Conjugation diagram • Donor cell produces
pilus. • Pilus attaches to recipient cell and brings the two cells together. • The mobile plasmid is nicked and a single strand of DNA is then transferred to the recipient cell. • Both cells synthesize a complementary strand to produce a double stranded circular plasmid and also reproduce pili; both cells are now viable donor for the F-factor. Several proteins coded for in the
tra or
trb locus seem to open a channel between the bacteria and it is thought that the traD enzyme, located at the base of the pilus, initiates membrane fusion. When conjugation is initiated by a signal, the
relaxase enzyme creates a
nick in one of the strands of the conjugative plasmid at the
oriT. Relaxase may work alone, or in a complex of over a dozen proteins known collectively as a
relaxosome. In the F-plasmid system, the relaxase enzyme is called TraI and the relaxosome consists of TraI, TraY, TraM and the integrated host factor IHF. The nicked strand, or
T-strand, is then unwound from the unbroken strand and transferred to the recipient cell in a 5'-terminus to 3'-terminus direction. The remaining strand is replicated either independent of conjugative action (vegetative replication beginning at the
oriV) or in concert with conjugation (conjugative replication similar to the
rolling circle replication of
lambda phage). Conjugative replication may require a second nick before successful transfer can occur. A recent report claims to have inhibited conjugation with chemicals that mimic an intermediate step of this second nicking event. s (yellow) on both the
F factor plasmid and the chromosome have similar sequences, allowing the F factor to insert itself into the
genome of the cell. This is called
homologous recombination and creates an Hfr (high frequency of recombination) cell. 2.The Hfr cell forms a pilus and attaches to a recipient F- cell. 3.A nick in one strand of the Hfr cell's chromosome is created. 4.DNA begins to be transferred from the Hfr cell to the recipient cell while the second strand of its chromosome is being replicated. 5.The pilus detaches from the recipient cell and retracts. The Hfr cell ideally wants to transfer its entire genome to the recipient cell. However, due to its large size and inability to keep in contact with the recipient cell, it is not able to do so. 6.a. The F- cell remains F- because the entire F factor sequence was not received. Since no homologous recombination occurred, the DNA that was transferred is degraded by enzymes. b. In very rare cases, the F factor will be completely transferred and the F- cell will become an Hfr cell. '
Spontaneous zygogenesis in E. coli''''' In addition to classical bacterial conjugation described above for
E. coli, a form of conjugation referred to as spontaneous zygogenesis (Z-mating for short) is observed in certain strains of
E. coli. In Z-mating there is complete genetic mixing, and unstable
diploids are formed that throw off phenotypically haploid cells, of which some show a parental
phenotype and some are true
recombinants. ==Conjugal transfer in mycobacteria==