Bacteriophages: Structure and Reproduction (Replication Cycle)

Bacteriophages, in short, are the viruses that infect bacteria. F. Twort (1915) and F. d ‘Herelle (1917) independently discovered bacteriophages.

Bacteriophages attacking Escherichia coli are called coliphages or T-phages. Max Delbruck (1938) numbered coliphages as T-even phages (T2, T4, T6 etc.) and T-odd phages (T1, T3, T5 etc.).

1. Structure:

The virion of T-even phage is binal or tadpole like structure with a polyhedral head connected to a helical tail through a short collar. The head composed of about 2000 capsomeres arid encloses a tightly packed dsDNA (50 nm long). The tail has an inner hollow tube called core, surrounded by a contractile sheath which consists of 24 annular rings. The distal end of the tube is connected to a hexagonal basal plate with spike or tail spin at each corner. Six long, flexible tail fibers also arise from the basal plate which helps in adsorption to bacteria (Fig. 10.5).

2. Reproduction (Replication cycle):

Bacteriophages exhibit two types of replication cycle – virulent or lytic cycle and temperate or lysogenic cycle (Fig. 10.6).

I. Virulent or lytic cycle:

The phages undergoing lytic cycle are called lytic phages or virulent phages, e.g., T-series bacteriophages. In lytic cycle, a lytic phage infects and kills the host cell to release progeny virions.

The whole process involves following steps:

(a) Adsorption or infection

(d) Virion assembly

(b) Penetration or injection

(e) Lysis or release

(c) Synthesis of phage components

Step-1. Adsorption or infection:

The lytic cycle begins with a collision between T-phage virion and a susceptible host cell i.e. Escherichia coli. The process of attachment of a virion on the host cell surface is called adsorption. The tips of tail fibers bind or adsorb to specific receptors on the surface of E. coli.

The viral receptors may be F-pili, lipoproteins, iron transport proteins etc. The T-phage virion adsorb to specific receptors by the tip of tail fibers. For example, T4 and T7 coliphages bind to lipopolysaccharides.

Step-2. Penetration or Injection:

The tail fibers of virion bend to bring the spikes and basal plate in contact with the surface of bacterial wall. The tail sheath contracts so that the hollow tail core (inner tube) penetrates the bacterial wall and injects the viral genome into the cytoplasm. After penetration, the empty capsid that remains outside the bacterium is called the ghost or doughnut.

Step-3. Synthesis of phage components:

Immediately after penetration, the phage DNA (genome) synthesizes early proteins. Some early proteins break down the bacterial (host) DNA and take the control of the bacterial cell machinery. The other early proteins used as enzymes for replication of phage DNA. The newly synthesized phage DNAs produces late proteins, which are the protein subunits of the phage capsid (head and tail).

Step-4. Virion assembly:

The capsid proteins assemble to form empty head and a condenced viral DNA is packed inside it. Finally the separately assembled tail joins to head to form a daughter or progeny virion.

Step-5. Lysis or release:

During assembly of progeny virions, the bacterial cell becomes spherical. The phage enzymes weaken the cell wall which ultimately burst or lyse to release about 100-200 progeny virions.

II. Temperate or lysogenic cycle:

The phages that exhibit lysogenic cycle are called temperate phages or non-virulent phages. For example, λ, (Lambda)- phages attacking, E. coli. During lysogenic cycle, the phage DNA integrates into the bacterial DNA and is now called as prophage. The host bacterium containing prophage is called a lysogenic bacterium or lysogen. The prophage passively replicates along with the host DNA for many generations. When a lysogenic bacterium exposed to UV-light or a chemical, the prophage withdraw from the host DNA to undergo lytic cycle. This conversion of a prophage into a lytic phage is called induction.

Virion:

A complete viral particle, consisting of RNA or DNA surrounded by a protein shell capsid and constituting the infective form of a virus. It is also known as virus is ready for infection. The capsid protects the interior core that includes the genome and other proteins. All virions have genomic nucleic acid: this maybe either RNA or DNA, ss (single stranded) or ds(double stranded). After the virion binds to the surface of a specific host cell, its DNA or RNA is injected into the host cell and viral replication occurs with eventual spread of the infection to other host cells.

Virusoids:

Virusoids are circular single-stranded RNAs dependent on plant viruses for replication and encapsidation. The genome of virusoids consists of several hundred nucleotides and only encodes structural proteins. Virusoids are similar to viroids in size, structure and means of replication .Virusoids while being studied in virology, are not considered as viruses but as subviral particles.

Prion:

It is an infectious protein particle similar to a virus but lacking nucleic acid; thought to be the agent responsible for scrapie and other degenerative diseases of the nervous system. The term Prion was coined in 1982 by Neurologist Stanley Prusiner. Prions are infectious proteinaceous particles that lack nucleic acid. Prions are said to be in the border zone between nonliving and living things because they have no need to metabolize or the capacity to reproduce but they are capable of replication within the body of a human or of some mammals.

Prions can gain entry into the body mainly by ingestion, e.g. of contaminated human. Growth Hormone or of contaminated blood or blood products. Prions may also arise from a mutation in the gene that encodes the protein. They not only fold into unusual shapes but also seem to have the ability to cause other (normal) proteins to alter their shape as well.

Viroid:

Viroid an infectious agent that consists solely of a single strand of RNA and causes disease in certain plants. It was discovered by T.O .Diener in 1971 .Viroids lack the protein coat (known as capsid) of viruses and are the smallest known infectious agents containing only about 250 to 375 base pairs. They are much smaller than the smallest genomes of viruses and have no genes for encoding proteins. Viroids are believed to cause disease by interfering with the host cell’s gene regulation. They are destructive to many important commercial plants, including potatoes, Lycopersicom, Cucumis, Cocos, and Chrysanthemum etc.