Tuesday, 9 February 2021

Transduction of Bacteria

Transduction is a Horizontal gene transfer event in which bacteriophages function as a Vactor/Vehical to transfer DNA from donor bacteria to recipient bacteria.

Transfer of bacterial genes by bacteriophagephage (Transduction) was discovered by Zinder & Lederberg in 1951 in bacterium Salmonella-  typhimurium.

Transduction is of two types :
1) Generalized Transduction
2) Specialized Transduction.

The bacteriophages containing the bacterial DNA of donor is called transducing phage.

Bacteriophage :

Bacteriophage is a virus that infect and replicate within a bacterium.
Bacteriophages has two types :
1) Virulent bacteriophage
2) Temperate bacteriophage.

• Virulent bacteriophage follow lytic cycle and they are capable of causing bacterial infection and eventually destruction and death of bacterial cell. E.g. T4 phage
Temperate bacteriophage does not cause disruptive infection, instead phage DNA incorporated into bacterial DNA and replicate lysogenic cycle and after some time become virulent cause lysis of bacterial cell. E.g. λ phage.

* Generalized Transduction :

In Generalized Transduction a DNA fragment is transferred from one bacterium to another by a lytic phage which carry donor bacterial DNA due to an error.
Virulent phage act as vehicle for transduction.

Generalized TransductionSteps

Steps of Generalized Transduction :

1). A lytic bacteriophage infect a susceptible bacterium.
2) when phage genome enters the bacterium it cause degradation of bacterial host DNA into fragments.
3). Phage genome replicate using host replication machinary and synthesize enzymes and coat protein.
4). During maturation and packaging of virus particles, few phage heads may envelope fragments of bacterial DNA by error that is double stranded. So bacterial (host) DNA is present in the transducing phage.
5). Transducing phage carrying bacterial(host) DNA or donor will infect another cell and transfer the donor DNA to the recipient bacterial cell.
6). When bacterial DNA of donor is introduced into the bacterial Chromosome of host and transfer several bacterial gene at one time forming recombinant DNA.
7). Bacteria(recipient) multiply with new genetic material.

Due to the phage gene are very small only genes that are located close together will be transduced together
Typical example of generalized transduction include P1 in E.coli and P22 in Salmonella sp.

Generalized transduction may be abortive or complete

Aborative transduction :

- The transient expression of one or more donor without formation of recombinant progeny
- The donor DNA fragment exogenate does not integrate with endogenate and also not replicate.
- Only one bacterium contain exogenate and other bacterium is like parent among the progeny.

Complete transduction :

- Production of stable recipient recombinants that inherits donor genes and retain the ability to express them.

The frequency of abortive transduction is greater than complete transduction.

* Specialised transduction :

- A DNA fragment is transferred from one bacterium to another by a temperate bacteriophage (lysogenic), Which carry donor DNA along with phage genome to an error.
- In specialised transduction phage insert donor genome at specific site and perform site specific recombination.
- Specialised transduction only occurs when lysogenic (temperate) infected donor bacteria enter into lytic cycle and release phage progeny.

Specialised Transduction

- The only bacterial genes that can be transduced are very near to the site at which pro phage is integrated.
E.g. the only site at which λ phage integrated is between genes for galactose fragmentation (gal gene) and biotin (bio)synthesis gene. So at the time of abnormal integration of prophase only gal or bio gene could be transduced.
Specialised transduction always occur by aberrant excision of λ lysogen

Steps for specialised transduction :

- A temperate phase infect a susceptible donor bacterium and it's DNA into its genome, it called pro-phage.
- When bacteriophage enters lytic cycle, occasionally during spontaneous induction a small piece of donor bacterial DNA is picked up during detachment as a part of phage genome.
- When the λ Phage dettaches  from bacterial DNA it contains some non essential parts of bacterial DNA which is called defective phage.

λ d-gel defective phage lack phage gene required to grow in lytic cycle but pro phase that still contain head/tail gene help in its packing for lysis.
  When this defective phage infects recipient cell(bacteria), due to lack of genes it could not transduced because lack of essential genes.
  So specialised transduction occur if a recipient bacteria is double infected with a wild type λ phase or λ d-gal. wild type phage supply the functions missing in defective phase and progeny will contain about numbers of both type.

1) Low frequency transducing lysate -

It forms when initial abberent excision of lysogen yield less than 1 phage per 1,000,000 wild type.

2) High frequency transducing lysate -

It forms by combination of λ d-gal and λ phase coinfect and it results into dilysogen.

Co transduction :

During transduction, only a specific size of DNA segment can be packaged by bacteriophagephage. If two genes are close along the chromosome, a phase may package a single piece of chromosome that carries both genes and transfer that piece to another bacterium. This is called co-transduction.

Frequency of Co-transduction is inversely proportional to distance between 2 genes.
Formula : C=(1-d/L)³ 
C= Frequency,
d= distance in minutes
L= size of transducing fragment in minutes.


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