Specialized transduction

Mozhan Haji GhaffariTeacher: Dr. Ashraf Hosseini

Transduction

• Transduction is the phage-mediated transfer of genetic material.

• There are two types of transduction:

1. Generalized transduction

2. Specialized transduction

• The key step in transduction is the packaging of DNA into the phage heads during lytic growth of the phage:

Often highly specific for phage DNA.

However, with some phages, errors can be made and fragments of bacterial DNA may be packaged with phage DNA.

This process is called Generalized Transduction

Since essentially any gene has an equal chance of being transduced.

Capable of infecting a recipient cell, since the information necessary for attachment and injection of DNA is carried by the proteins of the phage particle, irrespective of the nucleic acid it contains

Specialized Transduction

• Temperate phages lysogeny phase

Expression of phage genes and replication of the phage is repressed (prophage is inserted into the bacterial DNA )

• Lysogeny lytic cycle

Prophage excised from the chromosome by recombination between sequences at each end of the integrated prophage

Recombination in the wrong place

An adjacent region of bacterial DNA is incorporated into the phage DNA

Specialized (or restricted) transduction

Specialized transduction in Lambda phage

Lambda DNA is linear, but the two cos sites at terminal ends of the linear DNA join

Circular plasmid in the bacterial cytoplasm.

1. The lambda plasmid, creating about 100 copies of the virus using rolling-circle replication

2. Bactera Lyses under direction of the viral DNA

3. Releasing the copies of the virus into the environment.

4. This autonomous replication is refered to as the "lytic" cycle; it takes about 50 minutes for lambda, at 37°C.

1. Becomes covalently bonded to the bacterial DNA. (Lambda DNA so bonded is then called a prophage) at specific binding sites (attP phage

site lines up with attB bacterial site

2. Via Holliday structures, the Lambda DNA changes conformation into a supercoiled helix,

using gyrase and is integrated into the Bacterial DNA ( Lysogeny )

Holiday junction

• Lambda has three genes cI, cII, and cIII that are necessary for lysogenization (D. Kaiser, 1957)

• The prophage that is quiescent can be induced by UV radiation (deactivates CI) to enter the lytic cycle.

• CI represses the creation of cytoplasmic viral particles, simultaneously preventing reinfection by lambda

• A cI mutation was found that makes the repressor thermolabile

• Thus the prophage is quiescent but will lyse and enter into the productive state if the temperature is raised to a specific threshold

• UV radiation the less stable prophage may be excised from the bacterial chromosome

• The excised DNA of the prophage replicates, making multiple copies of the viral DNA, as well as capsids and tails, all assembled under a gene sequence into new lamba virus

• The prophage lyses from the bacterial DNA, may take along ADDITIONAL, nearby, neighboring parts of the bacterial DNA as a unit of transduction.

• These new sequences of bases replace parts of the lambda DNA.

• The excised portion of the lambda plus new Bacterial DNA must be able to fit into a capsid (either too small, or too large, and a defective virus may result).

• λdg was used to denote the case where there is "defective galactose" transduction.

Specialized transduction in Mu bacteriophage

• The name Mu is short for mutator (E. coli cells which carry this phage show an abnormally high rate of mutation):

1. The Mu virus injects its linear DNA into the bacterial host (E. coli) cytoplasm(DNA remains linear)

2. Integrates into the host DNA as a "prophage".

3. By replicative transposition, multiple copies of the Mu prophage are created in the host DNA (directly, not in the cytoplasm)!

4. Excision multiple copies of the Mu DNA are then assembled into capsids

5. The host cell bursts releasing the multiple copies of Mu virus into the environment to further infect new host bacteria.

• The excision of Mu DNA copies from the chromosome starting from a fixed point to the left of the Mu DNA insert (thus including some bacterial DNA).

• Creating a wide range of specialized transducing phages with Mu can be used both in genetic mapping and in mutagenesis.

References:

• Molecular Genetics of Bacteria 4th Edition Jeremy W. Dale Simon F. Park University of Surrey, UK

• An Introduction to Genetic Analysis. 7th edition.Griffiths AJF, Miller JH, Suzuki DT, et al.

• http://www.estherlederberg.com/Clark_MemorialEMZL/Transduction2%20EMZL.html

Thanks for your patience