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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:

• OriT (Origin of Transfer): The sequence which marks the starting point of conjugative transfer.
• OriV (Origin of Replication): The sequence starting with which the plasmid-DNA will be replicated in the recipient cell.
• tra-region (transfer genes): Genes coding the F-Pilus and DNA transfer pores.
• IS (Insertion Elements): so-called "selfish genes" (sequence fragments which can integrate copies of themselves at different locations).

The F factor is an episome and can exist as an independent plasmid or integrate into the bacterial cell's genome. There are several names for the possible states:

• Hfr bacteria possess F factor integrated into the bacterial genome. After integration into the host chromosomal DNA, these plasmids possess portions, or entire, F factor and portions of the bacterial genome.

• F⁺ bacteria possess F factor as a plasmid independent of the bacterial genome. The F plasmid contains only F factor DNA and no DNA from the bacterial genome.

• F' (F-prime) bacteria possess an F plasmid that also includes some DNA taken from the bacterial genome. Sometimes formed by incorrect excision from the chromosome.

• F^- bacteria do not contain F factor.

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.