4. History
Discovery: Joshua Leaderberg & Edward Tatum in
1946.
They experimented with two Auxotropic strain of
E.coli K12 denoted by Strain A and Strain B.
Strain A ( met- bio- thr+ leu+ thi+) Strain B ( met+
bio+ thr- leu- thi-) were plated on minimal medium
and incubated overnight( control), no growth
observed. Also Strain A and B were mixed together
and when plated on minimal medium resulted in
prototrophs.
5.
6. Conjugation requires physical contact
• The evidence for cell to cell contact was provides by Bernard
Davis in 1950.
• The to arms of the U tube are separated by a filter.
• On the right side is medium containing auxotrophic strain A
while on the left side is medium containing auxotrophic strain
B.
• The filter allows only the medium to move but not the cells
on either side.
• When culture was plated from both sides on minimal
medium, no prototroph growth was observed as in Lederberg
and Tatum’s experiment.
9. Terminology
HFR ( F+) ( F-) Mixed culture
♪ The donor
♪ F+
Fertility
Factor
♪ Contains
genetic
material for
gene exchange
‘Male’ cell.
♪ The recipient
♪ F-
No fertility
factor
♪ Receives
genetic
material
‘Female’ cell.
♪ The conjugate
♪ HFR
High frequency
recombinant
♪ Resulting genetic
recombinant
♪ Transfers entire
genomic DNA
when mixed with
F-
16. 3. DNA synthesis
• complementary strands of DNA are replicates in both
F+ and F- cells.
• Targeted DNA incorporated into F- genome.
17. 4. Separation
• Cells separate.
• Form two F+ cells with complete recombinant plasmids
• Both severe as donor cells for further conjugation.
18. Bacterial conjugation
in E.coli strains
HFR ( F+) ( F-) Mixed culture
Able to produce
amino acids
Thr, Leu, Thi
Sensitive to
Streptomycin
HFR strain used
for experimental
purposes
Requires amino
acids
Thr, Leu, Thi
Resistant to
streptomycin.
Able to produce
Thr and Leu
Thi not produce
Resistant to
streptomycin