Conjugation: Transfer of Chromosomal DNA by HFr Strains

Conjugation is the process of the transfer of genetic material from one bacteria to another. It requires close contact between the two bacteria. The transfer of plasmid DNA (deoxyribonucleic acid) occurs during conjugation. The donor bacteria (F⁺) strain transfers the plasmid into the recipient bacteria (F^–) strain. This is the general mechanism of conjugation.

The F⁺ strain which contains the plasmid gene as an episome (i.e.  F plasmid becomes integrated into the host cell genome at one of the several possible sites of a chromosomal gene) induces more than a thousand times the number of genetic recombination than seen in F⁺ and F^– cells.  Such a donor strain is called a high frequency of recombination (HFr) strain. The HFr strain is responsible for transferring specific part of chromosomal DNA.

Mechanism of Transfer of HFr (high frequency recombination) Strains

1. The HFr strain is able to transfer some of the host genes to the recipient.
2. Firstly, the sex pilus contacts the recipient F^– cell and pulls the cells together
3. The donor chromosome is transferred as SS DNA starting at the origin of transfer. Gene that is closest to the origin transfers first.
4. Segments of the integrated plasmid are at the beginning and the end of the DNA being transferred (note: yellow part is integrated plasmid.)
5. To transfer the entire plasmid, the entire bacterial gene must be transferred first but the donor and recipient cells separate prior to the complete transfer of genome and plasmid
6. Usually, only part of the F plasmid called the initiating segment is transferred along with adjacent chromosomal genes.
7. Transferred DNA becomes double-stranded
8. The donor DNA is integrated into the recipient cell’s DNA by homologous recombination.
9. The recipient now carries transferred genes but remains F^– whereas the donor cell remains HFr.
10. The recipient cell does not become an F⁺ donor cell, as only a part of the F plasmid is transferred but the donor cells remain as Hfr strains.

References

1. Sana, T. G., Laubier, A., & Bleves, S. (2014). Gene transfer: conjugation. Methods in molecular biology (Clifton, N.J.), 1149, 17–22. https://doi.org/10.1007/978-1-4939-0473-0_3 
2. Llosa, M., Gomis-Rüth, F. X., Coll, M., & de la Cruz Fd, F. (2002). Bacterial conjugation: a two-step mechanism for DNA transport. Molecular microbiology, 45(1), 1–8. https://doi.org/10.1046/j.1365-2958.2002.03014.x