Lecture Notes: Transformation and Transduction



Lecture Notes: Temperate phages and Transduction

Evolution of bacteria is significantly affected by the ability of the bacteria to exchange genetic information in the absence of sexual reproduction. Genetic material can move from one cell to another via several different mechanisms, either in the form of naked DNA or DNA encapsidated in a virus particle.

One way that is increasingly being recognized as an important mechanism by which bacteria acquire new traits is by the formation of lysogens of temperate bacteriophage (Fig. 10.16). One important example of a temperate phage is lambda. Temperate phages make a choice upon infecting a host cell: lysis or lysogeny. If lysis is chosen, lytic infection genes (e.g., those involved in DNA replication and capsid production) are expressed, as in any normal lytic infection cycle. However, if there is significant expression of a repressor for these lytic genes, the phage DNA might integrate into the host chromosome (forming a lysogen with an integrated prophage). Once a temperate phage has become integrated into the genome of the host cell, there will be a few prophage genes expressed in the host cell. The expression of prophage genes can give the host cell new characteristics, including immunity to reinection by the same phage, or toxin production. The ability of a prophage to give a host cell new characteristics is called phage conversion.

Temperate bacteriophages are defined as those with the ability to infect a host cell and then follow a lytic or lysogenic path. (They all contain DNA genomes.)

Bacteriophages can also mediate the transfer of chromosomal DNA from one bacterial cell to another via transduction (two distinct types, as described below).

TRANSDUCTION- results from an error in phage DNA replication or phage DNA packaging.

Specialized transduction: (Fig. 11.18) formation of this type of phage particle occurs when a temperate phage like lambda excises incorrectly from the prophage state. If some host DNA accompanies the replication phage DNA during excision, the host and phage DNA enter a lytic replication cycle. Ultimately, these DNA molecules are packaged into phage capsids, and can be injected into the next susceptible host cell, where any chromosomal DNA from the previous (now dead) host may recombine with the chromosome of the newly infected host. Generation of specialized transducing particle is the result of an incorrect excision of prophage. In a specialized transducing phage, only chromosomal DNA adjacent to the original prophage attachment site can be transduced.

Generalized transduction Examples: phage P22, host Salmonella typhimurium or P1 from E. coli. This can occur as a result of an error in packaging phage DNA into capsid structures. For certain phages that use "headful" mechanism a for packaging their DNA into the capsid, occasionally a section of the host DNA can be packaged rather than the phage DNA (Fig. 11.17). Any portion of the chromosome can be packaged into the phage capsids as long as it is of the correct size. Generalized transducing phage particles are not capable of lytic phage infection.

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