F-plasmid

The Fertility factor (also known as F factor or sex factor) is a bacterial DNA sequence that allows a bacterium to produce a sex pilus necessary for conjugation. It contains 20 tra (for "transfer") genes and a number of other genetic sequences responsible for incompatibility, replication, and other functions. 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.

• 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.