Transfer of DNA Between Bacterial Cells
The genetic information can be transferred from one bacterium to another. There are three general methods for genetic exchange in bacteria: (a) transformation, (b) transduction, and (c) conjugation.
Transformation is a process of the transfer of DNA itself fromone bacterium to another. This may occur either in nature or in a laboratory. In nature, DNA is released from a bacterium by lysis, which may be taken up by recipient bacterium that must be competent. This natural process of transfer of genetic material appears to play no role in disease. In laboratory condi-tions, DNA may be extracted from one type of bacterium and introduced into genetically different bacteria. The cell walls of bacteria in vitro are made more permeable for DNA uptake by using substances, such as calcium chloride.
Griffith (1922) in his classical experiment on mice demon-strated that neither of the mice died when injected separately with a live, noncapsulated Pneumococcus (nonvirulent) and heat-killed, capsulated Pneumococcus (nonvirulent), but the mice died when they were injected with a mixture of both these strains. From the dead mice, he could isolate live, capsulated pneumococci, which were virulent. He demon-strated that some factor in heat-killed, capsulated pneumo-cocci had transferred the material for capsule synthesis in the noncapsulated strains of the bacteria, making them virulent (Fig. 7-1).
McLeod and McCarthy in 1944 demonstrated that DNA extracted from encapsulated, smooth pneumococci could transform nonencapsulated, rough pneumococci into capsulated, smooth organisms. They demonstrated the transforming principle of DNA. The experimental use of transformation was the first experiment to reveal important information about DNA and was the first example of genetic exchange in bacteria.
Another bacterium where transformation is observed is Haemophilus influenzae.
The transfer of a portion of DNA from one bacterium to another mediated by a bacteriophage is known as transduc-tion. During replication of virus within the cell, a piece ofbacterial DNA is incorporated into the bacteriophage and is carried into the recipient bacterium at the time of infection. The phage DNA within the recipient bacterial cell integrates into the cell DNA during a process called lysogenic conversion. The process of lysogenic conversion confers a new property to the bacterial cell; for example, by lysogenic conversion non-pathogenic bacteria can become pathogenic. Bacteriophages encode diphtheria toxin, botulinum toxin, cholera toxin, and erythrogenic toxin and can be transferred from one bacte-rium to another by transduction (Fig. 7-2). Transduction is of two types: (a) generalized transduction and (b) specialized transduction.
This occurs when a small fraction of the phage virions produced during lytic cycle are aberrant and contain a random fragment of the bacterial genome instead of phage DNA. Each individ-ual transducing phage carries a different set of closely linked genes, representing a small segment of the bacterial genome. Transduction mediated by populations of such phages is called generalized transduction. Each part of the bacterial genome hasapproximately the same probability of being transferred from donor to recipient bacteria.
Generalized transduction involves any segment of the donor DNA at random. This occurs because the cell DNA is fragmented after such infection and pieces of same DNA, the same size as viral DNA, are incorporated into the bac-terial DNA. This occurs at a frequency of about 1 in every 1000 viruses. Generalized transduction may be complete or abortive:
Complete transduction is characterized by production ofstable recombinants that inherit donor genes and retain the ability to express them.
Abortive transduction refers to the transient expressionof one or more donor genes without formation of recombi-nant progeny. The donor DNA fragment does not replicate in abortive transduction, and only one bacterium contains the donor DNA fragment among the progeny of the origi-nal transductant. The donor gene products become progres-sively diluted in all other progeny after each generation of bacterial growth until the donor phenotype can no longer be expressed.
On selective medium, abortive transductants produce minute colonies that can be distinguished easily from colonies of stable transductants. The frequency of abortive transduction is typically one to two times more than the frequency of generalized trans-duction. This indicates that most cells infected by generalized transducing phages do not produce recombinant progeny.
Specialized transduction results from lysogenization of the recipient bacterium by the specialized transducing phage and expression of the donor genes. Specialized transducing phages are formed only when lysogenic donor bacteria enter the lytic cycle and release phage progeny.
