The bacteriophages or viruses that attack bacteria for their survival follow two main cycles such as lytic and lysogenic cycles. These two cycles possess differences. However, few phages may switch over from lysogenic to the lytic cycle. These smarter entities are capable of surviving in either of the ways. There is one more astonishing feature of a bacteriophage. It is capable of transferring genetic material from one bacterium to another bacterium. The process known as transduction is a phage-mediated DNA transfer between the two bacterial cells. Therefore a bacteriophage acts like a vector. The capacity of the phage particle to carry the genetic material is less than one percent of the capacity of a bacterial chromosome. You might be wondering exactly what a phage gains out of this activity. The bacteriophage may or may not gain anything out of this activity. The bacteriophage involved in transduction is a defective phage. The bacterial DNA instead of the phage DNA gets packed into the capsid. Therefore a defective phage transfers the bacterial DNA into another bacterium out of its mistake. In a population of normal phages, 1 in 105 transducing phages are present.
The genetic material gets transferred from one bacterium to another with the help of a defective phage. The fragment of genetic material transferred from one bacterium to another is known as a transducing element. Two types of transduction such as generalized and specialized transduction are carried out by a transducing phage. The phages may possess the ability to transduce any genes in a bacterial chromosome. Such phages undergo generalized transduction. In generalized transduction, the phage is flexible according to the transducing genes. Such phages are non-selective, meaning they transfer the genes without selection. Specialized transduction is a restricted one. A temperate phage undergoes a faulty looping out. There are loci near the attachment sites. The time and way of transduction vary as per the type of transduction and the type of phage. It is possible to map the genes based on the data obtained from transduction.
Generalized transduction:
An infected E.coli cell is best to study generalized transduction. Phage P1 mainly infects the bacteria for transduction. This phage undergoes a lysogenic cycle. However, it gets switched over to the lytic cycle in specific environmental conditions. During the lytic cycle, the bacterial DNA gets degraded before getting into the phage capsid. Such phages are known as transducing phages. A transducing phage is known as a transducing particle because it carries a part of the host genome in place of the phage genome. Hence the defective nature of the phage acts in favor of bacterial gene transfer.
Consider a bacterium having m+, n+ and q+ genes on its chromosome. As soon as the phage DNA enters inside, the bacterial DNA is fragmented. Each fragment has one gene say m+, n+, and q+ respectively. Bacteriophages reproduce normally. However, an error usually happens during the assembly. Defective phages pack the bacterial genes such as m+, n+ and q+ into their heads. When the progeny phages are released, some may have a normal genome while others may have a bacterial gene m+, n+ or q+ respectively. These phages, later on, infect other bacteria. In this case, the bacterial cells act like recipients since the transducing phages carrying donor genes infect them. Consider a transducing phage carrying an m+ gene segment. When this phage infects a recipient bacterium, there can be a genetic exchange of donor m+ gene with the recipient m gene. It occurs by a double cross-over. The transduced bacterium is stable and is known as a transductant. Suppose we try designing an experiment by selecting the markers in the donor and the recipient cells. Such cells exhibit prototrophic transductants. It is possible to transduce two or more genes simultaneously. A process of simultaneous transduction is known as cotransduction. Two or more marker genes can be cotransduced. There are two types of cotransduction. In one type of cotransduction, the genes closer to each other are packed. In another type of cotransduction, the distant genes are packed due to infection of the bacterial cell by two phages. However, this type of cotransduction is extremely rare. Thus, cotransduction occurs mostly in closely linked genes.
Image 1: Generalized transduction
Consider a bacterial chromosome with x+, y+ and z+ genes.
Of the x+ transductants, 50% of the transductants were z+. Only 2% exhibited y+ transductants. So, x+ and z+ genes are cotransduced on the same DNA.
Of the y+ transductants, 5% were x+ transductants and 0.05% were z+ transductants.
The representation of the data is as follows:
Main transductants
|
Maximum percentage transductants
|
Minimum percentage transductants
|
x+ transductants
|
50% z+
|
2% y+
|
y+ transductants
|
5% x+
|
0.05% z+
|
In conclusion, z+ and x+ are close to each other and so the x+ and y+ genes. The order of genes is y+ x+ z+
The multiplication of the number of single gene transductants out of total gene transductants with 100% gives the map distance.
Specialized transduction:
The phages transduce only a specified region of the bacterial gene or chromosome. Hence the process is known as specialized transduction. Lambda phage is a specialized transducing phage. It follows a lysogenic cycle. It integrates into own genome into the bacterial genome at a specific site between the gal and the bio regions producing a lysogen. The bacterial attachment site is homologous to an att site in lambda DNA. It integrates with a single cross-over. Now the prophage is maintained. Production of initial lysate involves outlooping. A precise excision produces a normal lambda phage chromosome by normal outlooping. In case of rare abnormal outlooping, crossing over occurs at sites other than homologous recombination. Therefore, it creates an abnormal lambda chromosome known as lambda d gal (λdgal). The letter “d” indicates defective nature. Not all phage genes are present. The lysate resulting from low-frequency transduction is known as low frequency transducing lysate (LFT).
Image 2: Specialized transduction
Two types of specialized transductants are as follows:
Type I specialized transductants are unstable meaning they are induced to initiate the lytic cycle. In type I transductants, the wild-type lambda integrates at its normal att lambda site with a simultaneous integration of defective lambda. It occurs by a cross-over thereby producing a double lysogen. Therefore a double lysogen involves both the transductants integrated together. The bacteria with a double lysogen is heterozygous, having a complete set of genes. Hence the outlooping and replication are well controlled. In type I transduction, the phage is a helper phage. It supplies all the genes or properties that are absent in the defective phage. Hence the helper phage helps the defective phage to multiply. The wild-type phage becomes a helper phage. This type of lysate containing defective and helper phages is known as high frequency transducing lysate (HFT).
Type II specialized transductants are stable. They never get switched over to the lytic cycle. It does not involve a double lysogen. Only the defective phage known as lambda d gal infects the cell. The gene present on the defective phage chromosome gets exchanged with that on the bacterial chromosome through a double crossover. Two crossovers occur in a chromosomal region. The type II specialized transductants are stable because lambda chromosome is unable to replicate since few genes are missing. Plus there is no helper phage to help it in replicating.
References:
[1] Microbial Genetics, Keya Chaudhari
[2] Molecular Genetics of Bacteria, Jeremy W. Dale, Simon F. Park
[3] Genetics, G. Ivor Hickey
[4] IGenetics, Peter Russell, second edition
[4] IGenetics, Peter Russell, second edition
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