orit

oriT (origin of transfer) is a short noncoding DNA sequence found on many conjugative and mobilizable plasmids in bacteria. It marks the site where DNA nicking initiates plasmid transfer during bacterial conjugation. The locus itself does not encode proteins but serves as a recognition and processing site for the relaxase enzyme and other components of the transfer machinery.

During conjugation, a relaxase binds to oriT and introduces a site-specific nick at a defined nic site. The relaxase remains covalently attached to the 5' end of the nicked DNA strand, forming a relaxosome complex with accessory proteins. The nicked single strand is guided through the type IV secretion system into the recipient cell, where it is replicated and converted into double-stranded DNA, while the complementary strand is synthesized in the donor.

oriT sequences are typically short and located adjacent to transfer gene clusters, including relaxase genes (often referred to as mob or tra genes). Mobilizable plasmids carry oriT but lack a complete transfer apparatus; they rely on a co-resident conjugative plasmid to supply the mating pair formation and transfer functions. In such cases, the recipient receives the transferred strand initiated at oriT.

Relaxases are grouped into MOB families, which define the recognition of specific oriT sequences and compatibility with transfer systems. Variation in oriT sequences can affect transfer efficiency and host range, influencing the scope of horizontal gene transfer. Consequently, oriT plays a central role in spreading genetic traits such as antibiotic resistance among bacteria and is also exploited in laboratory methods to mobilize plasmids via conjugal transfer.