RNA polymerase. Direct evidence for a unique topographical site for initiation.

The compound 9-(3'-azido-3'-deoxy-beta-D-xylofuranosyl)adenine 5'-monophosphate is an inhibitor (Ki = 330 microM) of the initiation binding site of the DNA-dependent RNA polymerase derived from Escherichia coli. The alpha-32P derivative of this photo-labile compound is used to derivatize a site on the sigma subunit of the holoenzyme (E sigma) using either T7 delta D111 or poly[d(A-T)] as a DNA template. The incorporation of the 32P label into the sigma subunit could be prevented by the addition of either 5'-AMP or 5'-ATP. The results are suggested to support the existence of a unique initiation binding site, topographically distinct from the sites employed during the elongation phase.