Viruses temperate

Viruses are discussed more fully elsewhere (Chapter 3). However, there are certain groups of viruses, called bacteriophages (phages), which can attack bacteria. This attack involves the injechon of viral DNA into baeterial eells which then proceed to make new virus particles and destroy eells. Some viruses, known as temperate viruses, do not cause this catastrophic event when they infect their host, but can pass genetic material from one cell to another. 

The temperate virus does not exist in its mature, infectious state inside the cell, but rather in a latent form, called the provirus or prophage state. In considering virulent viruses we learned that the DNA of the virulent virus contains information for the synthesis of a number of enzymes and other proteins essential to virus reproduction. The prophage of the temperate virus carries similar information, but in the lysogenic cell this information remains dormant because the expression of the virus genes is blocked through the action of a specific repressor coded for by the virus. As a result of a genetic switch, the repressor is inactivated, virus reproduction occurs, the cell lyses, and virus particles are released. 

A lysogenic culture can be treated so that most or all of the cells produce virus and lyse. Such treatment, called induction, usually involves the use of agents such as ultraviolet radiation, nitrogen mustards, or X rays, known to damage DNA and activate the SOS system. However, not all prophages are inducible in some temperate viruses, prophage expression occurs only by natural events. 

Figure 5.21 Consequences of infection by a temperate bacteriophage. The alternatives upon infection are integration of the virus DNA into the host DNA (lysogenization) or replication and release of mature virus (lysis). The lysogenic cell can also be induced to produce mature virus and lyse.

Consequences of temperate virus infection What happens when a temperate virus infects a nonlysogenic organism The virus may... 

Viruses may also cause latent infection of a host. In a latent infection, there is a delay between infection by the virus and the appearance of symptoms. Fever blisters (cold sores), caused by the herpes simplex virus, result from a latent viral infection the symptoms reappear sporadically as the virus emerges from latency. The latent stage in viral infection of an animal cell is generally not due to the integration of the viral genome into the genome of the animal cell, as is the case with latent infections by temperate bacteriophages. 

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 added attraction of the transition from myoblast to myotube is the synchrony with which differentiation occurs in vitro. Myogen-esis will occur in primary cultures of skeletal muscle (e.g. 6.12) but can also be induced in diploid myoblast lines (Richter and Yaffe, 1970) which has allowed the selection of mutants (Chapter 13) that exhibit drug resistance or temperature-sensitive differentiation (Loomis et al., 1973 Somers et al., 1975). Holtzer et al. (1975) and Fiszman and Fuchs (1975) have developed a myoblast line transformed with a temperature-sensitive virus. At the permissive temper-... 

The theoretical models considering the influence of (algal) viruses on the carbon cycle that exist to date are steady-state models assuming a fixed percentage of the algal population dying due to viral lysis. A bloom of Phaeocystis in, for example, temperate eutrophic coastal waters is, however, clearly not a steady-state situation. Based on the ecosystem model by Ruardij et al. (2005), we established a carbon budget for the main players... 

Baudoux A-C, Noordeloos AAM, Veldhuis MJW, Brussaard CPD (2006) Virally induced mortality of Phaeocystis globosa during a spring bloom in temperate coastal waters. Aquat Microb Ecol 44 201-217 Bergh 0, Bprsheim KY, Bratbak G, Heldal M (1989) High abundance of viruses found in aquatic environments. Nature 340 467 168... 

West Nile (WN) virus has emerged in recent years in temperate regions of Europe and North America, presenting a threat to public, equine, and other animal health. The most serious manifestation of WN virus infection is fatal encephalitis (inflammation of the brain)... 

Mansonia mosquitos as well as some ticks, with birds as intermediate hosts (see Table 23.7—Clinical Profile of West Nile Fever). Historically, WNV was endemic to Africa, West Asia, and the Middle East. Scientists from the CDC believe the virus migrated to the eastern United States in the summer of 1999 or possibly earlier. The continued expansion of the virus in the United States indicates that it is permanently established in the Western Hemisphere (refer to Figure 23.7—2005 West Nile virus activity in the United States.). In the temperate zones, cases occur primarily in the late summer or early fall, whereas in the southern climates the virus can be transmitted year round (CDC, 2005e). Birds, mosquitoes, and equines serve as sentinel animals that could alert health officials to the occurrence of human disease. 

In temperate climates, natnrally occnrring smallpox had a seasonal incidence similar to that of chickenpox and measles, with more cases occurring in the late winter and spring. Most likely, the association of increased survival of the virus at lower temperature and hnmidity contributed to the seasonal pattern. In tropical areas, there was less seasonal variation and the illness occurred year-round (25). 

Verbascum thapsus L. (whole plant) Amentoflavone (1). Used in temperate Himalaya for the treatment of asthma and other lung eomplaints. Antiviral activity against herpes virus type 1. Hussain et al., 2009[327]. 

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