Regulation of coliphage T3 and T7 RNA polymerases by the lac repressor-operator system

The single-polypeptide RNA polymerases that are encoded by bacteriophage T7 and its relatives form the basis of highly specific and efficient transcription systems. Here, we describe the regulation of transcription from phage promoters by the lac repressor-operator system of Escherichia coli. A synthetic oligodeoxyribonucleotide that contains the core sequence of the lac operator (lacO) was cloned at various distances downstream from the transcription start point (tsp) of the T3 and T7 promoters. The ability of lac repressor to prevent transcription from the phage promoters in vitro was dependent on the position of the operator. Efficient repression was observed when the center of the operator was placed between +14 and +27 (+1 being the tsp), whereas the repressor had little effect when bound to operators centered at +64. For in vivo studies, the chloramphenicol acetyltransferase (CAT)-encoding reporter gene was placed under the control of various promoter-operator constructs, and introduced into bacterial cells containing the genes for the lac repressor and T3 or T7 RNA polymerase. As with in vitro studies, high levels of repression (greater than 4000-fold) of T3 and T7 RNA polymerase activity were achieved, and repression was reversed by the inducer isopropyl-beta-D-thiogalactopyranoside. When the T3 promoter-lacO constructs are used to regulate the expression of a target gene in combination with an inducible RNA polymerase gene under control of the lacUV5 promoter, the doubly regulated system provides extremely tight levels of repression, yet allows high levels of expression after induction. In such a system, we observed a greater than 10(5)-fold increase in CAT activity within 30 min after induction. This system should prove useful in cloning and expressing genes that are potentially toxic to the host cells.