ADVERTISEMENTS:
In this article we will discuss about the morphology and growth cycle of cyanophages.
Cyanophages are the viruses that attack on cyanobacteria i.e. members of the blue-green algae in general. For the first time Safferman and Morris (1963) isolated a virus from the waste stabilization pond of Indiana University (U.S.A) that attacked and destroyed the three genera: Lyngbya, Plectonema and Phormidium.
Therefore, they named the virus by using the first letter of the three genera as LPP. Thereafter, several serological strains of LPP were isolated and named as LPP-1, LPP-2, LPP-3, LPP-4 and LPP-5. The viruses are commonly called as blue-green algal viruses or cyanophages. They screened 78 host organisms and found the cyanophages only in 11 filamentous cyanobacteria.
After the discovery of LPP-1, a large number of cyanophages was discovered by the other workers including R.N. Singh and coworkers from Banaras Hindu University, India. Padan and Shilo (1973) reviewed different types of cyanophages.
Morphology of Cyanophages:
Morphology of LPP-1 has been studied in detail as compared to the other cyanophages. The cyanophages differ morphologically (Fig. 18.26) as well as in physico-chemical properties.
Sources of some of the cyanophages are given in Table 18.7. The LPP-1 group of cyanophages has an icosahedral head and a tail and are similar to T3 and T7 bacteriophages, whereas the N-1 group resembles with T2 and T4 phages.
ADVERTISEMENTS:
Like T-even phages the tail may be contractile or non-contractile. In some groups the tail is absent. The AS-1 group has the largest cyanophages. The group G-III and D-1 are serologically related but do not show any relationship with T-phages.
Growth Cycle of Cyanophages:
Like bacteriophages, the cyanophages too follow the same one-step growth curve. The salient features of a few important cyanophages are summarized in Table 18.8. The growth cycle resembles with that of T4 phages, however, the latent and rise period in different cyanophages differ. Replication of genetic material of cyanophages has been reviewed by Padan and Shilo (1973) and Sherman and Brown (1978).
LPP-1 is adsorbed on host surface and the DNA is injected into the host cell leaving the protein coat outside the cell wall. How does the DNA is injected, its mechanism is not known. However, soon after injection of the genome the rate of protein synthesis is reduced and gradually blocked at the end of 5 hour of injection. The phage multiplies in the invaginated photosynthetic lamellae or in virogenic stroma.
After injection the following three types of proteins are formed:
(i) The earliest proteins soon after injection upto 4th hour,
(ii) Earlier proteins after two hours of injection to completion of lysis,
(iii) Late proteins or structural proteins after 4th hour to host lysis.
After three hours of infection, degradation of the host DNA begins and by the end of 7th hour it is converted into acid soluble material. However, complete degradation of the host DNA does not occur. Sufficient amount of der-graded DNA material is used up in building of viral DNA. It has also been demonstrated that in virogenic stroma synthesis of viral DNA takes place.
ADVERTISEMENTS:
The latent period differs in different viruses, for example 7 hours in LPP-1 and N-1. Thereafter, there starts the rise period which also varies with the viral types. At the end, after maturation and assembly, the progeny cyanophages are released almost from each cell leaving aside the lysed cell.
The burst size for different cyanophages follows the following sequence: 350 plaques forming units (pfu) in LPP-1, 100 pfu in N-1, 50 pfu in AS- 1, 100 pfu in SM-1. The plaques can be observed either on algal lawn growing on nutrient agar or in broth cultures. After infections, several physiological processes are disturbed such as respiration, photosynthesis, host DNA metabolism and nitrogen fixation.
Waste stabilization ponds, eutrophic lakes and polluted water support the luxuriant growth of cyanobacteria. These can be obnoxious bloom in water reservoirs like lakes and result in fish mortality. Therefore, the cyanophages can play a significant role in control of blooms. So far the problems with them are that they are specific to genus and difficult to isolate.
