Cryo-EM Structure of a Relaxase Reveals the Molecular Basis of DNA Unwinding during Bacterial Conjugation

Cell. 2017 May 4;169(4):708-721.e12. doi: 10.1016/j.cell.2017.04.010. Epub 2017 Apr 27.


Relaxases play essential roles in conjugation, the main process by which bacteria exchange genetic material, notably antibiotic resistance genes. They are bifunctional enzymes containing a trans-esterase activity, which is responsible for nicking the DNA strand to be transferred and for covalent attachment to the resulting 5'-phosphate end, and a helicase activity, which is responsible for unwinding the DNA while it is being transported to a recipient cell. Here we show that these two activities are carried out by two conformers that can both load simultaneously on the origin of transfer DNA. We solve the structure of one of these conformers by cryo electron microscopy to near-atomic resolution, elucidating the molecular basis of helicase function by relaxases and revealing insights into the mechanistic events taking place in the cell prior to substrate transport during conjugation.

Keywords: TraI; bacterial conjugation; cryo-electron microscopy; relaxase; structural biology; type IV secretion system.

MeSH terms

  • Conjugation, Genetic*
  • Cryoelectron Microscopy
  • DNA Helicases / chemistry
  • DNA Helicases / metabolism*
  • DNA Helicases / ultrastructure*
  • DNA, Bacterial / chemistry
  • DNA, Bacterial / ultrastructure
  • DNA, Single-Stranded / chemistry
  • DNA, Single-Stranded / metabolism
  • Escherichia coli / enzymology
  • Escherichia coli / genetics*
  • Escherichia coli / metabolism
  • Escherichia coli Proteins / chemistry
  • Escherichia coli Proteins / metabolism*
  • Escherichia coli Proteins / ultrastructure*
  • Models, Molecular


  • DNA, Bacterial
  • DNA, Single-Stranded
  • Escherichia coli Proteins
  • TraI protein, E coli
  • DNA Helicases