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Journal of Bacteriology, July 2002, p. 3501-3507, Vol. 184, No. 13
0021-9193/02/$04.00+0 DOI: 10.1128/JB.184.13.3501-3507.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.
Division of Biological Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,1 Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, Massachusetts 016052
Received 31 January 2002/ Accepted 10 April 2002
Nitric oxide (NO.) is involved in neurotransmission, inflammation, and many other biological processes. Exposure of cells to NO. leads to DNA damage, including formation of deaminated and oxidized bases. Apurinic/apyrimidinic (AP) endonuclease-deficient cells are sensitive to NO. toxicity, which indicates that base excision repair (BER) intermediates are being generated. Here, we show that AP endonuclease-deficient cells can be protected from NO. toxicity by inactivation of the uracil (Ung) or formamidopyrimidine (Fpg) DNA glycosylases but not by inactivation of a 3-methyladenine (AlkA) DNA glycosylase. These results suggest that Ung and Fpg remove nontoxic NO.-induced base damage to create BER intermediates that are toxic if they are not processed by AP endonucleases. Our next goal was to learn how Ung and Fpg affect susceptibility to homologous recombination. The RecBCD complex is critical for repair of double-strand breaks via homologous recombination. When both Ung and Fpg were inactivated in recBCD cells, survival was significantly enhanced. We infer that both Ung and Fpg create substrates for recombinational repair, which is consistent with the observation that disrupting ung and fpg suppressed NO.-induced recombination. Taken together, a picture emerges in which the action of DNA glycosylases on NO.-induced base damage results in the accumulation of BER intermediates, which in turn can induce homologous recombination. These studies shed light on the underlying mechanism of NO.-induced homologous recombination.
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