The ban operon of bacteriophage P1. Localization of the promoter controlled by P1 repressor

Repression of a strong promoter localized 5' to the P1 ban gene is a prerequisite for cloning the ban operon in the multicopy plasmid pBR325. Repression is brought about by the binding of P1 repressor to the operator of the ban operon (Heisig, A., Severin, I., Seefluth, A. K., and Schuster, H. (1987) Mol. Gen. Genet. 206, 368-376). Binding of RNA polymerase in vitro overlaps with the operator and is inhibited by P1 repressor as shown by electron microscopy. The mutant P1 bac, which renders ban expression constitutive, contains a single base pair exchange within the operator. As a consequence, more repressor is required (i) for the inhibition of binding of RNA polymerase, and (ii) for the electrophoretic retardation of a P1 bac DNA fragment when compared to the corresponding bac+ fragment. A P1 ban recombinant plasmid containing a 4-base pair deletion close to the operator still allows binding of repressor but not of RNA polymerase. By that means, a repressible promoter is located at the P1 map position 72 in a distance of about 2.5 kilobase pairs to the beginning of the ban gene.