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

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.