III. Host range
- Cell-cell interactions
- DNA metabolism
- DNA transfer
IV. Comparison of Type IV secretion systems
Conjugation conferred by certain types of genetic elements, particularly plasmids. Transfer of plasmid from donor to recipient does not require plasmid DNA to recombine with recipient DNA. Conjugation is conservative -- donor retains a copy of the plasmid after transfer, indicating that plasmid replication occurs during conjugation.
Plasmid replication requires a "mating bridge" between the donor and recipient cells. The mating bridge is a region of contact between the donor and recipient cells where the DNA is presumably transferred via a pore (although the pore has not yet been realiably visualized). Before the mating bridge can form:
These two events do not necessarily occur simultaneously.
- donor must recognize recipient cell
- donor must make contact with recipient cell
Multiple genes are required for conjugation. For example, for the TiC58 plasmid the trb operons encode the gene products required for recognition of the recipient and mating pair formation. Genes in this cluster encode the sex pilus. The sex pilus is essential for conjugation. The sex pilus can be very long (e.g. the pilus produced by F-plasmid) or very short (e.g. the pilus produced by plasmid RP4). DNA is not transferred via the sex pilus. Although the receptor is yet not known for any conjugal plasmid, the sex pilus is required for recognition of the recipient cell. Plasmids like F that code for long flexous pili mate well in liquid media, but plasmids that code for short stiff pili usually mate well only on solid surfaces. Following contact, the long flexous pilus of F acts as a retractile motor -- the donor and recipient cells are pulled together as the pilus subunits depolymerize into the cytoplasmic membrane of the donor cells. In contrast, the retractile nature of other pili has not been demonstrated.
The tra operons encode gene products required for the DNA replication functions. DNA replication requires several steps.
- Initiation of rolling circle replication requires nicking of the covalently closed circular plasmid DNA. The plasmid DNA is nicked at a specific cis-acting site (called nic) within the origin of transfer (oriT). The enzyme that catalyzes nicking is called the "nickase" or "strand transferase". Upon nicking the DNA, the nickase remains covalently attached to the 5' phosphate group, leaving a free 3'OH group. Accessory proteins facilitate binding of the nickase to oriT, and this complex is called a "relaxosome".
- DNA replication initiates at the free 3'OH, and proceeds in a 5' to 3' direction
- The relaxasome complex binds to the transfer pore.
- Rolling circle DNA replication in the recipient pushes the ssDNA into the recipient cell.
- The ssDNA is converted to dsDNA in the recipient by lagging strand DNA synthesis.
See the following link for a model of the mechanism of plasmid conjugation.
In the absence of recipient, the relaxasome both nicks and religates the plasmid DNA. Although the mechanism is not yet known, contact with a recipient cell somehow stimulates nicking and subsequent DNA transfer.
The host range for conjugation can be determined by the range of hosts that can serve as recipients for DNA transfer or the range of hosts that the plasmid can replicate within. The host range for conjugation is typically determined by the ability of the plasmid to replicate within a specific host. For example, consider the following two conjugative plasmids:
Plasmid Conjugation host range Replication host range Transfer host range F narrow (Gram- enterics) narrow broad RP4 broad (Gram- and Gram+) broad broadThe host range of some conjugal plasmids is very broad, including transfer to recipients that are bacteria, yeast, plant cells, and mammalian cells. However, plasmid replication is often limited to certain hosts.
The genes encoding conjugal transport systems share considerable DNA sequence homology between different conjugal plasmids, and with genes involved in Type IV protein export systems. The sequence similarity suggests that these systems are closely related and probably evolved from a common ancestor. This sequence similarity raises questions about whether the role of the primordial Type IV system was DNA transfer or protein translocation.REFERENCES:
This page is based in part upon a lecture on conjugation by Prof. Steve Farrand at the University of Illinois, Urbana.
This page is maintained by Stanley Maloy, please send comments, suggestions, or questions to firstname.lastname@example.org
Last modified October 1, 2004