γ³²P]GTP. These measurements show that there is one site per 40,000 nucleotide pairs on DNA and one-tenth that number on chromatin. This is in contrast to the bacterial RNA polymerase, which has many more initiation sites both on DNA and on chromatin.