TABLE 9.1 Satellites and Satellite Viruses
Genome size
Helper virus
dsDNA satellite
Bacteriophage P4
11.5 kb (1015 genes)
P2 bacteriophage
All structural proteins from P2
ssDNA satellite cirusesa
Dependovirus (AAV)
4.7 kb
See Table 7.16
dsRNA satellites
M satellites of yeast
1 to 1.8 kb
Encode "killer" proteins; encapsidated in
helper coat protein
ssRNA satellite viruses
Chronic bee-paralysis virus
3 RNAs, each 1 kb
Chronic bee-
associated satellite
paralysis virus
Tobacco necrosis virus satellite
1239 nt
Tobacco necrosis virus
ssRNA satellites
Encode two forms of δ antigen
Hepatitis delta virus
1.7 kb
Hepatitis B virus
encapsidated by helper proteins
B-type mRNA satellites
0.8 to 1.5 kb
Various plant viruses
Encode nonstructural proteins, rarely
modify disease syndrome
C-type linear RNA satellites
<0.7 kb
Various plant viruses
Commonly modify disease caused by helper
D-type circular RNA satellites
~350 nt
Various plant viruses
Self-cleaving molecules
When a satellite encodes its own coat protein, it is known as a satellite virus.
satellite. One of the best studied satellite systems is tobacco
divided among two or more segments that are separately
necrosis virus (TNV) and its satellite, tobacco necrosis virus
encapsidated into different particles, a situation that does
satellite (TNVS). TNV has a plus-strand RNA genome of
not occur among animal viruses. Evidently, the mechanisms
about 3.8 kb. The TNV virion is icosahedral with T=3, and
by which plant viruses are transmitted allow the infection
contains 180 copies of a single-coat protein species of about
of a plant, and of individual cells within a plant, by multi-
30 kDa. Associated with many isolates of TNV in nature is
ple particles that together constitute a virus or that consti-
TNVS. TNVS has an RNA genome of 1239 nucleotides.
tute a virus and its satellites. Transmission of animal viruses
The TNVS virion is a T=1 icosahedral structure formed by
between hosts or among the cells of a host does not appear to
60 molecules of a single species of capsid protein encoded
allow multiple infections with sufficient frequency to main-
by the satellite RNA. The satellite RNA encodes only this
tain virus systems that are constituted by multiple particles,
with the exceptions of hepatitis δ virus, described later, and
single protein that encapsidates its own RNA. All of the
functions required to replicate the RNA are provided by the
dependoviruses, which have evolved ways to persist within
helper TNV.
a cell until the helper virus comes along (Chapter 7). The
Some satellites of RNA plant viruses encode only a non-
defense mechanisms of the animal host may also play a role
structural protein required for RNA replication, and the RNA
in this restriction.
is encapsidated by the capsid protein of the helper virus. In
other cases, the satellite is not translated into protein and
depends on the helper for all of its functions, in which case
it is functionally analogous to DI RNAs. A distinct class of
satellite RNAs, called virusoids, consist of viroid-like RNAs
Viroids are small, circular RNA molecules that do not
that are encapsidated in the capsid protein of the helper virus.
encode any protein and that are infectious as naked RNA
These are discussed in the next section.
molecules. Sequenced viroids range from 246 to 375 nucle-
Although satellites are quite common among plant
otides and possess extensive internal base pairing that results
viruses, they are almost unknown among animal viruses. It
in the RNA being rodlike and about 15 nm long. A partial
is very common among plant viruses to have the genome
listing of viroids is given in Table 9.2. All known viroids
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