Regulation of futile ligation during early steps of BER in M. tuberculosis is carried out by a β-clamp-XthA-LigA tri-component complex

Ankita Shukla, Mohammad Afsar, Taran Khanam, Nelam Kumar, Faiz Ali, Sanjay Kumar, Farheen Jahan, Ravishankar Ramachandran (Lead / Corresponding author)

Research output: Contribution to journalArticlepeer-review


The Class-II AP-endonuclease (XthA) is a mycobacterial DNA base excision repair (BER) pathway enzyme that functions in the initial steps. It acts on DNA substrates that contain abasic sites to create nicks with 3'-hydroxyl (OH) and 5'-deoxyribose phosphate (5'-dRP) moieties. The NAD+-dependent DNA ligase (LigA) is the terminal player in mycobacterial BER and seals such nicks efficiently. Here, we demonstrate that the Mtbβ-clamp-MtbXthA complex that exists in the initial steps of BER engages with MtbLigA to form a novel tri-component BER complex. Size exclusion chromatography (SEC) experiments analysis show that the three proteins interact with equimolar stoichiometry. Small angle X-ray scattering (SAXS) analysis and associated studies reveal that the apo tri-component BER-complex adopts an extended conformation where MtbXthA is sandwiched between the Mtbβ-clamp and MtbLigA. The studies support that in the apo-complex MtbXthA binds subsite-I of Mtbβ-clamp through 239QLRFPKK245 motif and to MtbLigA by 104DGQPSWSGKP113 motif simultaneously. However, the complex adopts a less-extended conformation in the presence of substrate DNA, where MtbXthA interactions switch from predominantly subsite-I to subsite-II of the Mtbβ-clamp. Overall, the novel tri-component complex prevents futile ligation activity of MtbLigA on the product of MtbXthA and ensures forward progression of the pathway and productive mycobacterial BER interactions.

Original languageEnglish
Pages (from-to)442-453
Number of pages12
JournalInternational Journal of Biological Macromolecules
Early online date14 Nov 2022
Publication statusPublished - 15 Jan 2023


  • Base excision repair
  • Futile ligation
  • MtbLigA
  • MtbXthA
  • Mtbβ-clamp
  • Small angle X-ray scattering

ASJC Scopus subject areas

  • Molecular Biology
  • Structural Biology
  • Biochemistry


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