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E M Witkin

Publications and source records attributed to E M Witkin.

At least 37 records · Page 2Linked to original sources
PubMed1977

Induction of lambda prophage and of mutations to streptomycin resistance in separate small fractions of a lysogenic derivative of Escherichia coli B/r by very low doses of ultraviolet light.

The number of induced mutations to streptomycin resistance is compared at doses of ultraviolet (UV) light between 0.2 and 6.4 J/m2 in a Uvr- (excision-deficient) derivative of E. coli B/r, strain WU, and in its lambda lysogen, strain WU(lambda). At UV doses up to about 1 J/m2, which converts about 5% of the lysogenic population into enfective centers, no difference is observed in the number of mutations to streptomycin resistance produced by the two strains. It is concluded that the capacity to produce UV-induced mutations is not coupled with lysis due to the induction of lambda prophage at low doses of UV radiation. At UV doses above 1 J/m2, the number of mutatants detected in the lysogenic strain decreases appreciably compared to the number detected in the nonlysogen, and is only about 10% as high at UV doses of 3 J/m2 and higher, doses which cause maximal induction of prophage. The results are compatible with the operation of a common "all-or-none" induction signal resulting in expression of UV-inducible functions at high UV doses, but not at low doses.

Coliphages↗
PubMed1976

Partial suppression of the LexA phenotype by mutations (rnm) which restore ultraviolet resistance but not ultraviolet mutability to Escherichia coli B/r uvr A lexA.

In Escherichia coli, lexA mutations eliminate expression of UV-inducible functions, causing pleiotropic effects which include sensitivity to ultraviolet (UV) light and loss of UV mutability. Selection for UV resistance, after 5-bromouracil (BU) treatment of E. coli B/r uvrA lexA-102, has yielded derivatives more resistant than lexA but still refractory to UV mutagenesis. The mutation responsible for the UV-resistant UV-nonmutable phenotype (rnm) is cotransducible with malB to about the same extent as is lexA-102 and is tightly linked to lexA-102 in at least one strain. The rnm mutation may therefore be an intragenic partial suppressor of the LexA phenotype. In addition to increased UV resistance and lack of UV mutability, rnm strains show improved ability to perform postreplication repair and to control postirradiation DNA degration compared to the lexA parent. We ascribe the properties of rnm mutants to their having reacquired control of Exonuclease V activity without having reacquired UV-inducible error-prone postreplication repair. We relate our results to current interpretations of UV mutagenesis and to models of coordinate regulation of UV-inducible functions.

Escherichia coli↗
PubMed1975

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.

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.

Animals↗
PubMed1975

Thermal enhancement of ultraviolet mutability in a dnaB uvrA derivative of Escherichia coli B/r: evidence for inducible error-prone repair.

DNA damage triggers coordinate expression of a cluster of diverse functions in Escherichia coli, including prophage induction, filamentous growth, and "aberrant" reintiation of DNA replication at the chromosomal origin. The "SOS repair" hypothesis proposes that one of these coordinately inducible functions is an error-prone system of DNA repair ("SOS repair") which is responsible for ultraviolet mutagenesis. In dnaB strains, incubation of 42 degrees C stops DNA synthesis and induces lambda prophage and should, therefore, also induce the postulated error-prone repair activity. Thermal posttreatment of a dnaB urvA derivative of E. coli B/r is found to enhance the yield of ultraviolet-light-induced mutations as much as 50-fold, while having no such effect in the dnaB+ parent strain. The results support the SOS repair hypothesis. The possibility is discussed that the inducible repair system is a mutagenic DNA polymerase.

Coliphages↗
PubMed1975

Ultraviolet light-induced responses of an mfd mutant of Escherichia coli B/r having a slow rate of dimer excision.

A mutant of Escherichia coli B/r designated mfd has drastically reduced ability to exhibit "mutation frequency decline" (MFD) the irreversible loss of potential suppressor mutations which occurs when protein synthesis is briefly inhibited after irradiation with UV. We have found that the initial rate of thymine dimer excision in the mfd mutant is only about one-third that of its mfd+ parent strain after a UV dose of 400 erg/mm-2. The yield of UV-induced Tyr+ revertants is 4-10 times higher in the mfd strain than in the mfd+ strain. This is comparable to the level of UV-mutability in the mfd+ strain in the presence of caffeine, an inhibitor of dimer excision. UV-mutability, prophage induction and Weigle reactivation of irradiated gamma phage occur to a greater extent at low UV doses (10-50 erg/mm-2) in the mfd strain compared to the mfd+ strain. We propose that the slow excision repair in the mfd mutant results in a shift in the induction threshold for these UV-inducible functions toward lower UV doses.

Caffeine↗
PubMed1975

Elevated mutability of polA derivatives of Escherichia coli B/r at sublethal doses of ultraviolet light: evidence for an inducible error-prone repair system ("SOS repair") and its anomalous expression in these strains.

The SOS repair hypothesis proposes that UV-induced mutations in E. coli are caused by an inducible error-prone repair system (SOS repair) which is normally induced coordinately with such other recApluslexplus-dependent functions as filamentous growth, prophage induction and W-reactivation-2 in response to UV or other inhibitors of DNA synthesis. Since polA-l strains induce these functions at unusually low doses of UV, the SOS hypothesis predicts elevated UV mutability for such strains at these low doses (50 ergs per mm2 and below) Strain WP6, a polA-1 derivative of B/r, exhibits the predicted high UV mutability in this dose range, producing ten times as many Trp+ mutations as its polA+ counterpart, strain WP1, at a dose of 12.5 ergs per mm-2. The UV sensitivity of a lex polA double mutant, in which the lex and polA mutations fail to exert their individual effects on UV sensitivity additively, also confirms a prediction generated by the SOS hypothesis.--The uvrA polA-1 strain WP67 also shows elevated UV mutability at very low UV doses (5 ergs per mm-2 and below), producing about ten times as many Trp+ mutations at a dose of 0.6 ergs per mm-2 as its uvrA polA+ parent strain WP2s. It is proposed that the double mutant induces the SOS repair system (and probably other "reclex" inducible functions) at unusually low doses of UV as a consequence of its relatively inefficient repair of DNA damage when plated on broth-supplemented media.

Culture Media↗
PubMed1974

Thermal enhancement of ultraviolet mutability in a tif-1 uvrA derivative of Escherichia coli B-r: evidence that ultraviolet mutagenesis depends upon an inducible function.

Inhibition of DNA replication in E. coli results in coordinate expression of a cluster of diverse functions, including prophage induction and filamentous growth, many of which are thermally inducible, without inhibition of DNA synthesis, in strains carrying a mutation in the "tif" locus. It has been proposed that one of the inducible functions in this cluster is responsible for the ultraviolet mutability of E. coli. As a test of this hypothesis, the effect of postirradiation temperature elevation on ultraviolet mutability was compared in strains differing by a tif mutation. In a tif(+) strain, raising the temperature to 42 degrees after irradiation did not change the frequency of induced mutations to tryptophan-independence. Similar treatment of a tif-1 strain resulted in as much as a 10-fold increase in the induced mutation yield. Thermal enhancement of ultraviolet mutability in the tif-1 strain was prevented or promoted by agents known to exert parallel effects on thermal induction of lambda prophage in tif lysogens. The results support the hypothesis that ultraviolet mutability in E. coli depends upon an inducible function that is normally expressed only when DNA replication is inhibited.

Adenine↗