During the establishment of an
infection, bacterial pathogens encounter oxidative stress resulting in the production of DNA lesions. Majority of these lesions are repaired by base excision repair (BER) pathway. Amongst these, abasic sites are the most frequent lesions in
DNA. Class II apurinic/apyrimidinic (AP)
endonucleases play a major role in BER of damaged
DNA comprising of abasic sites. Mycobacterium tuberculosis, a deadly pathogen, resides in the human macrophages and is continually subjected to oxidative assaults. We have characterized for the first time two AP
endonucleases namely
Endonuclease IV (End) and
Exonuclease III (XthA) that perform distinct functions in M.
tuberculosis. We demonstrate that M.
tuberculosis End is a typical
AP endonuclease while XthA is predominantly a 3'→5'
exonuclease. The
AP endonuclease activity of End and XthA was stimulated by Mg(2+) and Ca(2+) and displayed a preferential recognition for abasic site paired opposite to a
cytosine residue in
DNA. Moreover, End exhibited
metal ion independent 3'→5'
exonuclease activity while in the case of XthA this activity was
metal ion dependent. We demonstrate that End is not only a more efficient
AP endonuclease than XthA but it also represents the major
AP endonuclease activity in M.
tuberculosis and plays a crucial role in defense against oxidative stress.