Nic site

The nic site or nick region is found within the origin of transfer (oriT) site and is key in starting bacterial conjugation.[1]

Purpose
A single strand of DNA, called the T-strand, is cut at nic by an enzyme called relaxase. This single strand is eventually transferred to the recipient cell during the process of bacterial conjugation. Before this cleavage can occur, however, it is necessary for a group of proteins to attach to the oriT site. This group of proteins is called the relaxosome. It is thought that portions of the oriT site are bent in a way that creates interaction between the relaxosome proteins and the nic site.

Cleaving the T-strand involves relaxase cutting a phosphodiester bond at the nic site. Interestingly, the cleaved strand is left with a hydroxyl group at the 3' end, which may allow for the strand to form a circular plasmid after moving into the recipient cell.