The specialized transducing phages are rare recombinants that lack part of the normal phage genome. They contain part of the bacterial chromosome present adjacent to the site of pro-phage attachment. Many specialized transducing phages are defective. They cannot complete the lytic cycle of phage growth in infected cells unless helper phages are present to provide missing phage functions.
Specialized transduction differs from generalized transduc-tion in many ways. The former is mediated only by specific temperate phages and only a few specific donor genes can be transferred to recipient bacteria.
Conjugation is a process of transfer of DNA from the donor bacterium to the recipient bacterium during the mating of two bacterial cells. In conjugation, direct contact between the donor and recipient bacteria leads to formation of a cytoplasmic bridge between them and transfer of part or all of the donor genome to the recipient (Fig. 7-3). Conjugation takes place between two closely related species and occurs mostly in Gram-negative bacteria. Conjugation also occurs in Gram-positive bacteria.
Donor ability of bacteria is determined by specific conju-gative plasmids called fertility (F1) plasmids or sex plasmids. The F plasmid controls the mating process of bacteria. Pilus is the most important protein that forms the sex pilus or conjugation tube. The sex pilus produces a bridge between conjugating cells in Gram-negative bacteria. Mating occurs between the donor male bacterium carrying the F factor (F1) and the recipient female bacterium that does not contain F fac-tor (F2). It begins when the pilus of F1 bacterium attaches to a receptor on the surface of a female (F2) bacterium. The cells are then brought into direct contact by the link in the pilus. This is followed by an enzymatic cleavage of the F factor DNA in which one strand of bacterial DNA is transferred into the recipient cell through the conjugation bridge. The synthesis of the complementary strand to form a double-stranded F-factor plasmid in both the donor and recipient cells completes the process of conjugation. The recipient cell becomes F1 male that is capable of transmitting the plasmid to other F2 cells.
High-frequency recombination (Hfr): Long length of DNAcan be transferred by process of conjugation. Hfr strain is a type of F+ cells that have an F plasmid integrated into the bacterial DNA. Hence they acquire the capability of transferring the chromosome to another cell.
A whole chromosome can be transferred
if it is integrated with F plasmid. In this process, the single strand of DNA
that enters the recipient F2 cell contains a part of the F factor at one end, followed by the
bacterial chromosome, and then by the remainder of the F factor. The bacterial
genes adjacent to the leading piece of F factor are the most frequently
transferred. The newly acquired DNA recombines with the recipient DNA and
becomes an integral component of genetic material. The complete transfer of the
bacterial DNA is usually completed in approximately 100 minutes.
In matings between F+ and F− bacteria, only the F plasmid is transferred with high efficiency to recipients. Chromosomal genes are transferred with very low efficiency, which is medi-ated by the spontaneous Hfr mutants in F1 populations. In matings between Hfr and F2 strains, the segment of the F plas-mid containing the tra region is transferred last, after the entire bacterial chromosome has been transferred. Most recombi-nants produced after matings between Hfr and F2 cells fail to inherit the entire set of F-plasmid genes and are phenotypi-cally F2. In matings between F1 and F2 strains, the F plasmid spreads rapidly throughout the bacterial population and most recombinants are F1.
Conjugation also occurs in Gram-positive bacteria. Gram-positive donor bacteria produce adhesions that cause them to aggregate with recipient cells, but sex pili are not involved. In some Streptococcus spp., recipient bacteria produce extracellular sex pheromones that facilitate conjugation. Table 7-1 shows a comparison of transformation, transduction, and conjugation.
After the DNA is transferred from one donor bacterium to the recipient through transformation, transduction, or conjugation, it combines with the chromosome of the bacterium by a process called recombination.
Recombination is of two types: homologous and nonho-mologous. Homologous recombination takes place between two pieces of DNA showing extensive homologous regions. This results in pairing up and exchange of pieces by the pro-cesses of breakage and reunion. The nonhomologous recom-bination takes place between two pieces of DNA showing little or no homology.