The fertility factor (first named F by one of its discoverers Esther Lederberg; also called the sex factor in E. coli or the F sex factor; also called F-plasmid) allows genes to be transferred from one bacterium carrying the factor to another bacterium lacking the factor by conjugation. The F factor was the first plasmid to be discovered. Unlike other plasmids, F factor is constitutive for transfer proteins due to a mutation in the gene finO. The F plasmid belongs to a class of conjugative plasmids that control sexual functions of bacteria with a fertility inhibition (Fin) system. Discovery Esther M. Lederberg and Luigi L. Cavalli-Sforza discovered "F," subsequently publishing with Joshua Lederberg. Once her results were announced, two other labs joined the studies. "This was not a simultaneous independent discovery of F (I names as Fertility Factor until it was understood.) We wrote to Hayes, Jacob, & Wollman who then proceeded with their studies." The discovery of "F" has sometimes been confused with William Hayes' discovery of "sex factor", though he never claimed priority. Indeed, "he [Hayes] thought F was really lambda, and when we convinced him [that it was not], he then began his work." Structure 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 Vegetative 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 process. IS (Insertion Elements) composed of one copy of IS2, two copies of IS3, and one copy of IS1000: so-called "selfish genes" (sequence fragments which can integrate copies of themselves at different locations). Some F plasmid genes and their Function: traA: F-pilin, Major subunit of the F-pilus. Relation to the genome The episome that harbors the F factor 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 the entire F episome integrated into 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 are formed by incorrect excision from the chromosome, resulting in F plasmid carrying bacterial sequences that are next to where the F episome has been inserted. F− bacteria