Lytic cycle

Figure 5.25 Two back-to-back promoters in the region of cl and cro control the genetic switch. When cl is present, it activates its own synthesis and blocks transcription of cro. When cl is inactivated, transcription of cro can occur, resulting in the lytic cycle. The cl (repressor) protein combines with the operator, 0 .

Of the 50 genes present in native X, only about half are necessary for replication in the lytic cycle. Thus, it is possible to delete about 1 / 3 of the genome to make room for more passenger DNA. However, to form mature phage particles the length of the DNA must be at least 75% of the native length. No more than 110% of the native amount may be present. The total DNA must fall between 38 and 53 kb in length. 

Cro Protein Prevents Buildup of cl Protein during the Lytic Cycle... 

About half of the time when A infects a cell it adopts a dormant lysogenic state in which the virus is linearly integrated into the host genome. This state is maintained by moderate amounts of the A-encoded cl repressor. The cl repressor prevents the lytic cycle from developing by inhibiting two promoters the PL promoter for early leftward transcription and PR the promoter for early rightward transcrip-... 

As the lytic cycle progresses, the phage DNA replicates. The increase in the number of gene copies can result in overexpression of the cl, which could shut down the lytic cycle. This effect is overcome by the regulatory protein cro, which is specifically designed to inhibit cl synthesis during late infection. 

The cro protein and the cl protein bind to exactly the same sites on the DNA. Despite this fact, the cl protein is required for lysogeny, whereas the cro protein is required for lysis. These requirements can be shown with mutants. A cl mutant invariably undergoes lysis, whereas a cro mutant can lysogenize but cannot complete the lytic cycle. This remarkable difference in the behavior of cro and cl results from the fact that although they bind to the same sites, they do so with totally different relative affinities (fig. 30.23). Cro binds preferentially to 0R3 and less strongly to 0R, and Or2. 

Segment of the A genome showing the three operators 0RI, 0R2, and OR3 around the Prm and the PR promoters. The cl and cro regulatory proteins bind to these operators with different relative affinities. The net result of these differing affinities is that cl is required for lysogeny and cro is required for the lytic cycle. 

Viruses borrow heavily on the host enzymatic machinery to obtain energy for synthesis, as well as for replication, transcription, and translation. The virus infective cycle is strongly irreversible. Virus infection is followed by the gradual turning on of viral genes. Viral enzymes are the first viral gene products in late infection, the virus structural proteins are favored. The irreversible lytic cycle of the virus is directed by a cascade of controls. 

Lysogenic virus. A virus that can adopt an inactive (lysogenic) state, in which it maintains its genome within a cell instead of entering the lytic cycle. The circumstances that determine whether a lysogenic (temperate) virus adopts an inactive state or an active lytic state are often subtle and depend on the physiological state of the infected cell. 

The human immunodeficiency virus (HIV) is a retrovirus. It infects (and normally kills) helper T lymphocytes that are a vital part of the immune system (see Topic Dl). This greatly impairs the immune system and may lead to acquired immune deficiency syndrome (AIDS). However, the HIV virus can also exist in a dormant state in T cells as a provirus until activated to enter the lytic cycle at a later time. The existence of this dormant state makes it difficult to design an effective strategy to overcome the HIV virus in an infected individual. 

FIGURE 16.5 Lytic cycle of bacteriophage X. D decoration protein pV tail protein V. 

Infection of a cell by a virus has two outcomes, depending on the nature of the virus and the particular cell type. Some viruses go into a lytic cycle, which results in the replication of the virus and the lysis of the cell. This is the case with viruses such as poliovirus and vaccinia virus, both of which are used in vaccination. The use of live viruses expressing protein antigens in vaccination has been discussed in Chapter 3. In vaccination, the goal is the transient replication of viruses for a period sufficient to raise antibody response. Thus, the use of lytic viruses leading to transient, self-limiting lytic infection is acceptable or even desirable. 

---