Overexpression, purification, and characterization of the SbcCD protein from Escherichia coli

The sbcC and sbcD genes mediate palindrome inviability in Escherichia coli. The sbcCD operon has been cloned into the plasmid pTrc99A under the control of the strong trc promoter and introduced into a strain carrying a chromosomal deletion of sbcCD. The SbcC and SbcD polypeptides were overexpressed to 6% of total cell protein, and both polypeptides copurified in a four-step purification procedure. Purified SbcCD is a processive double-strand exonuclease that has an absolute requirement for Mn2+ and uses ATP as a preferred energy source. Gel filtration chromatography and sedimentation equilibrium analyses were used to show that the SbcC and SbcD polypeptides dissociate at some stage after purification and that this dissociation is reversed by the addition of Mn2+. We demonstrate that SbcD has the potential to form a secondary structural motif found in a number of protein phosphatases and suggest that it is a metalloprotein that contains the catalytic center of the SbcCD exonuclease.