An origin of transfer is a short sequence (up to 500 bp

Base pair

In molecular biology and genetics, the linking between two nitrogenous bases on opposite complementary DNA or certain types of RNA strands that are connected via hydrogen bonds is called a base pair...

) of DNA

DNA

Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...

 that is necessary for transfer of a bacteria

Bacteria

Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

l plasmid

Plasmid

In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...

 from a bacterial host to recipient during bacterial conjugation

Bacterial conjugation

Bacterial conjugation is the transfer of genetic material between bacterial cells by direct cell-to-cell contact or by a bridge-like connection between two cells...

. The oriT is cis-acting - it is found on the same plasmid that is being transferred, and is transferred along with the plasmid. The origin of transfer consists of three functionally-defined domains

Protein domain

A protein domain is a part of protein sequence and structure that can evolve, function, and exist independently of the rest of the protein chain. Each domain forms a compact three-dimensional structure and often can be independently stable and folded. Many proteins consist of several structural...

: a nicking domain, a transfer domain, and a termination domain.

At the beginning of conjugation, a multi-protein complex called the relaxosome

Relaxosome

Relaxosome is the complex of proteins that facilitates plasmids during bacterial conjugation. The proteins are encoded by the tra gene in the origin of transfer, oriT, region. The most important of these proteins is relaxase, which is responsible for beginning the conjugation process by cutting...

 assembles around the oriT, with each individual protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

 binding at specific sites on the oriT. A relaxase

Relaxase

A relaxase is a single-strand DNA transesterase enzyme produced by some prokaryotes and viruses. Relaxases are responsible for site- and strand-specific nicks in double-stranded DNA. Known relaxases belong to the Rolling Circle Replication initiator superfamily of enzymes and fall into two broad...

 (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

Tyrosine

Tyrosine or 4-hydroxyphenylalanine, is one of the 22 amino acids that are used by cells to synthesize proteins. Its codons are UAC and UAU. It is a non-essential amino acid with a polar side group...

 residue on the relaxase. Relaxase then moves in the 5' to 3' direction on the plasmid, unwinding the DNA in a helicase

Helicase

Helicases are a class of enzymes vital to all living organisms. They are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separating two annealed nucleic acid strands using energy derived from ATP hydrolysis.-Function:Many cellular processes Helicases are a...

-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.