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 able to transfer some of the host genes to the recipient.
- The sex pilus contacts the recipient F- cell and pulls the cells together
- The donor chromosome is transferred as SS DNA starting at the origin of transfer. Gene that is closest to the origin is transferred first.
- Segments of the integrated plasmid are at the beginning and the end of the DNA being transferred (Note: Yellow part is integrated plasmid.
- 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
- Usually, only part of the F plasmid called the initiating segment is transferred along with adjacent chromosomal genes.
- Transferred DNA becomes double-stranded
- The donor DNA is integrated into the recipient cell’s DNA by homologous recombination.
- The recipient now carries transferred genes but remains F- whereas the donor cell remains HFr.
- 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.
