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Mutagenesis Advance Access published online on November 5, 2009
Mutagenesis, doi:10.1093/mutage/gep052
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© The Author 2009. Published by Oxford University Press on behalf of the UK Environmental Mutagen Society. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

Lethal and mutagenic properties of MMS-generated DNA lesions in Escherichia coli cells deficient in BER and AlkB-directed DNA repair

Anna Sikora, Damian Mielecki, Aleksandra Chojnacka, Jadwiga Nieminuszczy, Michal Wrzesinski and Elzbieta Grzesiuk*

Institute of Biochemistry and Biophysics, Polish Academy of Sciences, Pawinskiego 5A, 02-106 Warszawa, Poland

Methylmethane sulphonate (MMS), an SN2-type alkylating agent, generates DNA methylated bases exhibiting cytotoxic and mutagenic properties. Such damaged bases can be removed by a system of base excision repair (BER) and by oxidative DNA demethylation catalysed by AlkB protein. Here, we have shown that the lack of the BER system and functional AlkB dioxygenase results in (i) increased sensitivity to MMS, (ii) elevated level of spontaneous and MMS-induced mutations (measured by argE3 -> Arg+ reversion) and (iii) induction of the SOS response shown by visualization of filamentous growth of bacteria. In the xth nth nfo strain additionally mutated in alkB gene, all these effects were extreme and led to ‘error catastrophe’, resulting from the presence of unrepaired apurinic/apyrimidinic (AP) sites and 1-methyladenine (1meA)/3-methylcytosine (3meC) lesions caused by deficiency in, respectively, BER and AlkB dioxygenase. The decreased level of MMS-induced Arg+ revertants in the strains deficient in polymerase V (PolV) (bearing the deletion of the umuDC operon), and the increased frequency of these revertants in bacteria overproducing PolV (harbouring the pRW134 plasmid) indicate the involvement of PolV in the error-prone repair of 1meA/3meC and AP sites. Comparison of the sensitivity to MMS and the induction of Arg+ revertants in the double nfo alkB and xth alkB, and the quadruple xth nth nfo alkB mutants showed that the more AP sites there are in DNA, the stronger the effect of the lack of AlkB protein. Since the sum of MMS-induced Arg+ revertants in xth, nfo and nth xth nfo and alkB mutants is smaller than the frequency of these revertants in the BER– alkB– strain, we consider two possibilities: (i) the presence of AP sites in DNA results in relaxation of its structure that facilitates methylation and (ii) additional AP sites are formed in the BER– alkB– mutants.

* To whom correspondence should be addressed. Tel: +48 22 592 33 37; Fax: +48 22 592 21 90; Email: elag{at}ibb.waw.pl

Received on July 28, 2009; revised on September 15, 2009; accepted on October 12, 2009.


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