Cyanophages are viruses that exclusively infect and target cyanobacteria, commonly known as blue-green algae.
Krauss (1961) was the first to propose that the disappearance of blue-green algae might be caused by algal viruses. Earlier, Lewin (1960) identified a phage that attacked the colorless blue-green alga Spirochaeta rosea.
In 1963, Safferman and Morris successfully isolated a virus from a waste stabilization pond at Indiana University, USA.
This virus was found to infect and destroy three genera of cyanobacteria: Lyngbya, Plectonema, and Phormidium.
They named the virus “LPP” after the initials of these three genera. Subsequently, several serological strains of LPP were identified and designated as LPP-1, LPP-2, LPP-3, LPP-4, and LPP-5.
Following the discovery of LPP-1, numerous cyanophages were isolated and studied by various phycologists.
Nomenclature
Regarding the nomenclature of Cyanophages, they are named based on their host organisms. The naming convention involves using the first letters of the hosts’ generic names, followed by Arabic numerals that indicate serological sub-groups.
For example, in SM-1, “SM” represents the hosts (Synechococcus elongatus and Microcystis aeruginosa), while “-1” denotes the serological subgroup (strain NRC-1).
Structure and Composition of Cyanophages
- Cyanophages are composed of nucleic acids and proteins.
- Their genetic material is a linear, double-stranded DNA approximately 45 kbp in length, with a size ranging from 13.2 nm to 24.3 nm.
- This genetic material is enclosed within a protective protein coat called a capsid.
- Similar to other bacteriophages, cyanophages have a protein-based head and tail structure.
- The molecular weight of the primary head proteins ranges from 13,000 to 39,000, while the major tail protein has a molecular weight of approximately 80,000.
- Among them, morphology of LPP-1 has been the most extensively studied. Lufting and Haselkorn proposed a structural model for LPP-1, describing its head as a polyhedron with 20 facets, exhibiting icosahedral symmetry and a diameter of 600±20 Å.
- The genetic material, always double-stranded DNA, is enclosed within the head, which is covered by a double membranous coat.
- The tail is a short, hollow, cylindrical structure, measuring 20–25 nm (200±25 Å) in length and 15 nm (150 Å) in width.
- It displays sixfold radial symmetry and consists of 2–4 rings made up of six subunits each.
- The upper end of the tail is attached to the protein head, while tail fibers likely extend from the lower end. The tail is non-contractile and features an outer tail sheath and an inner tail cavity.
- At the junction of the head and tail lies the tail capital, which is presumed to serve as a joint, anchoring the tail to the head, though its exact function remains uncertain.
Growth Phases (Cycle) of Cyanophages
The growth cycle of Cyanophages comprises four distinct stages that are – attachment, penetration, biosynthesis, maturation and release.
- Attachment and Penetration:
- They (LPP-1) attaches to a complementary receptor site on the cyanobacterial cell via a chemical interaction.
- The tail plays a critical role in this process, with the tail fibers facilitating attachment.
- When the tail fibers bind to the cell receptor, they trigger conformational changes in the multiprotein base plate of the tail.
- This leads to the contraction of the tail sheath, enabling the transfer of phage DNA from the capsid into the host cell through the tail cavity.
- Biosynthesis:
- Once inside the host cell, the cyanophage uses the host’s nucleotides and enzymes to replicate its DNA. Following this, the synthesis of viral proteins begins.
- A key early sign of infection is the displacement of photosynthetic lamellae to one side of the cell. In the initial stages, small particles, likely α-particles, appear between the lamellae.
- As the infection progresses, the virogenic stroma becomes filled with developing virus particles.
- During the early phase of infection, no complete cyanophages are detected; only separate components, such as DNA and proteins, are present.
- Maturation:
- The viral DNA migrates to the space between the photosynthetic lamellae, where DNA helices start to form.
- These helices are subsequently transported to the virogenic stroma, where they undergo final assembly, with a protein coat forming around the DNA helices to create complete viral particles.
- Release:
- The mature cyanophages are released when the cyanobacterial cell undergoes lysis.
- This release is essential for dispersing the new cyanophages into the environment, allowing them to infect other host cells and complete their life cycle.
Significance and Importance of Cyanophages
- Tracing evolutionary relationships: Cyanophages that infect multiple species of blue-green algae are considered to be evolutionarily related, as they exhibit specificity for similar host types.
- Exploring genetic exchange (horizontal gene transfer): They play a significant role in horizontal gene transfer, particularly in the exchange of photosynthetic genes, which highlights their importance in genetic studies.
- Maintaining ecological balance: The seasonal abundance of cyanophages reflects the ecological equilibrium of the ecosystem. They contribute to biogeochemical cycles, influence nutrient dynamics, and help regulate population levels within the environment.
- Controlling algal blooms: The can be utilized to combat water blooms caused by certain blue-green algae, such as Anabaenopsis, Spirulina, Oscillatoria, and Microcystis, effectively reducing algal blooms.
- Studying plant photosynthesis under viral infection: Since the photosynthetic process in cyanobacteria closely resembles that of higher plants, with minor differences, the algal-cyanophage system serves as a useful model for investigating how viral infections affect plant photosynthesis.
FREQUENTLY ASKED QUESTIONS (FAQS)
What are Cyanophages?
Cyanophages are viruses that exclusively infect and target cyanobacteria, commonly known as blue-green algae.
Discuss the Structure and Composition of Cyanophages.
Cyanophages consist of nucleic acids and proteins, with a linear, double-stranded DNA protected by a protein capsid. Their structure includes a protein-based head and a short, hollow, cylindrical tail.
The DNA is housed in the head, covered by a double membranous coat. The tail, featuring sixfold radial symmetry, is formed by 2–4 rings of six subunits each. It connects to the head via the tail capital, a joint-like structure with an uncertain function.
The non-contractile tail has an outer sheath, an inner cavity, and likely extends tail fibers from its lower end. This design enables host cell attachment and infection.
Point out the growth phases (cycle) of Cyanophages.
The growth cycle of Cyanophages comprises four distinct stages that are;
1. Attachment and Penetration
2. Biosynthesis
3. Maturation
4. Release.
List out the importance of Cyanophages.
1. Tracing evolutionary relationships
2. Exploring genetic exchange (horizontal gene transfer)
3. Maintaining ecological balance
4. Controlling algal blooms
5. Studying plant photosynthesis under viral infection
