Download Positive Strand RNA Viruses

Objectives
• To understand the general principles involved in
RNA replication discussed in Chapter 6 pages
175-192
• To use the following + stranded RNA viruses as
examples and to understand the different
processes that during replication by RNA viruses:– Picornavirus
– Togaviruses
– Flaviviruses
General Principles
• De novo initiation:
– RNA polymerase
– RNA template
– The initiating NTP and a second NTP
General Principles
• Primer Dependent Initiation
– Protein priming
– Capped RNA fragments
• All synthesis begins with the formation of a
complex of polymerase, template-primer, and
initiating NTPs
General Principles
• Template specificity
– Appears to be very strongly related to :
a)
b)
c)
d)
Affinity of RNA polymerase for the initiating NTP
The sequence
The structure of the viral RNA molecules
Encoding proteins that bind to genomic RNA
segments which then activates an endonuclease that
cleaves host cell RNAs.
More facts
• Unwinding the RNA template
– Encoded in genome of ds RNA viruses
– Prevent base pairing between template and new
strand.
• Role of host cell proteins
• Different RNA polymerases for mRNA
synthesis and genome replication
• Switching from mRNA to genome replication
7
Positive Strand RNA Viruses
PICORNAVIRUSES (PICORNAVIRIDAE)
• Properties
–
–
–
–
These are small (28nm),
naked
icosahedral viruses
RNA is single-stranded,
plus sense,
polyadenylated.
– functions as mRNA
immediately upon
infection
– E.g poliovirus
Adsorption and penetration
• A viral protein recognizes a receptor on the
host cell membrane (this is important in the
tropism of virus).
It seems that binding to the receptor alters
capsid structure in some way, a channel forms
across the cell membrane and the RNA is
released into cytoplasm. The mRNA is now
available for translation.
Synthesis of viral proteins
• Poliovirus virion RNA functions as an mRNA but does not have the
methylated cap structure typical of eucaryotic mRNAs
• It has a "ribosome landing pad" (known as the internal ribosome
entry site or IRES) which enables ribosomes to bind without having
to recognize a 5' methylated cap structure
• Most host cell translation is cap-dependent, so this inhibits a lot of
host protein synthesis but not viral protein synthesis.
Synthesis of Viral Proteins
•
•
•
The mRNA is translated into a single polypeptide (polyprotein), which is cleaved.
The cleavages occur before translation is complete ( i.e. on the nascent (=growing)
chain) and are carried out by virally coded proteases.
Products of cleavage include:
– An RNA polymerase (replicase)
– Structural components of the virion
– Proteases
RNA replication
•
•
•
•
•
•
We now have newly made viral proteins to support replication.
Viral RNA polymerase copies plus-sense genomic RNA into complementary minussense RNA requiring:
– VPg (Viral Protein genome-linked)
– Viral RNA polymerase (replicase)
– Certain Host proteins
VPg may act as a primer for RNA synthesis, this would explain why it is at the 5'
end of all newly synthesized RNA molecules
New minus sense strands serve as template for new plus sense strands
Again, poliovirus RNA polymerase and VPg are needed. VPg is linked to the 5' ends
of the new plus sense strands (again, it probably functions as a primer).
The new plus strand has three alternative fates:
i. It may serve as a template for more minus strands
ii. It may be packaged into progeny virions
iii. It may be translated into polyprotein (In this case VPg is usually removed prior
to translation)
Assembly
• When sufficient plus-sense progeny RNA and
virion proteins have accumulated, assembly
begins.
• Particles assemble with VPg-RNA inside and 3
proteins in the capsid [VP0,1 and 3].
• VP0 is then cleaved to VP2 and VP4 as the virions
mature and these mature virions are infectious.
• Virions are released following cell lysis.
• Excess capsids are formed and inclusion bodies
may be seen in the cytoplasm.
• NOTE: THE ENTIRE LIFE CYCLE OCCURS IN THE
CYTOPLASM
• THERE IS NO DIVISION INTO EARLY AND LATE
GENE EXPRESSION
NON-SEGMENTED NEGATIVE
STRAND VIRUSES