1. Rifamazine (AF/RP) a dimeric rifamycin, is active against bacterial DNA-dependent RNA polymerase and against viral RNA-dependent DNA polymerase.2. Rifamazine is active also against DNA-dependent RNA polymerase extracted from rifampicinresistant mutants of Escherichiu coli. It does not interfere with enzyme-template interaction or with RNA elongation. It blocks initiation.3. A comparison is made between the mechanism of action of rifamazine and that of rifampicin, and of AF/013 (octyloxime of 3-formylrifamycin SV), a C-class rifamycin. Our results show that the mechanism of action of rifamazine is more similar to that of rifampicin than to that of the octyloxime derivative.4. Activity of rifamazine against RNA polymerase from rifampicin-resistant mutants is thought to be due to binding of the dimer to both the rifamycin-specific binding site and to a second weak site.Rifamycins [l] are antibiotics active against bacterial DNA-dependent RNA polymerase [2 -41. Their mechanism of action on the wild-type enzyme is well understood. They interfere with RNA polymerase at the initiation step of RNA synthesis, without affecting the binding of enzyme to template or the elongation of RNA. They interact stoichiometrically with RNA polymerase; the ratio of rifamycin to enzyme is variously estimated to be between 1 : 1 and 2 : 1 [5-81. In an attempt to find rifamycins active against rifampicin-resistant bacteria, Lepetit chemists have synthesized a number of derivatives with bulky, lipophilic side-chains. These so called C-class rifamycins [9,10] have a large spectrum of activity. At concentrations higher than those inhibitory for wild-type RNA polymerase they also inhibit rifampicin-resistant bacterial RNA polymerase, and other nucleotide polymerases including RNA-dependent DNA polymerase of RNA tumor viruses [11 -131, eukaryotic DNA-dependent RNA and DNA polymerases (1 3 -16), viral RNA replicase 1171. They have been reported active against polynucleotide phosphorylase [18]. We have observed that, at still higher concenEnzymes. Glutamic-oxaloacetic transaminase (EC 2.6.1.1); glutamic-pyruvic transaminase (EC 2.6.1.2); alkaline phosphatase (EC 3.1.3.1).~-trations, they inhibit some non-polymerizing enzymes, such as glutamic-oxaloacetic transaminase, glutamicpyruvic transaminase and alkaline phosphatase [6].A typical C-class rifamycin, AFjOl3 (O-n-octyloxime of 3-formylrifamycin SV), is very active and has been much studied [6,14,17-211.In previous work we have shown that the mechanism of action of this drug against the rifampicinresistant (rif-r) RNA polymerase appears to be different from the mechanism of action of rifamycins on wild-type bacterial RNA polymerase, in that tested against rif-r RNA polymerase (a) it blocks a step preceding initiation ( i e . , it interferes with binding of enzyme to template); (b) at inhibitory concentrations many molecules of octyloxime derivative are bound per enzyme; (c) its hexahydro analog has the same activity against both rif-s and r$r enzymes [6]; (d) it inhibits some non-polymerizi...