Animal DNA‐Dependent RNA Polymerases

The inhibitory effect of various semi‐synthetic derivatives of rifamycin SV on calf thymus DNA‐dependent RNA polymerase AI and B activities was investigated. Some of these derivatives having highly hydrophobic side‐chains, like AF/013, completely inhibited both types of activities at a concentration of 20 to 40 μg/ml. Initiation, but not elongation, of RNA synthesis was inhibited by these derivatives. Studies on the mode of action of these inhibitors on B enzyme activity have shown that inhibition took place at two steps of the preinitiation process: both the primary binding of enzyme to DNA and a temperature‐and time‐dependent step were inhibited. After preincubation of B enzymes and calf thymus DNA at 37°C, RNA synthesis was almost completely resistant to AF/013, suggesting that the formation of the resistant complex was related to the opening of the two DNA strands over a short local region at the initiation site. Divalent cations, especially Mn 2+ , had a marked effect on the formation and the stability of the resistant complex. In the case of enzyme AI some AF/013‐resistant RNA synthesis was also observed. However, the same values were obtained whether the preincubation temperature was 37°C or 0°C, indicating that the preinitiation process on calf thymus DNA could be different for the two types of calf thymus RNA polymerase activities.