Int-constitutive mutants of bacteriophage lambda.

The constitutive production of small amounts of trpB enzyme in an Escherichia coli strain carrying lambdacI857 prophage within the trpC gene has been examined in derivatives of this strain from which portions of the prophage have been deleted. Enzyme production requires a site (p(I)) within the prophage close to the left prophage end. Selection for mutants of this lysogen that grow on low concentrations of indole yielded two types of mutations within the prophage: (a) v2-type, in which all phage genes controlled by the major leftward operator are derepressed; and (b) int-c type, in which the only phage gene derepressed is int. The int-c mutations lie in the same part of the prophage as p(I). All int-c mutants appear deficient in xis gene function, even when derepressed.