16. Which of the following mattings between E. coli strains would result in a high frequency of transfer of chromosomal genes?
(1) F+ X F– (2) F+ X F+
(3) Hfr x Hfr (4) Hfr X F–
In E. coli conjugation, the mating between Hfr and F- strains results in a high frequency of chromosomal gene transfer due to the integrated F plasmid in the Hfr donor initiating efficient chromosome mobilization to the F- recipient. This process contrasts with other matings where chromosomal transfer occurs at low rates or not at all. The correct answer is option (4) Hfr x F-.
Option Analysis
-
(1) F+ x F-: F+ donors carry an autonomous F plasmid that transfers primarily itself to F- recipients at high frequency, converting them to F+; chromosomal genes transfer rarely (less than 0.1%) as this requires rare Hfr-like events.
-
(2) F+ x F+: Both strains possess F plasmids, preventing stable conjugation due to surface exclusion; no significant chromosomal gene transfer occurs.
-
(3) Hfr x Hfr: Hfr donors transfer chromosomal DNA efficiently but recipients, also being Hfr, resist entry exclusion from another donor, resulting in minimal transfer.
-
(4) Hfr x F-: Hfr integrates F into its chromosome, enabling origin-specific, high-frequency (up to 100% early markers) linear transfer of chromosomal genes to receptive F- cells via conjugation bridge.
Conjugation Mechanism
Hfr strains form when F plasmid integrates into the E. coli chromosome, creating a transfer origin that mobilizes adjacent genes first during mating. Transfer proceeds unidirectionally but often interrupts before completion, yielding partial recombinants in F- recipients without converting them to donors. This defines “high frequency recombination” essential for bacterial mapping.
