Competition of rifampicin with binding of substrate and RNA to RNA polymerase.

The rate of formation of dinucleoside tetraphosphate, pppApU, from ATP and UTP by RNA polymerase on the A1 promoter of the mutant D111 of bacteriophage T7 is distinctly and specifically reduced not only by the third template-directed nucleotide, CTP, but also by CMP. The inhibitory effect of CMP is not changed when the enzyme contains prebound rifampicin. The synthesis of pppApU is also strongly reduced after preincubation of the enzyme with RNA. This inhibitory effect of RNA is, however, distinctly diminished by rifampicin bound to the enzyme prior to the addition of RNA. On the other hand RNA can suppress the specific binding of the antibiotic to the RNA polymerase subassembly alpha 2 beta.