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Transduction

Transduction is a process of transfer of bacterial genes through bacteriophages.Discovery

In 1951, Joshua Lederberg and Norton Zinder were repeating the experiments of

Lederberg and Tatum (1946), i.e. conjugation experiment in E.coli k12. They were

working on Salmonella typhimurium using the same technique-U arm tube withmembrane filter. The intent of their study was to confirm conjugation in Salmonella.

However, they happen to discover transduction in bacteria, a new means of

recombination in bacteria.

Lederberg & Zinder experiment

Transduction was first discovered in 1952 by Joshua Lederberg and Norton

Zinder Mixed two strains of bacteria

LA22 strain: phe- trp- met+ his+

LA2 strain :phe+ trp+ met- his-

Plated mixture on minimal media Media lacked these four amino acids

1 cell in 100,000 grew Genotypephe+ trp+ met+ his+

Genetic material had been transferred


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The researchers have found that when two strains were mixed, prototrophs were obtained

at a frequency 10-5. The prototrophs could synthesize all amino acids(phe+trp+met+his+). This demonstrated genetic recombination in S. typhimurium

conjugation.Confirmation that recombination was not through conjugation but through other

mode, transduction

In order to confirm that recombination occurred via process other than conjugation, the

researchers have prevented cell contact by using a filer between the two arm.

Each strain was added in U arm tube. The two arms of the U tube were separatedby a bacteria proof sintered glass filter, which will prevent contact between two bacteria

but will allow free movement of medium. The suction was applied so that culture

medium can pass from one arm to other. Thereafter, fluid was drawn from both arm and

plated on M.M. media. The prototrophs were recovered from fluid drawn from the armwhere LA22 culture was placed but not from the fluid drawn from arm where LA2

culture was placed. From LA2 culture, a filterable agent was isolated which was resistantto DNase. This later was identified as P22 virus. The filterable agent was sensitive to

antiserum of P22. In the presence of antiserum, no prototrophs were recovered from

strain LA22. This confirms the involvement of virus mediated gene transfer from one

strain of bacteria to the other strain of bacteria. Transduction is usually intragenic-involving closely related strains of bacteria but intergeneric transduction have also been

reported, i.e. E. coli and Salmonella, E. coli and Shigella.

Types of Ttransduction

1. Generalized Transduction

2. Specialized Transduction

1. Generalized Transduction: Transduction in which potentially any donor bacterial

gene can be transferred. It is shown by virulent phages or lysogenic phages during itslytic conversion where it picks a DNA fragment from a bacterium transfer it to

another bacterium during infection by the phage. During packaging instead of phage

DNA to be packaged, fragment of bacterial DNA is packaged inside phage head. P22

phages can package 1% of bacterial chromosome. The extent of packaging of bacterial DNA depends upon the phage head. If it is of larger size, it can

accommodate more bacterial genome. If the phage head is small only small portion of

bacterial chromosome can be packaged. The frequency of transduction ranged from10-5 to 10-7.

Steps in Generalized Transduction

1. Infection of Donor:

2. Phage replication and degradation of host DNA


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3. Assembly of phages particles: During a LYTIC infection, a transducing phage,such as P1 infecting E. coli, accidentally packages a piece of the bacterial

chromosome into a virus particle instead of its own viral DNA.

4. Release of phage

5. Infection of recipient:The phage carrying the bacterial DNA then delivers it to therecipient cell when it tried to infect again6.

7. Legitimate recombination:The injected bacterial DNA may then be inserted intorecipient chromosome by homologous recombination

Fig. CompleteTransduction by

transducing phage P1


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Abortive Transduction: In abortive transduction, a segment of bacterial DNA

(exogenote) is introduced into another bacterium by a bacteriophage. When the

exogenote is integrated into the recipient bacteria, this is called complete

transduction. But when the exogenote is not integrated into endogenote and remains

free, it is called abortive transduction. When the abortive transductant divide, out of

the two-only one daughter cells contain exogenote. The other cell do not containexogenote. As the time progresses, the exogenote is progressively diluted out through

subsequent cell divisions. This type of transduction is known as abortivetransduction.


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Normal Events Leading to LysogenyThe steps leading to lysogeny under normal circumstances follows as:


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Circularizati

Cohesive en

Site-specific

Site-specificrecombination


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Term

Induction


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Specialized Transduction-error in lysogenic cycle.

Error in

Excision of the


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During specialized transduction, only specific gene from donor bacterium is transferred

to the recipient bacterium through phages. Unlike random packaging of bacterial DNA,this form of gene exchange results from imprecise excision of an integrated phage

(prophage).

phage iphage i

recombinrecombin


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biobio oror galgalii

Bal


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Due to imprecise excision, lemda bio or lemda gal transducing

phage Phages that carry a very small part of the adjacent flanking bacterialgenome, either to the left or right of the att site. These phage canrecombine into the genome via phage mediated integration or

homologous recombination.

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