The chromosomes of bacteriophages, which are viruses that specifically infect bacteria, can also be used as vectors. These are linear double-stranded DNA molecules which contain the genes required for viral replication within the host cell. During infection the DNA molecules are inserted into the bacterial host. The most widely used bacteriophage is lambda (X) which infects E. coli. Within the E. coli lambda has two alternative modes of replication: the lytic and the lysogenic cycles. In the lytic cycle the DNA is incorporated into the E. coli chromosome. Replication takes place, resulting in the production of a large number of mature phage particles, which are released from the cell by cell lysis. In the lysogenic mode the lambda DNA is incorporated into the chromosomal DNA in such a way that replication of the lambda DNA occurs but it is not packaged into mature particles and there is no cell lysis. Instead the lambda genome is inherited by each daughter bacterial cell at cell division. Lambda is maintained in this prophage state through the action of a repressor protein, cl. If the repressor protein is inactivated the lambda DNA is excised from the host chromosome and a lytic cycle is activated.
In one mutated form of lambda the repressor protein is inactive at temperatures above 37 °C but active at lower temperatures. When this lambda is used as a vector the infected E. coli are grown first at 32 °C to allow replication of the DNA in the lysogenic cycle. The temperature is then increased to 37 °C to inactivate the repressor. This results in the excision of the lambda genome and release of lambda particles by lysis.
Another viral chromosome that can be used as a vector is that of the filamentous phage Ml3. The M13 chromosome is a single-stranded DNA molecule which when inserted into the bacterial host replicates outside the bacterial chromosome in the cytoplasm. The virus is then reassembled and released from the bacterial cell without cell lysis.
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