The Fertility factor (also known as the F factor, sex factor, or F-plasmid) is a bacterial DNA sequence that allows a bacterium to produce a sex pilus necessary for conjugation. The F plasmid belongs to a class of conjugative plasmids that control their sexual functions with a fertility inhibition (Fin) system in which a trans-acting factor, FinO, and antisense RNAs, FinP, combine to repress the expression of an activator, TarJ, of the tra operon. The tra operon encodes functions required for conjugal replication and transfer. The finO gene of the original F plasmid is interrupted by an IS3 insertion, resulting in constitutive tra operon expression, in E. coli K12.
The most common functional segments constituting F factors are:
When an F+ cell conjugates with an F− cell, the result is two F+ cells, both capable of transmitting the plasmid further by conjugation. Unlike other plasmids, F factor is constitutive for transfer proteins due to the gene traJ. This means that an F+ bacteria can always act as a donor cell. In the case of Hfr, the result are two Hfr cells. When F-prime plasmids are transferred to a recipient bacterial cell, they carry pieces of the donor's DNA that can become important in recombination. Bioengineers have created F plasmids that can contain inserted foreign DNA; this is called a fosmid.
The first DNA helicase ever described is encoded on the F-plasmid and is responsible for initiating plasmid transfer. It was originally called E. coli DNA Helicase I, but is now known as F-plasmid TraI.