Phage Biology

Phage Biology

- Phage infection can have two possible outcomes--a lytic cycle of growth, comparable to that of phage T4, or lysogenization, in which the viral chromosome circularizes and undergoes site-specific integration into the host-cell chromosome (Figure 25.17). In the lysogenic state, the phage chromosome is maintained in the host genome with repression of nearly all viral genes. This is accomplished by the binding of a encoded repressor to two operators in . Phage mutants defective in establishing or maintaining lysogeny have phenotypes comparable to those defective in lac regulation. The major phenotypes and comparisons to lac operator mutations are summarized in Table 26.2.

The transcriptional repression can be broken, however, leading to excision of the viral chromosome from the host genome as a circular DNA, followed by replication of viral DNA, followed by activation of genes needed to assemble virus particles.

The virus must rely on the following four distinct patterns of gene expression needed for its four physiological states:

1. Infection leading to lytic growth;

2. Infection leading to the establishment of lysogeny,

3. Long-term maintenance of lysogeny, and

4. The breaking of lysogeny with subsequent lytic growth.