Bacterial conjugation is a natural process of genetic exchange where bacteria share DNA through direct cell-to-cell contact. This horizontal gene transfer allows bacteria to rapidly acquire new traits, such as antibiotic resistance. The ability to initiate this process is governed by the Fertility factor (F factor), a genetic element that determines if a cell can act as a donor. Understanding the two primary donor types—the F+ donor and the High Frequency of Recombination (Hfr) donor—is essential to distinguish how genetic material is transferred.
The F Factor: The Genetic Element
The F factor is a circular, double-stranded DNA plasmid that exists independently of the main bacterial chromosome. Classified as a conjugative plasmid, it contains all the necessary genes to initiate and execute the DNA transfer process. The F plasmid carries a large cluster of genes known as the tra operon.
The tra operon encodes the proteins required to form the conjugative pilus, a filament that connects the donor cell to a recipient cell. It also contains the origin of transfer (oriT), the specific site where the DNA molecule is nicked to begin transfer. The relaxase enzyme, encoded in this region, nicks the DNA at the oriT site and facilitates the transport of a single DNA strand into the recipient cell.
The F+ Donor: Extrachromosomal Plasmid Transfer
An F+ donor cell harbors the F factor as a separate, self-replicating plasmid, suspended freely in the cytoplasm. The F+ cell uses the F plasmid proteins to form a pilus, attaching to a recipient cell (F- cell). The relaxase enzyme prepares the F plasmid for transfer by nicking one strand at the oriT site. The single-stranded DNA is threaded into the recipient cell. This transfer uses rolling circle replication in the donor, ensuring the donor retains a complete copy of the F plasmid. The F+ donor primarily transfers only the F plasmid DNA, not the chromosomal DNA.
The Hfr Donor: Integrated Chromosomal Transfer
The Hfr donor (High Frequency of Recombination) originates when the F factor integrates directly into the bacterial chromosome. This integration occurs via homologous recombination between shared genetic sequences on the F plasmid and the chromosome. The Hfr cell retains the ability to initiate conjugation because the F factor’s tra genes remain functional, but its location is fixed within the chromosome.
When an Hfr cell conjugates with an F- recipient, transfer begins at the oriT site, which is now part of the host chromosome. The transferred DNA first includes a portion of the integrated F factor, followed by the adjacent chromosomal DNA. Because the physical connection is fragile, the conjugation bridge usually breaks before the entire chromosome is transferred. Consequently, only a segment of chromosomal DNA and a partial copy of the F factor are delivered to the recipient.
Key Differences in Conjugation Outcomes
The difference between F+ and Hfr donors lies in the nature of the DNA transferred and the recipient cell’s resulting status. F+ donors transfer only the F plasmid. The outcome of F+ conjugation is that the recipient F- cell receives the entire F factor and becomes a new F+ donor cell capable of initiating conjugation.
In contrast, Hfr donors transfer chromosomal genes at a high frequency, but this transfer is usually incomplete. The transferred DNA is a linear segment of the bacterial chromosome, starting with a partial sequence of the integrated F factor. Since the full F factor is rarely transferred, the recipient cell remains F- and cannot initiate conjugation itself. The transferred chromosomal DNA must integrate into the recipient’s chromosome through recombination to confer new traits, which explains the “High Frequency of Recombination” designation.