Origin of transfer

An origin of transfer (oriT) is a short sequence (up to 500 bp) that is necessary for transfer of the DNA that contains it from a bacterial host to recipient during bacterial conjugation. The oriT is cis-acting — it is found on the same DNA that is being transferred, and it is transferred along with the DNA. The origin of transfer consists of three functionally defined domains: a nicking domain, a transfer domain, and a termination domain.

At the beginning of conjugation of plasmids, a multi-protein complex called the relaxosome assembles around the oriT, with each individual protein binding at specific sites on the oriT. A relaxase (one of the proteins of the relaxosome) nicks the plasmid at the nic site of the oriT, catalyzing a trans-esterification reaction that transfers the 5' end of the DNA at the nic site to a tyrosine residue on the relaxase. Relaxase then moves in the 5' to 3' direction on the plasmid, unwinding the DNA in a helicase-like fashion, until it comes a full circle back to the oriT where the relaxase recognizes a termination domain that causes it to dissociate from the DNA.