The transfer of genetic information from one cell to another can occur by three methods: conjugation, transduction, and transformation (Table 4–1). From a medical viewpoint, the two most important consequences of DNA transfer are (1) that antibiotic resistance genes are spread from one bacterium to another primarily by conjugation and (2) that several important exotoxins are encoded by bacteriophage genes and are transferred by transduction.
Table 4–1Comparison of Conjugation, Transduction, and Transformation ||Download (.pdf) Table 4–1 Comparison of Conjugation, Transduction, and Transformation
|Transfer Procedure ||Process ||Type of Cells Involved ||Nature of DNA Transferred |
|Conjugation || |
DNA transferred from one bacterium to another
Chromosomal or plasmid
DNA transferred by a virus from one cell to another
Any gene in generalized transduction; only certain genes in specialized transduction
Naked DNA in the immediate environment taken up by a cell
Conjugation is the mating of two bacterial cells, during which DNA is transferred from the donor to the recipient cell (Figure 4–2). The mating process is controlled by an F (fertility) plasmid (F factor), which carries the genes for the proteins required for conjugation. One of the most important proteins is pilin, which forms the sex pilus (conjugation tube). Mating begins when the pilus of the donor bacterium carrying the F factor (F+) attaches to a receptor on the surface of a recipient bacterium, which does not contain an F factor (F–), resulting in a direct connection between the cytoplasm of the donor and recipient cells. After an enzymatic cleavage of the F factor DNA, one strand is transferred across the conjugal bridge (mating bridge) into the recipient cell. The process is completed by synthesis of the complementary strand to form a double-stranded F factor plasmid in both the donor and recipient cells. The recipient is now an F+ cell that is capable of transmitting the plasmid further. Note that in this instance only the F factor, and not the bacterial chromosome, has been transferred.
Conjugation. An F plasmid is being transferred from an F+ donor bacterium to an F– recipient. The transfer is at the contact site made by the sex pilus. The new plasmid in the recipient bacterium is composed of one parental strand (solid line) and one newly synthesized strand (dashed line). The previously existing plasmid in the donor bacterium now consists of one parental strand (solid line) and one newly synthesized strand (dashed line). Both plasmids are drawn with only a short region of newly synthesized DNA (dashed lines), but at the end of DNA synthesis, both the donor and the recipient contain a complete copy of the plasmid DNA.
Some F+ cells have their F plasmid integrated into the bacterial DNA and thereby acquire the capability of transferring the chromosome into another cell. These cells are called Hfr (high-frequency recombination) cells (Figure 4–3). During this transfer, the single strand of DNA that enters the recipient F– cell contains a piece of the F factor at the leading end followed by the bacterial chromosome and then by the remainder of the F factor. The time required for complete transfer of the bacterial DNA is approximately 100 minutes. Most matings result in the transfer of only a portion of the donor chromosome because the attachment between the two cells can break. The donor cell genes that are transferred vary since the F plasmid can integrate at several different sites in the bacterial DNA. The bacterial genes adjacent to the leading piece of the F factor are the first and therefore the most frequently transferred. The newly acquired DNA can recombine into the recipient’s DNA and become a stable component of its genetic material.
High-frequency recombination. Top: A fertility (F) plasmid has integrated into the bacterial chromosome. Bottom: The F plasmid mediates the transfer of the bacterial chromosome of the donor into the recipient bacteria.
Resistance plasmids (R plasmids) can also be transferred by conjugation. R plasmids can carry one or more genes for a variety of enzymes that can degrade antibiotics and modify membrane transport systems. For example, R plasmids encode the β-lactamases of Staphylococcus aureus, Escherichia coli, and Klebsiella pneumoniae. In addition, they encode the proteins of the transport system that actively export sulfonamides out of the bacterial cell. Note that R plasmids can be transferred not only to cells of the same species, but also to other species and genera. (See Chapter 11 for more information about R plasmids.)
Transduction is the transfer of cell DNA by means of a bacterial virus (bacteriophage, phage) (Figure 4–4). During the growth of the virus within the cell, a piece of bacterial DNA is incorporated into the virus particle and is carried into the recipient cell at the time of infection. Within the recipient cell, the phage DNA can integrate into the cell DNA and the cell can acquire a new trait—a process called lysogenic conversion (see the end of Chapter 29). This process can change a nonpathogenic organism into a pathogenic one. Diphtheria toxin, botulinum toxin, cholera toxin, shiga toxin of E. coli and erythrogenic toxin (Streptococcus pyogenes) are encoded by bacteriophages and can be transferred by transduction.
Transduction. A: A bacteriophage infects a bacterium, and phage DNA enters the cell. B: The phage DNA replicates, and the bacterial DNA fragments. C: The progeny phages assemble and are released; most contain phage DNA, and a few contain bacterial DNA. D: Another bacterium is infected by a phage-containing bacterial DNA. E: The transduced bacterial DNA integrates into host DNA, and the host acquires a new trait. This host bacterium survives because no viral DNA is transduced; therefore, no viral replication can occur. (Another type of transduction mechanism is depicted in Figure 29–10.)
There are two types of transduction: generalized and specialized. The generalized type occurs when the virus carries a segment from any part of the bacterial chromosome. This occurs because the cell DNA is fragmented after phage infection and pieces of cell DNA the same size as the viral DNA are incorporated into the virus particle at a frequency of about 1 in every 1000 virus particles. The specialized type occurs when the bacterial virus DNA that has integrated into the cell DNA is excised and carries with it an adjacent part of the cell DNA. Since most lysogenic (temperate) phages integrate at specific sites in the bacterial DNA, the adjacent cellular genes that are transduced are usually specific to that virus.
Transformation is the transfer of DNA itself from one cell to another. This occurs by either of the two following methods. First, in nature, dying bacteria may release their DNA, which may be taken up by recipient cells. Certain bacteria, such as Neisseria, Haemophilus, and Streptococci, synthesize receptors on the cell surface that play a role in the uptake of DNA from the environment.
Second, in the laboratory, an investigator may extract DNA from one type of bacteria and introduce it into genetically different bacteria. The experimental use of transformation has revealed important information about DNA. In 1944, it was shown that DNA extracted from encapsulated smooth pneumococci could transform nonencapsulated rough pneumococci into encapsulated smooth organisms. This demonstration that the transforming principle was DNA marked the first evidence that DNA was the genetic material.