Cloning and expression of the bacteriophage T3 RNA polymerase gene

The gene that encodes the RNA polymerase of bacteriophage T3 (gene 1) has been cloned into a pBR322 derivative under the control of an inducible lacUV5 promoter. Large quantities of the protein are synthesized after induction of cells that carry this plasmid. RNA polymerase purified from these overproducing cells selectively initiates transcription from T3 promoter sequences as demonstrated by transcription of a dual promoter plasmid that carries both T3 and T7 promoters. Cells that carry the T3 RNA polymerase gene can complement amber mutants of T3 that are defective in gene 1 but not gene 1 amber mutants of T7, and vice versa; this experiment demonstrates the specificity of these enzymes in vivo.