title: The dimer of the beta subunit of Escherichia coli DNA polymerase III holoenzyme is dissociated into monomers upon binding magnesium(II).
abstract: 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.
mesh_terms: D004258:DNA Polymerase III / Q000378:metabolism*; D004259:DNA-Directed DNA Polymerase / Q000378:metabolism*; D004926:Escherichia coli / Q000201:enzymology*; D046911:Macromolecular Substances; D008274:Magnesium / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism
chemical_list: D046911:Macromolecular Substances; D012964:Sodium; D004258:DNA Polymerase III; D004259:DNA-Directed DNA Polymerase; D008274:Magnesium; D011188:Potassium