Persistence and decay of thermoinducible error-prone repair activity in nonfilamentous derivatives of tif-1, Escherichia coli B/r: the timing of some critical events in ultraviolet mutagenesis.

Abstract

Ultraviolet (UV) mutagenesis in E. coli is associated with a UV-inducible type of error-prone postreplication repair ("SOS" repair) which, in tif-1 strains, is thermo-inducible in coordination with other recA+ lexA+-dependent inducible functions, including filamentous growth. Mutants of E. coli B/r tif-1 strains have been isolated which retain thermoinducibility of SOS repair activity, but lack the thermosensitivity caused by filamentous growth at 42 degrees C. These strains have been used to determine: the kinetics of decay at 30 degrees C of thermally induced ability to enhance UV mutagenesis; the kinetics of thermal enhancement of spontaneous and UV-induced mutability at 42 degrees C, and the kinetics of decay at 30 degrees C of susceptibility to thermal enhancement of spontaneous and UV-induced mutability. Mutations from tryptophane requirement to prototrophy (Trp- to Trp+) were scored. UV doses were 0.2 J/M2 for excision repair-deficient (Uvr-) and 2J/m2 for Uvr+ strains. The results support the following conclusions. 1) thermally induced SOS repair activity decays at 30 degrees C to about 25% of its maximum level in 45 min, and is no longer detectable after 90 min. 2) Thermal enhancement of UV mutability occurs at sites produced primarily (perhaps exclusively) before completion of the first post-irradiation cell division. 3) UV-induced sites susceptible to thermally induced SOS repair are stable at 30 degrees C in cells not containing the error-prone repair system, and are refractory to constitutive error-free repair for at least 2-3 hours. 4) UV produces a potentially mutagenic type of photoproduct in DNA which can, without interacting with another UV lesion, provide a site susceptible to SOS repair, but which is not a sufficient signal for SOS induction. 5) 50-70% of the SOS-mutable SOS-noninducing UV photoproducts are photoreversible pyrimidine dimers. The results are discussed in relation to current models of UV mutagenesis and induction of UV-inducible functions.