VIRUSES OF ARCHAEA

 VIRUSES OF ARCHAEA

·       Viruses of Archaea represent one of the most enigmatic parts of the Virosphere.

·       A virus infecting Archaea was first described in 1974.

·  Viruses are among the most abundant biological entities on earth, outnumbering cells in some environments by more than an order of magnitude. Viruses of Archaea (termed Archaeal viruses) are some of the most unusual and least understood group of viruses.

·       Most of the characterized Archaeal viruses infect Extremophilic hosts and display remarkable diversity of Virion morphotypes, many of which have never been observed among Viruses of bacteria or Eukaryotes.

·       Morphology - Most of the Archaeal viruses have Head-tail morphologies and Linear double-stranded DNA genomes. Other morphologies have also been described including Spindle shaped, Rod shaped, Filamentous, Icosahedral, and Spherical.

Double Stranded DNA viruses infecting Archaea

·       Archaea can be infected by Double-stranded DNA viruses that are unrelated to any other form of virus and have a variety of unusual shapes.

·       Double-stranded DNA viruses have been studied in the most detail in Thermophilics, particularly the orders Sulfolobales and Thermoproteales.

·       Some of the Double Stranded DNA viruses infecting Archaea are

a)  Bacteriophages (viruses infecting bacteria) belonging to the families Tectiviridae and Corticoviridae have a lipid bilayer membrane inside the icosahedral protein capsid and the membrane surrounds the genome.

b)   Species of the order Ligamenvirales and the families Ampullaviridae, Bicaudaviridae, Clavaviridae, Fuselloviridae, Globuloviridae, and Guttaviridae infect Hyperthermophilic Archaea species of the Crenarchaeota.

c) Species of the Genus Salterprovirus infect Halophilic Archaea species of the Euryarchaeota.

Single Stranded DNA viruses infecting Archaea

·       Two groups of Single stranded DNA viruses that infect Archaea have been recently isolated.

·   One group is exemplified by the Halorubrum Pleomorphic Virus - 1 (Pleolipoviridae) infecting Halophilic archaea and the other one by the Aeropyrum coil-shaped virus.

·       Although around 50 Archaeal viruses are known, all but two have Double stranded genomes.

a)     The first archaeal ssDNA virus to be isolated is the Halorubrum Pleomorphic Virus 1.

ü  Pleomorphic enveloped virion.

ü  Circular genome.

ü  Infects Halophilic archaea.

ü  Defenses against these viruses may involve RNA interference from repetitive DNA sequences that are related to the genes of the viruses.

b)     The second single stranded DNA virus infecting Archaea is Aeropyrum coil-shaped virus (ACV).

ü  The genome is circular and with 24,893 nucleotides is currently the largest known ssDNA genome.

ü  The viron is non-enveloped, hollow, cylindrical, and formed from a coiling fiber.

ü  The morphology and the genome appear to be unique.

ü  Aeropyrum coil-shaped virus has been suggested to represent a new viral family tentatively called “Spiraviridae” (from Latin spira, “a coil”).

ü  The Aeropyrum coil-shaped virus infects a Hyperthermophilic Archaea (optimal growth at 90 – 95 °C) host.