Transduction..

Transduction is a mode of transfer of the genetic material from one bacteria to another by a bacteriophage (virus infecting bacteria). Other modes of transfer of genetic material in bacteria are conjugation and transformation.

In the process of tranduction, a phage infects a bacteria and its progeny phage(s) may pick up few of the genes from host bacterial genome during erroneous packaging. The cell from which the genes are picked up is called a ‘donor‘ cell (see Fig 1: steps 1 to 3) .

The progeny phage carrying donor genes, is released from the donor bacterial cell after lysis. This abnormal phage goes on to infect another bacterial cell, called the ‘recipient‘ cell. The phage injects the donor genes into the recipient cell (see Fig 1: steps 4 & 5). There could be a homologous recombination and recipient cell becomes recombinant (see Fig 1;step 6).

• Discovery of Transduction:

This mode of genetic material transfer was first observed by Joshua Lederberg and Norton Zinder in 1951. They were carrying out a recombination (conjugation like) experiment in the bacterium Salmonella typhimurium. The strain they chose were phe− trp− tyr− and met− his−. They found that mixing the two strain resulted in the wild-type bacteria (the recombinants).

The recombinants were also recovered from a U-tube experiment in which the two strains were separated by a filter separating the two arms (see fig 2). The filter pores were small enough to not allow the bacterial cells to pass through. Hence the two strains could not come in direct cell to cell contact. It was therefore clear, that the recombination was not due to conjugation but some ‘filtrable agent‘.

Fig 2: Representation of U tube with filter

In further tests, filtrable agent was found to exhibit many chemical, physical, and genetic properties, including adsorption specificity, time course of production, filtration endpoint, resistance to solvents, proteases, RNase and DNase and inactivation by formalin, similar to those of bacteriophage P22. Thus, the process of transduction was discovered.

(Just for info: read more on the Discovery of transduction)

• Types of Transduction:

There are two types of transduction: generalized transduction and specialized transduction.

1. Generalized Transduction:

Generalized tranduction is the type of transduction in which any random DNA fragment can be transferred from one bacteria to another by the phage. There is no specific part of DNA that is transduced.

Mechanism:

Usually during a lytic cycle, after the injection of the virulent phage DNA into the bacterial cell, the bacterial chromosome is cleaved.

The cleaved fragments of the host genome, with size similar to viral DNA is sometimes erroneously packaged into head of the progeny phage. Any part of bacterial genome can be packaged into the phage head. These phages carry only bacterial DNA. Such non-functional phages on infecting a bacteria do not replicate. There could be a homologous recombination between the donor DNA fragment and the recipient DNA region, resulting into a recombinant bacteria (as in Fig 1).

(Just for info: Know more about generalised transduction)

2. Specialized transduction:

In specialized transduction, bacteriophage transfer only a specific region of the host (donor) genome to recipient bacteria.

Mechanism:

Specialized transduction occurs when a temperate bacteriophage is involved. As seen in a previous post, temperate phage can reproduce through either lysogenic or lytic cycle. During the lysogenic cycle, the temperate phage DNA integrates into the host genome (here the phage DNA is called prophage) (see fig 4).

In some temperate phages the integration is very specific and occurs at a particular region. For example; λ always integrates between the gal region and the bio region of the host chromosome (see fig 5).

Fig 5: λ integrates between gal and bio region

We also saw in the previous post, that the temperate phage initiates lytic cycle after induction. During induction the phage DNA gets excised from the host genome.

Sometimes there occurs incorrect excision of the prophage, wherein it carries the adjacent bacterial genes along with the viral DNA (see Fig 6). In case of specialized transduction, the abnormal phage is packed with viral as well as the bacterial DNA.

During infection, the abnormal progeny phage transfers these adjacent genes to the next bacterial cell (recipient cell). This transduction is specific in that the genes transduced are the neighbouring ones.

Hence λ can transduce only the gal or bio genes.

(Just for info: Read more about transduction from the discoverer Norton Zinder’s perspective)

• Uses of transduction:

The transduction is useful in the molecular biology studies in bacteria. It is particularly useful in genetic mapping, site directed mutagenesis and strain construction. The small size of a transduced fragment allows recombination of short region of interest.

(Just for info: Read this paper where the researchers use of bacteriophage P1 as a vector for transposon insertion mutagenesis)

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Also read other posts by The Biotech Notes:

High Pressure Liquid Chromatgraphy (HPLC).

Detection and Measurement of Radioactivity: Geiger Muller Counter and more.

Chromosome Banding..

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