- Specialized transduction can be distinguished from generalized transduction by two properties.
- Only bacterial genes close to the attachment site of pro-phage can be transduced in this type.
- It also carries both bacterial genes and phage genes instead of only bacterial genes, like in generalized transduction.
- So, in specialized transduction certain temperate phage strains can transfer only a few restricted genes of the bacterial chromosome.
- For this reason, it is also called restricted transduction.
- It occurs when a bacteriophage genome, after becoming integrated as pro-phage in the DNA of host bacterium, again becomes free of induction and takes with it into the phage head a small adjacent piece of the bacterial chromosome.
- In this way when a phage infects a cell, it carries with it the group of bacterial genes that has become a part of it.
- Such genes can be recombined with the homologous DNA of infected cell.
- Phage lamda (λ) of E. coli is the first and the best studied specialized transducing phage.
- The location of λ pro-phase in the bacterial chromosome is almost always between the bacterial genes ‘gal’ and ‘bio’.
- Whenever the phage genome comes out of it, or is excised from the bacterial chromosome, it sometimes takes with it ‘gal’ or ‘bio’ genes.
- When the phage carrying the ‘gal’ or ‘bio’ genes infect a new host, recombination with the ‘gal’ or ‘bio’ genes of the host can occur.
- In a λ lysogen of E. coli, the λ pro-phage is integrated close to and between ‘gal’ and ‘bio’ genes in the chromosome.
- The ‘gal’ gene products degrade galactose for use as a carbon and energy source and ‘bio’ gene product make the vitamin biotin.
- Specialized transduction can occur when a phage picks up neighbouring bacterial genes during induction of pro-phages.
- Specialized transducing phage carrying the gal genes, called λ gal, forms as the result of mistake during excision recombination.
- Recombination sometimes occurs by mistake between the pro-phage DNA and a neighbouring site in the bacterial DNAs.
- The DNA later packaged into head will include some bacterial sequences.
- Such transducing phages are very rare, because the erroneous recombination that gives rise to them is extremely infrequent.
- Furthermore, the recombination must by chance, occur between two sites that are approximately a λ genome length apart or the DNA wouldn’t fit into phage head.
- In contrast to generalized transduction, specialized transduction can allow extremely efficient DNA transfer while allowing a small region of bacterial chromosome to be selectively transduced.
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- Because of the rarity of this transducing phage, powerful selection techniques are required to detect them.
- To select λ phage carrying ‘gal’ genes of the host, induced phase are used to infect Gal– recipient bacteria and Gal+ transductants are selected on plates with galactose as sole carbon source.
- In the rare Gal+ transductants, a λ phage carrying gal genes may have integrated into the chromosome, providing the gal gene product that mutant lacks.
- If such a Gal+ lysogen is colony purified and the pro-phage is induced from it, all of resultant phage progeny will carry the ‘gal’ genes.
- Such a lysogenic strain produces phage that can transduce bacterial genes at a very high frequency.
- Because the λ phage head can hold DNA of only a certain length, the transducing particles of necessity have lost some phage genes to make room for the bacterial genes.
- The properties of transducing particles will be determined by lost phage genes.
- E.g., λ dgal phage lacking head and tail genes cannot multiply without λ helper phage.
- Transducing phage in which the ‘int’ and ‘xis’ genes have been replaced by ‘bio’ genes can multiply but cannot form a lysogen without the help of wild type phage.
- Specialized transducing phage particles played a major role in first isolation of genes and the discovery of IS elements in bacteria.
- They can also be used to map phage genes.
Specialized transduction and its process