RecBCD (Exonuclease V) is inhibited by DNA adducts produced by cisplatin and ultraviolet light.

The presence of adducts on the DNA double-helix can have major consequences for the efficient functioning of DNA repair enzymes. E. coli RecBCD (exonuclease V) is involved in recombinational repair of double-strand breaks that are caused by defective DNA replication, DNA damaging agents and other factors. The holoenzyme possesses a bipolar helicase activity which helps unwind DNA from both 3'- and 5'-directions and is coupled with a potent exonuclease activity that is also capable of digesting DNA from both 3'- and 5'-ends. In this study, DNA sequences were damaged with cisplatin or UV followed by RecBCD treatment. DNA damaging agents such as cisplatin and UV induce the formation of intrastrand adducts in the DNA template. It was demonstrated that RecBCD degradation was inhibited by either cisplatin-damaged or UV-damaged DNA sequences. This is the first occasion that RecBCD has been demonstrated to be inhibited by DNA adducts induced by cisplatin or UV. In addition, we quantified the amounts of DNA remaining after RecBCD treatment and observed that the level of inhibition was concentration and dose dependent. A DNA-targeted 9-aminoacridinecarboxamide cisplatin analogue was also found to inhibit RecBCD activity.

Costs and benefits of natural transformation in Acinetobacter baylyi.

BACKGROUND: Natural transformation enables acquisition of adaptive traits and drives genome evolution in prokaryotes. Yet, the selective forces responsible for the evolution and maintenance of natural transformation remain elusive since taken-up DNA has also been hypothesized to provide benefits such as nutrients or templates for DNA repair to individual cells. RESULTS: We investigated the immediate effects of DNA uptake and recombination on the naturally competent bacterium Acinetobacter baylyi in both benign and genotoxic conditions. In head-to-head competition experiments between DNA uptake-proficient and -deficient strains, we observed a fitness benefit of DNA uptake independent of UV stress. This benefit was found with both homologous and heterologous DNA and was independent of recombination. Recombination with taken-up DNA reduced survival of transformed cells with increasing levels of UV-stress through interference with nucleotide excision repair, suggesting that DNA strand breaks occur during recombination attempts with taken-up DNA. Consistent with this, we show that absence of RecBCD and RecFOR recombinational DNA repair pathways strongly decrease natural transformation. CONCLUSIONS: Our data show a physiological benefit of DNA uptake unrelated to recombination. In contrast, recombination during transformation is a strand break inducing process that represents a previously unrecognized cost of natural transformation.