DNA blockade by rifampicin-inactivated Escherichia coli RNA polymerase, and its amelioration by a specific mutation.

1. Partially diploid strains of Escherichia coli containing both rifampicin-sensitive and rifampicin-resistant RNA polymerase are, in general, sensitive to the drug: of the two copies of the rpoB gene present in such strains, that which codes for sensitive enzyme is dominant. 2. RNA polymerase purified from a normal sensitive strain of E. coli, and inactivated by rifampicin, can "blockade" bacteriophage T7 DNA in vitro, inhibiting its transcription by drug-resistant enzyme molecules. 3. A mutation, rcs-40, reverses the normal dominance relationship in vivo, without detectably affecting the concentrations of resistant and sensitive RNA polymerase in the diploid cell. I show that rcs-40 is closely linked to the rpoB gene which codes for the rifampicin-sensitive enzyme. 4. Rifampicin-sensitive RNA polymerase purified from E. coli rcs-40, although indistinguishable from the normal enzyme by many criteria, is significantly less efficient in the production of drug-dependent DNA blockade.