The fertility factor allows genes to be transferred from one bacterium carrying the factor to another bacterium lacking the factor by conjugation. The F factor is carried on the F episome, the first episome to be discovered. Unlike other plasmids, F factor is constitutive for transfer proteins due to a mutation in the gene finO. The F plasmidbelongs toa class of conjugative plasmids that control sexual functions of bacteria with a fertility inhibition system.
Discovery
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 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 thought F was really lambda, and when we convinced him , he then began his work."
Structure
The most common functional segments constituting F factors are:
OriT : The sequence which marks the starting point of conjugative transfer.
OriV : The sequence starting with which the plasmid-DNA will be replicated in the recipient cell.
tra-region : Genes coding the F-Pilus and DNA transfer process.
IS composed of one copy of IS2, two copies of IS3, and one copy of IS1000: so-called "selfish genes".
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' 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 do not contain F factor and act as the recipients.
Function
When an F+ cell conjugates/mates with an F− cell, the result is two F+cells, both capable of transmitting the plasmid to other F− cells by conjugation. A pilus on the F+ cell interacts with the recipient cell allowing formation of a mating junction, the DNA is nicked on one strand, unwound and transferred to the recipient. The F-plasmid belongs to a class of conjugative plasmids that control sexual functions of bacteria with a fertility inhibition system. In this system, a trans-acting factor, FinO, and antisense RNAs, FinP, combine to repress the expression of the activator gene TraJ. TraJ is a transcription factor that upregulates the traoperon. The tra operon includes genes required for conjugation and plasmid transfer. This means that an F+ bacteria can always act as a donor cell. The finO gene of the original F plasmid is interrupted by an IS3 insertion, resulting in constitutive tra operon expression. F+ cells also have the surface exclusion proteins TraS and TraT on the bacterial surface. These proteins prevent secondary mating events involving plasmids belonging to the same incompatibility group. Thus, each F+ bacterium can host only a single plasmid type of any given incompatibility group. In the case of Hfr transfer, the resulting transconjugates are rarely Hfr. The result of Hfr/F− conjugation is a F− strain with a new genotype. 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 bacterial artificial chromosome. The first DNA helicase ever described is encoded on the F-plasmid and is responsible for initiating plasmid transfer. It was originally called E. coliDNA Helicase I, but is now known as F-plasmid TraI. In addition to being a helicase, the 1756 amino acid F-plasmid TraI protein is also responsible for both specific and non-specific single-stranded DNA binding as well as catalyzing the nicking of single-stranded DNA at the origin of transfer.
