Cyanophages: Distribution and Nomenclature

In this article we will discuss about:- 1. Introduction to Cyanophages 2. Distribution of Cyanophages 3. Nomenclature 4. Ultrastructure 5. Infection and Growth Cycle.

Introduction to Cyanophages:

Sufferman and Morris (1963) were the first to report the discovery of viral agent which attacks and lysis blue-green algae. Since then many more viruses have been found which attack a wide range of blue-green algae.

These viral agents attacking algae have been differently designated as phycoviruses, algal phages, blue-green algal viruses and cyanophages.

To the majority of the investigators, the latter term (Cyanophages) appears more appropriate for two reasons namely:

(i) Close relationship between the blue-green algae and bacteria (Echlin and Morris, 1965), and

(ii) Similarity to a considerable extent in structure, infection and growth cycle to a bacteriophage.

The term Cyanophage thus denotes a viral agent which is capable of infecting, reproducing in and lysing a blue-green alga.

Distribution of Cyanophages:

Cyanophages have been reported from many countries including India and thus are considered to be worldwide in their distribution. They occur almost throughout the year in fresh-water bodies such as streams, rivers, lakes, industrial storage tanks, farm and residential ponds.

Singh, R.N., and P.K.Singh (1967) reported cyanophages from India in sewage, ponds and Singh P.K. reported cyanophages (1973) from rice fields.

Nomenclature of Cyanophages:

The cyanophages, known at present are placed under six groups namely, LPP_1-2, SM-1, C-1, AR-1, AS-1 and N-1. Each group is given a code name according to its host range. The code name is composed of the initials of the generic names to the known hosts.

The Arabic numerals are added to the code names to denote seriological subgroups. For instance, LPP group of Cyanophages is subdivided into two sub-groups LPP₁ and LPP₂. It means another group of viruses with the same host range has since been discovered and designated LPP₂.

The code names of the main cyanophage groups, with their host range and important morphological features, are given in the above Table.

Ultrastructure of Cyanophage (Fig. 19.9):

The fine structure of the intact particles (virions) of LPP, group has been thoroughly studied. They are similar in structure to one another. Like the bacteriophage they are of a head-tail type. The cyanophage virion is differentiated into 3 regions namely, head, tail capital and tail.

(i) Head:

The LPP₁ cyanophage head is polyhedron. It appears hexagonal in outline and has many (20) facets and thus is described as icosahedral in form. The short dia. of the viral head is .50.6 ± 2.0 nm (586 ± 20A°).

It is differentiated into an outer protein coat or sheath which encloses a cavity filled with the genetic material called the nucleic acid. The latter which forms the core of the cyanophage head is the infective agent. The protein sheath which encloses it is protective in function. It is inert and non-genetic.

Technically the protein sheath constitutes the head capsid. It is 8-10 nm thick. The major proteins of the head capsid are the 39,000 and 13,000 species. The nucleic acid in the cyanophages is always a DNA molecule.

It is a single thread-like, double stranded macromolecule of DNA with a contour length of 13.2 ± 0.5 nm. It just fills the cavity of the head capsid.

(ii) Tail:

The short, hollow cylindrical tail is 20- 25 nm long and 15 nm wide. The upper end of the tail is attached to one of the vertices of the head capsid and from the lower end probably protrude the tail fibres (Shilo, 1967).

The tail is non-contractile.

It is differentiated into an outer protein sheath enclosing a cavity. The former is designated tail sheath and the latter tail cavity. The tail sheath is made up of 2-4 rings of six protein subunits each.

(iii) Tail Capital:

Located at the junction of the tail with the head capsid is the tail capital about 10 nm long and 25 nm in dia. It protrudes into the head capsid. The function of the tail capital is uncertain. Probably it serves as a joint to fix the tail to the head capsid.

Infection and Growth Cycle of a Cyanophage:

(i) Infection process:

The LPP-1 Cyanophage particle in the aquatic environment and close to its blue-green algal host Plectonema affixes (adsorbs) itself to the prokaryont host cell probably at a specific receptor site which the tail end of the phage tail happens to touch on the host cell wall.

According to Smith et al. (1967), the tip of the phage tail appears to penetrate the host cell wall at the point of attachment. It is presumed that the cyanophage then injects its DNA into the host cell protoplast through the hollow tail.

The empty protein sheath or coat of the phage called the ghost remains sticking to the host cell surface. The steps outlined above describing the infection process also constitute the adsorption and penetration phase.

(ii) Latent Period Phase:

Alter completion of the infection process which takes about 1 hr. (Goldstein et al., 1967) starts the latent period phase. The latter lasts for about 6 to 6.50 or even 7 hrs. The first three hours of this phase constitute eclipse period during which the injected virus DNA, as such disappears.

It is not seen in the host cell. Three hours after infection, the thylakoids arranged in the form of a sheath at the periphery of the host cell of Plectonema begin to recede from the plasma membrane.

The space thus formed between the replaced thylackoids and plasma membrane is termed the virogenic stroma. The virogenic stroma becomes distinct by the 4th hour corresponding to the end of the eclipse period.

The formation of virogenic stroma is the first visible sign of infection. The cyanophage DNA, after infection, seems to reach nucleoplasm of the host cell and takes control of the host cell genetic machinery.

Synthesis of host cell proteins and replication of algal DNA ceases within hours after infection. Sherman and Haselkorn (1970) reported that about half of the host DNA is broken down to provide acid soluble precursors of the viral DNA.

The replicated viral DNA first migrates into the spaces between the thylakoids and then into the virogenic stroma in the form of long helices. Viral heads have been seen in the virogenic stroma by the end of the latent period (7th hr.) where assembly of cyanophage virions begins by the 10th hour and is complete within 14 hours after infection.

The release of mature cyanophage particles is achieved by cell lysis. The average burst size is 100 virions per cell. The cyanophage reproductive cycle is of longer duration as compared with that of the bacteriophage.