In this article we will discuss about:- 1. Discovery of Cyanophages 2. Morphology of Cyanophages 3. Replication.
Discovery of Cyanophages:
Cyanophages are the phages (Fig. 13.14) that attack cyanobacteria. Cyanophages were first discovered by Safferman and Morris (1963) from a waste stabilization pond of Indiana University (USA). The first cyanophage studied by Safferman and Morris was the cyanophage attacking Lyngbya, Plectonema and Phormidium.
They named the virus as LPP-1 using the first letter of the three genera. Thereafter, several serological strains of LPP were isolated from different parts of world and named LPP-1, LPP-2, LPP-3, LPP-4 and LPP-5. Besides LPP groups of cyanophages, a large number of other cynophages such as SM-1, AS-1, N-1, C-1, AR-1. A-1, etc. have been reported in recent years (Table 13.1).
Waste stabilization ponds, eutrophic lakes and polluted water support the luxurient 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 that they are specific to genus and difficult to isolate.
Morphology of Cyanophages:
LPP group of cyanophages resemble T3 and T7 bacteriophages as they possess icosahedral head (580 Å diam.) and short (20 x 15 nm) tail. N-1 cyanophages resemble T2 and T4 phage because their head (550 Å diam.) is icosahedral but the tail is long (110 x 10 nm).
SM-1 cyanophages have tailless icosahedral head (880 Å diam.) whereas As-1 viruses posses hexagonal head (900 Å diam.) and long tail (243 x 22 nm). Like R-even phages, the tail may be contractile or non-contractile. AS-1 group has the largest cyanophages.
Replication of Cyanophages:
Cyanophages resemble T-even bacteriophages in their growth cycle. However, cyanophage LPP-1 is much more studied and our discussion on growth cycle is based on this virus. After the LPP-1 is adsorbed on the host cyanobacterium, the viral-DNA is injected into the host cytoplasm; the injection mechanism is still obscure.
Unlike bacteriophages, it does not takes over the complete charge of host cell machinery immediately; the depression of protein synthesis starts soon after the injection of viral DNA but its complete blockage occurs by the end of 5th hours.
The synthesis of viral DNA starts immediately after injection and continues till the lysis of host. The host DNA is not completely degraded (about 50% of it is converted to acid soluble material by the end of 7th hour of infection) and most of the degraded material is further incorporated into viral DNA.
Therefore, it is considered that LPP-1 does not completely arrest the host DNA synthesis. It has been established further that synthesis of viral DNA takes place in the virogenic stoma or invaginated photosynthetic lamellae. However, finally, complete virus-like structures consisting of head and tail are formed and they are released upon the lysis of the cyanobacterial cell.