Restricted Transduction of Bacterial genome

 Restricted transduction is mediated by certain temperate bacteriophages, like the λ-phage (lambda phage) of E. coli, which enters into a lysogenic relationship with the host bacterium. This type of phages normally does not cause a lysis of the host, instead the phage DNA is integrated into the host chromosome where it is replicated as a part of the chromosome for many generations in the prophage state.

The release of the phage DNA from the host chromosome — known as excision — is, on rare occasions, imperfect, so that a region of host DNA situated on either side of the prophage DNA may be included into the excised phage DNA. When such a defective phage DNA is packaged into a phage head, the progeny phage on being released and on infecting another host cell transmits the bacterial DNA along with its own DNA into the infected host cell.

On integration with the host DNA, the phage DNA along with the portion of bacterial DNA carried by it becomes part of the host chromosome, thereby causing transduction. Since this type of bacteriophages can be integrated at only specific sites of the host chromosome, they can cause transfer only of specific segments of host DNA from one cell to another.

Hence, this type of transduction is designated as restricted or specialized to distinguish from generalized transduction. Restricted transduction occurs only at a low frequency, generally in the order of 10-6 to 10-7 which indicates that incorrect excision is a rare phenomenon.

The best known agent mediating restricted transduction is the λ -phage of E. coli. This phage contains a double-stranded linear DNA molecule as its genome in the virion and has 4,650 base-pairs with 12 base-pair long single-stranded cohesive ends. When the phage infects an E. coli cell, the injected linear DNA circularizes with the complimentary single stranded cohesive segments and this circular DNA is integrated into the E. coli chromosome having about 4700 x 103 base-pairs. Integration results in extending the length of the bacterial chromosome by about 1%.

The integration of the two circular DNA molecules, viz. λ-DNA and circular E. coli chromosome, occurs at specific sites of both DNAs and results in linear insertion of the λ-DNA into the circular chromosome. The attachment sites of λ-DNA and E. coli chromosome are known as POP’ and BOB’, respectively. During integration crossing-over takes place between these sites and the gene order of the λ-DNA is reversed (Fig. 9.102).

After integration, the λ-DNA remains as a prophage in the host cell which is then called a lysogen. In the lysogen, the λ-genes are kept in a repressed state by a repressor produced by a λ-gene which is the only viral gene which remains functional in the lysogenic condition.

The lysogenic host can grow indefinitely without the expression of the other λ-genes. However, the phage genes can be occasionally activated either spontaneously or by external agents, like UV-light — through a process known as induction. On induction, the phage DNA is excised from the bacterial chromosome again in a circular form by reversing the process of integration. While integration is catalysed by the enzyme integrase, excision is catalysed by excisionase. Excisionase binds to the integrase making it to reverse the process.

As a consequence, the original attachment sequences, POP’ and BOB’ are recreated and the λ-DNA is released from the chromosome in its circular form. Although excision is by and large accurate, on rare occasions in one cell per million or ten million (10-6 to 10-7) it may be inaccurate.

When such an event takes place, either the gal locus or the bio locus of E. coli chromosome may be included into the λ-DNA. Many of these inaccurate excisions produce DNA segments which are either too large or too small to be packaged into λ-head and they are lost. But aberrant excisions sometimes produce a DNA segment of correct length to fit in the phage head. The phage, progeny particles in such case contain either gal or bio loci in their heads along with λ-DNA.

However, these phage particles are often defective, because some parts of phage DNA may not be included into the DNA segment packaged into their heads. These gal or bio transducing λ-phages are unable to complete their life-cycle by themselves, because they are unable to perform certain functions controlled by the missing phage genes.

With the help of normal λ-phages, the specialized (restricted) defective gal or bio transducing phages, can grow in a host bacterium and integrate the gal or bio loci into its chromosome causing transduction of these genes. Aberrant excision producing λ-gal and λ-bio transducing phages and restricted transduction are diagrammatically shown in Fig. 9.103.