Conjugation leads to one DNA transfer from a male (or donor) cell to a female (or recipient) cell via the sex pilus. The existence (male) or lack (female) of a conjugative plasmid, like the F-plasmid of E. coli, determines the cell’s mating type (sex).
The fertility or F factor is a defining characteristic of F-plasmids. Due to its role in the genetic material transfer from one bacteria to another, it is also known as the “sex factor.” The F factor is primarily present in bacteria, and the exchange occurs during conjugation. In other words, two F+ cells are produced due to conjugation between F+ and F– cells.
Table of Contents
- Definition of F-Plasmid
- Structure
- F-Plasmid Integration
- Functions of F-Plasmid
- Frequently Asked Questions (FAQs)
Definition of F-Plasmid
A plasmid is referred to be an F prime (F′) plasmid if chromosomal DNA fragments have been incorporated into it. The F-plasmid is called conjugative because it possesses all the genes required for its transfer, including the capacity to produce sex pili and activate DNA synthesis at the plasmid’s transfer origin (oriT).
When the F-plasmid is transferred, the recipients develop into F+ male cells. It transmits that fragment into the recipient cell, transforming it into an F′ male. The cell is referred to as a Hfr (high-frequency recombination) cell if the F-plasmid sequence is incorporated into the bacterial chromosome.
The F factor integration results in high-frequency recombination by causing a relatively frequent transference of the bacterial chromosome from the donor into the recipient cell.
Structure
The following functional segments make up F factors:
- OriT (Origin of Transfer) is the sequence that acts as the origin of conjugative transfer.
- OriV (Origin of Vegetative Replication) is the sequence where the plasmid-DNA will replicate in the recipient cell.
- DNA transfer and the F-Pilus are coded for by the tra-region (transfer genes).
- IS (Insertion Elements), also known as “selfish genes”, consist of one copy of IS1000, a single copy of IS2, and two copies of IS3.
The F-plasmid, found on the surface of F+ bacterial cells, codes for the sex-pili or F-pili. There are 23 pili in an F+ cell. They are 2-3 m long, hair-like projections that protrude from the cell wall of the bacteria. Pilin is a single-subunit protein encoded by gene tra A polymerises “pilus”.
F-Plasmid Integration
Insertion sequences (IS) are specific DNA regions where the F factor is integrated into the bacterial chromosome. The F factor contains various IS regions, including IS 2, IS 3, and V-δ. The 20 IS regions on the E. coli chromosome are dispersed across the chromosome in clockwise and anti-clockwise directions.
A single crossover event between the IS elements and the bacterial chromosome is required to integrate the F factor into the chromosome. They are the locus for homologous pairing between the bacterial chromosome and the F factor. The chromosome bearing an F-plasmid is termed as Hfr chromosome.
The Hfr factor is firmly attached to the host chromosome at a specific location. Different Hfr strains can be produced by breaking the bacterial chromosome and orienting the transfer at various places.
Functions of F-Plasmid
When an F+ cell and an F– cell combines, they produce two F+ cells capable of passing the plasmid to other F– cells through conjugation. A mating junction is formed when a pilus on the F+ cell contacts the recipient cell. The DNA is snipped on one strand, unfolded, and transported to the recipient during this process.
The F-plasmid is a member of a group of conjugative plasmids that use the fertility inhibition (Fin) system to regulate bacterial sexual behaviour. An antisense RNAs called FinP and a trans-acting protein called FinO work together in this system to suppress the production of the activator gene TraJ.
This transcription factor, TraJ, increases the expression of the tra operon. The tra operon contains the genes necessary for plasmid transfer and conjugation. As a result, the F+ bacteria always serve as donor cells.
The F-plasmid contains the first-ever DNA helicase responsible for plasmid transfer initiation. Originally known as E. coli DNA Helicase I, it is now referred to as F-plasmid TraI.
The 1756 amino acid F-plasmid TraI protein is a helicase in addition to being a binding site for non-specific and specific single-stranded DNA and a catalyst for the nicking of single-stranded DNA at the transfer origin.
Related Links:
Main Page: BYJU’S NEET
Comments