A mutant of Escherichia coli showing constitutive expression of the lysogenic induction and error-prone DNA repair pathways.

A mutant of E. coli (designated the STS mutant) has been isolated in which the phage induction and error-prone DNA repair pathways appear to be expressed constitutively without the cells having received an inducing signal. Phage lambda was not able to lysogenize this mutant, whereas a noninducible mutant of lambda, lambdacIind-, known to synthesize a repressor that is insensitive to the induction mechanism, lysogenized it normally. This result suggested that normal phage repressor was synthesized in the STS mutant but was then inactivated by the induction mechanism. The STS strain also had mutator characteristics, and showed spontaneous, error-prone repair of UV-damaged phage lambda. Derived from a lexA tif sfiA parent strain, the STS mutant carried an additional mutation spr at the lexA locus that resulted in a high level of expression of the induction pathways. The properties of this and related strains provide additional evidence that induction of phage and induction of error-prone DNA repair occur by a similar mechanism, and further suggest a model for the regulation of these pathways.