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
Conjugation or conjugate may refer to:
Linguistics
*Grammatical conjugation, the modification of a verb from its basic form
* Emotive conjugation or Russell's conjugation, the use of loaded language
Mathematics
*Complex conjugation, the change ...
. The F factor was the first
plasmid
A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; howev ...
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
Joshua () or Yehoshua ( ''Yəhōšuaʿ'', Tiberian: ''Yŏhōšuaʿ,'' lit. 'Yahweh is salvation') ''Yēšūaʿ''; syr, ܝܫܘܥ ܒܪ ܢܘܢ ''Yəšūʿ bar Nōn''; el, Ἰησοῦς, ar , يُوشَعُ ٱبْنُ نُونٍ '' Yūšaʿ ...
. 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 William Hayes may refer to:
In politics
*William Hayes (Irish politician), Irish Sinn Féin politician
*William Hayes (Canadian politician) (1879–1939), member of the Legislative Assembly of Alberta
* William P. Hayes (1866–1940), American la ...
' discovery of "sex factor", though he never claimed priority. Indeed, "he
ayesthought F was really lambda, and when we convinced him
hat it was not
A hat is a head covering which is worn for various reasons, including protection against weather conditions, ceremonial reasons such as university graduation, religious reasons, safety, or as a fashion accessory. Hats which incorporate mecha ...
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
An episome is a special type of plasmid, which remains as a part of the eukaryotic genome without integration. Episomes manage this by replicating together with the rest of the genome and subsequently associating with metaphase chromosomes during ...
that harbors the F factor can exist as an independent
plasmid
A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; howev ...
or integrate into the bacterial cell's
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding ...
. 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 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 (Fin) system. In this system, a trans-acting factor, FinO, and antisense RNAs,
FinP
FinP encodes an antisense non-coding RNA gene that is complementary to part of the TraJ 5' UTR. The FinOP system regulates the transfer of F-like plasmids. The traJ gene encodes a protein required for transcription
Transcription refers to the ...
, combine to repress the expression of the activator gene
TraJ. TraJ is a
transcription factor
In molecular biology, a transcription factor (TF) (or sequence-specific DNA-binding factor) is a protein that controls the rate of transcription of genetic information from DNA to messenger RNA, by binding to a specific DNA sequence. The fu ...
that upregulates the ''tra''
operon
In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo spli ...
. 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 (in
E. coli K12) 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 (Inc) 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 A bacterial artificial chromosome (BAC) is a DNA construct, based on a functional fertility plasmid (or F-plasmid), used for transforming and cloning in bacteria, usually '' E. coli''. F-plasmids play a crucial role because they contain partition ...
.
The first DNA
helicase
Helicases are a class of enzymes thought to be vital to all organisms. Their main function is to unpack an organism's genetic material. Helicases are motor proteins that move directionally along a nucleic acid phosphodiester backbone, separat ...
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. In addition to being a helicase, the 1756 amino acid (one of the largest in ''E. coli'') 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.
See also
*
FlmB RNA
*
Fosmid
*
Hfr cell
References
External links
*
*
{{DEFAULTSORT:Fertility Factor
Bacteriology
Molecular genetics