Transduction: Generalized and Specialized

Transduction is the mechanism by which fragments of chromosomal DNA is introduced into a cell through the use of a virus, otherwise known as the bacteriophage. Transduction can be classified into two categories; generalised and specialised transduction. The difference between generalised and specialised transduction is that specialised transduction transfers a restricted set of bacterial genes.

Generalised transduction is the action where bacterial DNA is inserted into a capsid by chance and then relocated to another cell where recombination takes place. Generalized transduction is concentrated on in this experiment; the bacteriophage P1vir is used to transduce the E-coli K-12 as the recipient strain with a nagC-tetR mutation.

The process of generalised transduction begins when the donor cell undergoes the lytic cycle and becomes infected by a virus. Proteins in the bacteriophage tail adhere to a specific receptor on the surface of the donor cell, resulting in the phage injecting its DNA into the cytoplasm of the cell. Once the phages DNA is in the donor cell, the terminally redundant ends of the bacteriophage DNA undertake site-specific recombination resulting in the circulation of the genome.

The single genome is packaged into newly synthesised phage heads and reproduces numerous copies of the bacteriophage through the rolling circle mode of replication. Cleavage of the bacteriophage genome occurs at the Pac, a specific sequence on the bacterial DNA. A second cleavage occurs at a specific distance from the Pac to produce a molecule bigger than the genome. The enzyme Nuclease is expressed by the bacterial genome which cuts the DNA into random fragments. This is significant because the DNA fragments of the bacterial genome will have a similar shape to the bacteriophage genome and therefore the bacterial genome will get inserted into a capsid.

Enzymes that package bacteriophage DNA cannot differentiate between bacterial and phage DNA. Therefore, there are phage head capsids that contain bacterial DNA transducing particles which can result in defective bacteriophage or transducing particles. This is because transduction can only occur when bacterial DNA is packaged into the phage head. Capsids containing bacterial DNA become transducing particles and capsids with bacteriophage DNA become defective.

The bacterial cell is lysed to release the bacteriophage into the cytoplasm this happens when the phage proteins create holes in the plasma membrane and cell wall. Once this occurs, water diffuses into the cell causing it to become turgid and eventually burst. This releases the bacteriophage into the surroundings allowing it to infect other cells.

The linear bacterial DNA must experience two homologous recombination events with the host genome for transduction to occur two homologous recombination events between the host cell’s genome and the bacterial cell’s genome to produce a transductant. A section of the host cell’s DNA is replaced by the bacterial DNA, therefore altering the host cell’s genome so that the insertion of the bacterial DNA can be passed onto other cells.