Oxidative stress effects on conjugational recombination and mutation in catalase-deficient Escherichia coli

Crossing over between regions of limited homology in Escherichia coli. RecA-dependent and RecA-independent pathways

We have developed an assay for intermolecular crossing over between circular plasmids carrying variable amounts of homology. Screens of Escherichia coli mutants demonstrated that known recombination functions can only partially account for the observed recombination. Recombination rates increased three to four orders of magnitude as homology rose from 25 to 411 bp. Loss of recA blocked most recombination; however, RecA-independent crossing over predominated at 25 bp and could be detected at all homology lengths. Products of recA-independent recombination were reciprocal in nature. This suggests that RecA-independent recombination may involve a true break-and-join mechanism, but the genetic basis for this mechanism remains unknown. RecA-dependent crossing over occurred primarily by the RecF pathway but considerable recombination occurred independent of both RecF and RecBCD. In many respects, the genetic dependence of RecA-dependent crossing over resembled that reported for single-strand gap repair. Surprisingly, ruvC mutants, in both recA(+) and recA mutant backgrounds, scored as hyperrecombinational. This may occur because RuvC preferentially resolves Holliday junction intermediates, critical to both RecA-dependent and RecA-independent mechanisms, to the noncrossover configuration. Levels of crossing over were increased by defects in DnaB helicase and by oxidative damage, showing that damaged DNA or stalled replication can initiate genetic recombination.

Background and radiation-induced 8-hydroxy-2'-deoxyguanosine in gamma-irradiated Escherichia coli

The DNA base damage product, 8-hydroxy-2'-deoxyguanosine (7-hydro-8-oxo-2'-deoxyguanosine, or 8-OHdG) was measured in 2'-deoxyribonucleoside digests of DNA from irradiated E. coli or model DNA using HPLC separation and electrochemical detection (LCED). The LCED technique enabled the specific quantitation of 25 fmol of 8-OHdG (signal: noise ratio = 4:1) among a variety of DNA base lesions within the sample. A radiation dose-yield relationship for 8-OHdG was observable in cells that were rapidly lysed after irradiation using a chloroform-saturated, sarkosyl and EDTA solution at 4 degrees C (Tilby and Loverock 1983). The observed yield of 8-OHdG was 7.05 +/- 0.27 pmol J-1 and was against a background of 120 8-OHdG per E. coli genome. This yield translates to an average of 12 additional 8-OHdG lesions per genome at the mean lethal dose. The low radiochemical yield and high background suggests that factors other than the toxicity of 8-OHdG per se--such as its participation in clusters of base lesions (e.g. Ward 1985)--must contribute significantly to cell killing. Although exposure of DNA to phenol mixtures has often been reported to increase 8-OHdG, we found that a more significant effect of phenol mixtures was the potentiation of the radiochemical yields of 8-OHdG in model DNA from 5.26 +/- 0.48 to 158 +/- 12.2 nmol J-1. Other factors, such as the exposure of dried 2'-deoxyribonucleoside digests to ambient humidity in the presence of buffer components, were shown to contribute more significantly to the background of 8-OHdG.