The dimer of the beta subunit of Escherichia coli DNA polymerase III holoenzyme is dissociated into monomers upon binding magnesium(II)

Biochemistry. 1988 Jul 12;27(14):5210-5. doi: 10.1021/bi00414a040.


The beta subunit of Escherichia coli DNA polymerase III holoenzyme binds Mg2+. Reacting beta with fluoresceinmaleimide (FM) resulted in one label per beta monomer with full retention of activity. Titration of FM-beta with Mg2+ resulted in a saturable 11% fluorescence enhancement. Analysis indicated that there was one noncooperative magnesium binding site per beta monomer with a dissociation constant of 1.7 mM. Saturable fluorescence enhancement was also observed when titration was with Ca2+ or spermidine(3+) but not with the monovalent cations Na+ and K+. The Mg2+-induced fluorescence enhancement was specific for FM-beta and was not observed with FM-glutathione, dimethoxystilbenemaleimide-beta, or pyrenylmaleimide-beta. Gel filtration studies indicated that the beta dimer-monomer dissociation occurred at physiologically significant beta concentrations and that the presence of 10 mM Mg2+ shifted the dimer-monomer equilibrium to favor monomers. Both the gel-filtered dimers and the gel-filtered monomers were active in the replication assay. These and other results suggested that the fluorescence increase which accompanies beta dissociation is due to a relief from homoquenching of FM when the beta dimer dissociates into monomers.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • DNA Polymerase III / metabolism*
  • DNA-Directed DNA Polymerase / metabolism*
  • Escherichia coli / enzymology*
  • Macromolecular Substances
  • Magnesium / metabolism*
  • Potassium / metabolism
  • Sodium / metabolism


  • Macromolecular Substances
  • Sodium
  • DNA Polymerase III
  • DNA-Directed DNA Polymerase
  • Magnesium
  • Potassium