In vitro synthesis of enzymes of the tryptophan operon of Escherichia coli. Evidence for positive control of transcription.

A protein fraction, called At (= anti termination) factor, has been isolated from extracts of E. coli and partially purified. The At factor stimulates the synthesis in vitro of anthranilate synthetase, an enzyme encoded by two genes of the tryptophan (trp) operon, but has no effect on the synthesis of T7 RNA polymerase and other T7- and T4 coded proteins. The At factor stimulates the synthesis of trp mRNA; it has no effect on the translation of trp mRNA. We conclude that in vitro transcription of the trp operon is positively controlled.