The β and β′ subunit of RNA polymerase are thought to be controlled by a translational feedback mechanism regulated by the concentration of RNA polymerase holoenzyme. To study this regulation in vivo, an inducible RNA polymerase overproduction system was developed. This system utilizes plasmids from two incompatibility groups that carry RNA polymerase subunit genes under lac promoter/operator control. When the structural genes encoding the components of core RNA polymerase (α, β and β′) or holoenzyme (α, β, β′ and σ 70 ) are present on the plasmids, induction of the lac promoter results in a two fold increase in the concentration of functional RNA polymerase. The induction of RNA polymerase overproduction is characterized by an initial large burst of ββ′ synthesis followed by a gradual decrease as the concentration of RNA polymerase increases. Overproduction of RNA polymerase in a strain carrying an electrophoretic mobility mutation in the rpoB gene results in the specific repression of ββ′ synthesis off the chromosome. These results indicate that RNA polymerase feedback regulation controls ββ′ synthesis in vivo. © 1986 Springer-Verlag.