Genus Corticovirus

Type Species	Alteromonas phage PM2	(PM2)

Virion Properties

Morphology

Virions exhibit icosahedral symmetry (T = 12 or T = 13) and are about 60  nm in diameter. They have no envelope. Capsids are complex and consist of an outer and an inner protein shell with a lipid bilayer in-between. Brush-like, 5-6  nm long spikes protrude from each apex (Fig. 1).

Physicochemical and Physical Properties

Virion Mr is 49 10⁶; buoyant density in CsCl is 1.28  g/cm³; S_20w is 230S. Virions are stable at pH 6-8, and are very sensitive to ether, chloroform, and detergents.

Nucleic Acid

Virions contain a single molecule of covalently closed, circular dsDNA about 9  kbp in size (Mr 6.3-6.4 10⁶). The DNA contains a large number of superhelical turns. The DNA comprises 14.3% of virion weight and is coiled within the inner shell in association with protein IV. The G+C content is 42-43%. Parts of the Alteromonas phage PM2 (PM2) genome have been sequenced.

Proteins

Four structural proteins are present. Protein II makes up 65% of the total protein. Proteins III and IV behave as lipoproteins. Transcriptase activity is associated with the virion.

Lipids

Particles are composed of 13% lipid by weight (5 species). Lipids form a bilayer between the outer and the inner shell. About 90% are phospholipids, mainly phosphatidylglycerol and phosphatidylethanolamine. The lipid composition of phages differs from that of the host plasma membrane.

Carbohydrates

Not known.

Genome Organization and Replication

Genome structure and modes of transcription and translation are largely unknown. DNA replication proceeds unidirectionally and counterclockwise. Replicative intermediates include rings, nicked circular molecules, and double-branched rings. Phages are assembled at the plasma membrane without formation of inclusion bodies. The inner shell assembles first in the presence of protein IV, and is filled with DNA. Virions are completed by progressive addition of lipids, the outer shell, and spikes, and are liberated by lysis.

Antigenic Properties

Not known.

Biological Properties

Phages are virulent and adsorb to the bacterial cell wall. Host range is limited to a marine bacterium of the genus Alteromonas.

List of Species Demarcation Criteria in the Genus

Not applicable.

List of Species in the Genus

Official virus species names are in italics. Tentative virus species names, alternative names ( ), strains or serotypes are not italicized. Virus names and assigned abbreviations ( ) are:

Species in the Genus

Alteromonas phage PM2	(PM2)

Tentative Species in the Genus

Not known.

List of Unassigned Viruses in the Family

Vibrio phage 06N-58P	(06N-58P)

Phylogenetic Relationships within the Family

Not applicable.

Similarity with Other Taxa

Corticoviruses have similarities to tectiviruses (capsid size, presence of lipids, sensitivity to ether, chloroform, and detergents). They differ from corticoviruses by the absence of an inner vesicle and a tail-like nucleic acid ejection device.

Derivation of Names

Cortico: from Latin cortex, corticis, “bark, crust”.