Environmental Microbiology of Aquatic and Waste Systems

98 4 Taxonomy, Physiology, and Ecology of Aquatic Microorganisms

Phospholipid bilayer

dsDNA genome

 ViralZone 2008

Swiss Institute of Bioinformatics

Spike protein P2

T = 21

Capsid protein P1

1 CORTICOVIRIDAE

Structure
Phages consist of a capsid and an internal lipid membrane. Virus
capsid is not enveloped. Internal lipid membrane located between
outer and inner protein shell. Capsid/nucleocapsid is round and
exhibits icosahedral symmetry (T=12), or 13). The capsid is iso­
metric and has a diameter of 60 nm (or more). The capsid shells of
virions are composed of three layers. Capsids appear hexagonal in
outline. The capsid surface structure reveals a regular pattern with
distinctive features. The capsomer arrangement is clearly visible.

Surface projections are distinct, brush­like spikes protruding from

the 12 vertices. Capsids all have the same appearance.

The genome is not segmented, constitutes 13% of the virus’s

weight and contains a single molecule of circular, supercoiled,

double-stranded DNA of 9500-12000 nucleotides in length. The

genome has a g + c content of 43%.

Host

Their hosts are members of the Phylum Proteobacteria.

 ViralZone 2008

Swiss Institute of Bioinformatics

Protein P1 RdRp P2

Protein P8 Protein P4 Protein P6 Protein P3

OUTER CAPSID T=13

INNER CAPSID T=2

SURFACE

2 CYSTOVIRIDAE

Structure
Enveloped, spherical virion of 85 nm in diameter. The virion has
a double capsid structure: Outer capsid a has an icosahedral
T=13 symmetry, inner capsid an icosahedral symmetry T=2.
All cystoviruses are distinguished by their three strands (analo­
gous to chromosomes) of double stranded (ds) RNA, totalling ~14
kb in length and their protein and lipid outer layer. No other bacte­
riophage have any lipid in their outer coat, though the Tectiviridae
and the Corticoviridae have lipids within their capsids.
Members of the Cystoviridae appear to be most closely
related to the Reoviridae but also share homology with the

Totiviridae. Cystoviruses are the only bacteriophage that are

more closely related to viruses of eukaryotes than to other

phage.

Host

Most identified cystoviruses infect Pseudomonas species, but

this is likely biased due to the method of screening and enrich­

ment. The type species is Pseudomonas phage F 6 , but there are

many other members of this family. F7, F8, F9, F10, F11,

F12 and F13 have been identified and named, but other cystovi­

ruses have also been isolated.