MobA, the DNA strand transferase of plasmid R1162: the minimal domain required for DNA processing at the origin of transfer.

MobA is a DNA strand transferase encoded by the plasmid R1162 and required for plasmid DNA processing during conjugal transfer. The smallest active fragment was identified using phage display and partial enzymatic digestion of the purified protein. This fragment, consisting of approximately the first 184 amino acids, is able to bind and cleave its normal DNA substrate, the origin of transfer (oriT). Smaller fragments having one of these activities were not obtained. An active intermediate consisting of MobA linked to DNA was isolated and used to show that a single molecule of MobA is sufficient to carry out all of the DNA processing steps during transfer. These results, along with those obtained earlier, point to a single large, active site in MobA that makes several different contacts along the oriT DNA strand.