A group of three proteins tightly bound to some E. coli supercoiled plasmids that convert supercoiled DNA to a nicked open circle. When heated or treated with alkali, proteolytic enzymes, or detergents, one of these proteins nicks one strand at a specific site, thereby relaxing the supercoil to a nicked open circular form. During relaxation, the two smaller proteins are released, but the largest protein becomes covalently attached to the 5′-P end of the nick. Nicking plays a role in transfer of the plasmid during conjugation. The site of the nick establishes the transfer origin.
Subjects: Genetics and Genomics.