chromosome of its host's genome. Each time the host chromosome is duplicated, so is the integrated viral DNA. In many cases these viruses express genes that keep the viral DNA dormant; that is, the virus does not immediately replicate.

The best characterized model for a specialized transducing virus is phage Lambda of Escherichia coli. The Lambda virus begins its life cycle in much the same way as P22. A tail fiber on the end of the virus specifically binds to a receptor, called the maltose binding protein, on the surface of the E. coli cell. The viral DNA is then injected into the host cell. On the chromosome of the virus is a section of DNA that is almost identical to a DNA sequence found on the bacterial chromosome. The recombination enzymes of the E. coli host break and rejoin the viral DNA and host DNA together at this site, thus integrating the virus genome into the chromosome of the host bacterium.

A viral gene, the repressor gene, is then expressed and keeps the virus from activating its own replication. This integrated virus, called a prophage, can be maintained stably as a part of the host chromosome as long as the host cell remains healthy. If the host cell becomes damaged, enzymes are activated that destroy the Lambda repressor protein. Without this protein, the viral DNA will break out of the host chromosome and begin to replicate itself, much as a lytic virus does. Ultimately, the virus particles are packaged, released, and move on to infect a new host cell.

Specialized transduction occurs when the enzyme that cuts the viral DNA out of the host chromosome makes a mistake and cuts in the wrong place, removing some, but not all, of the viral genes. Since Lambda cap-sids fill by a "headful" mechanism, small bits of the host chromosome are packaged along with part of the viral genes. These viral particles are called defective particles because some of the viral genes are missing in the package and thus, when the virus infects a new host cell, not all the genes needed for viral replication are present. Lacking the ability to replicate, the virus cannot kill its new host. Because of this mechanism of viral packaging, specialized transducing viruses can pick up genes only on either side of the site where the virus integrates into the bacterial chromosome. Thus, while generalized transducing viruses can move any genes, specialized tran-ducing viruses move only specific genes.

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