Yeast Rad27 is a 5'→3' exonuclease and a flap endonuclease. Apn1 is the major apurinic/apyrimidinic (AP) endonuclease in yeast. The rad27 deletion mutants are highly sensitive to methylmethane sulfonate (MMS). By examining the role of Rad27 in different modes of DNA excision repair, we wish to understand why the cytotoxic effect of MMS is dramatically enhanced in the absence of Rad27. Base excision repair (BER) of uracil-containing DNA was deficient in rad27 mutant extracts in that (i) the Apn1 activity was reduced, and (ii) after DNA incision by Apn1, hydrolysis of 1-5 nucleotides 3' to the baseless sugar phosphate was deficient. Thus, some AP sites may lead to unprocessed DNA strand breaks in rad27 mutant cells. The severe MMS sensitivity of rad27 mutants is not caused by a reduction of the Apn1 activity. Surprisingly, we found that Apn1 endonuclease sensitizes rad27 mutant cells to MMS. Deleting the APN1 gene largely restored the resistance of rad27 mutants to MMS. These results suggest that unprocessed DNA strand breaks at AP sites are mainly responsible for the MMS sensitivity of rad27 mutants. In contrast, nucleotide excision repair and BER of oxidative damage were not affected in rad27 mutant extracts, indicating that Rad27 is specifically required for BER of AP sites in DNA.
|Number of pages||7|
|Journal||Nucleic Acids Research|
|State||Published - Feb 15 1999|
Bibliographical noteFunding Information:
We thank Grigory Dianov, Errol Friedberg and Thomas Donahue for providing us with purified E.coli endonuclease IV, yeast strains SX46Arad27∆ and KG119, respectively. We thank Bruce Demple for the apn1 deletion plasmid construct and yeast strain DRY370. We thank Elena Braithwaite for critical review of this manuscript. These studies were supported by a research grant CA67978 from NIH and a start-up fund from the University of Kentucky.
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