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The mechanism of inhibition of RNA polymerase-catalyzed synthesis of RNA by actinomycin D and the phenan-thridinium derivatives ethidium bromide and 3,8-diamino-6-ethylphenanthridinium bromide (DEMB) is examined. A general kinetic equation describing the dependence of RNA synthesis on DNA template concentration is derived and distinct expressions corresponding to various possible mechanisms of inhibition are subsequently obtained by introducing into the equations assumptions as appropriate for the individual mechanisms. The fitting of the experimental results of inhibition into the resulting equations suggested that the ethidium bromide and DEMB inhibit RNA polymerase by forming an inhibitor-template complex which interferes with enzyme recognition of, and binding to, appropriate sites on the template (binding inhibition). The fitting of the dependence of the rate of RNA synthesis on the bound-inhibitor to DNA ratios to the derived kinetic expressions also allows a tentative distinction to be made as to whether ethidium bromide and DEMB interfere with RNA synthesis by a mechanism of 'partial' or 'complete' inhibition.

The rate of RNA synthesis catalysed by DNA-dependent RNA polymerase shows a Michealis-Menten-type saturation curve with increasing template concentration. However, the apparent Km is proportional to enzyme concentration, indicating that the reaction does not obey a simple kinetic scheme. The action of inhibitors also indicates a more complex interaction between the enzyme and the DNA template; many inhibitors of RNA synthesis either decrease Vmax. without affecting Km, or increase Km without affecting Vmax. All of these observations can be accounted for quantitatively by a reaction pathway in which the non-specific binding sites of the viral DNA template inhibit competitively the binding of the enzyme to the initiation sites. In terms of this pathway the two classes of inhibitors of RNA synthesis must then act predominantly either on the rate of elongation or on the availability of the binding sites respectively.

T7 late promoters have a 22-base pair sequence in common. The 22 base pairs are necessary and sufficient for recognition and initiation by T7 RNA polymerase.