Repression of induction by u.v. of λ phage byexrAmutations inEscherichia coli

Abstract

The effect of ultraviolet radiation (u.v.) onλlysogens ofexrAstrains ofEscherichia coliwas studied.exrAstrains could be lysogenized with, as well as support the vegetative reproduction of, λ. However, though spontaneous induction of λ occurred inexrA(λ) strain at 10% the frequency ofexrA⁺(λ) strain,exrA(λ) strains were not induced by u.v. Because λ was not induced inexrA(λ) strains, lysogens of these strains were no more sensitive to u.v. than were non-lysogens.The heat-inducible mutant λhcI857could be induced inexrAstrains at elevated temperatures. Furthermore, u.v. irradiation ofexrA(λhcI857) strain did not prevent the heat induction of this λ mutant. TheexrAmutation appeared to interfere only with the inactivation of λ repressor by u.v.Among theexrAstrains studied was strainBs1(exr A uvrB). Whereas the λ lysogen of strain Bsl could not be induced by u.v. and was no more sensitive to u.v. than its non-lysogen, theexrA⁺uvrB(λ) derivative of strain Bsl could be induced by u.v. and was more sensitive to u.v. than its non-lysogen.