[Genetic system of selecting point mutations of bacteriophage T7 RNA polymerase]

Random mutagenesis of the bacteriophage T7 gene 1 was used to delineate the functionally important amino acids residues of its product--T7 RNA polymerase. A two-plasmid system has been constructed for the phenotypic selection of the mutants. Nine mutants were isolated; the RNA polymerase with Tyr-639----Asp substitution has been shown to be completely inactive. Methods were developed for the rapid purification of plasmid-encoded mutant proteins, facilitating their future biochemical analysis.