dsRNA Viruses

Question 1

baltimore classification of dsRNAviruses

Answer 1

dsRNA viruses belong to Class III according ot the Baltimore classification of viruses

Question 2

What is the most important group of dsRNA viruses

Answer 2

Reoviruses

Question 3

Introduction to Reoviruses

Answer 3

• They are icosahedral shaped virus with segmented genome.
• They usually infect the respiratory and enteric tracts of humans and animals.
• Originally, the name ‘’reoviruses’’ came about based on early observations about the virus as follows:

Found in respiratory tract: R
Found in enteric tract: e
The disease was unknown at the time (ophan): o

The suffix ‘’virus’’ was added to make up the name Reovirus.

•Later, similar viruses have been observed to infect and cause disease in birds, mammals, fish, invertebrate (suck as insects), plant and fungi. Nevertheless, the group still retains the name reovirus

Question 4

epidemeology of Reoviruses

in birds lol

Answer 4

• Reovirusesbelong to the family Reoviridae.
• Several genera have been named under the family for which the original reoviruseshave been incorporated into the genus Orthoreovirus.
• The genus Orthoreovirusinfect birds and mammals.
• The avian (bird) viruses are important pathological members of the genus orthoreovirus.
• Animal infection: majority of orthoreovirusinfecting animals are asymptomatic (show no symptom).
• Humaninfection: Most persons are exposed to orthoreovirusesinfection at childhood and have developed specific serum antibodies by early adult hoo

Question 5

Reovirus(genus: Phytoreovirus) infecting plant

Answer 5

• Most of the reoviruses(Genus-Phytoreovirus) infecting plants are transmitted by vectorsthrough feeding. The virus undergo circulative transmission (insect saliva –body circulation-saliva) in the vector.
• Acquired virus replicatein the body circulation of the vector before returning to the saliva, ready to be injected into a new hosts.
• The vector suffers little or no harm in the process while the plant hosts suffer disease conditions due to the virus

Question 6

Rotavirus infecting animals

Answer 6

ds RNA virus

Origin: The rotavirus was first discovered in faecelsamples from monkeys and mice using the electron microscope in 1963.

Human origin: In 1973, the rotavirus was discovered for the first time in faecalsamples from children with symptom of diarrhea using electron microscopy.

Shape: They are spherical shaped virionsmeasuring approximately 75 nm in diameter. Hence, the name rotavirus was derived from the shape of the virus which is round (rotain latin)

Question 7

Rotavirus Capsid

Answer 7

• The capsid of rotaviruses has an icosahedral shape made up of three layers (triple layer).
• Each of the 3 capsid layers are made up of different virus proteins (VP).

1. Inner layer: Have perforated channels made up of VP2.
2. Middle layer: Have perforated channels made up VP6. It is arranged similar to a wheel’s spokes and serves as the major component of the virion.
3. Outer layer: It is a glycosylated layer made up of VP7. VP7 is taken from a membrane within the cell and incorporated unto the surface of the virion.

• Innercore: Two protein species are associated with the inner core (VP1 and VP3) of rotaviruses.
• Spikes: VP4 forms the 60 spikes protruding from the surface of the rotavirus virion.

Question 8

Rotavirus structure // enzymes

Answer 8

Figure 2: Rotavirus structureThe seven virionproteins are arranged based on sizes.

The 3 largest proteins are arranged toward the center of the virion

:VP2 is associated with the inner capsid layer.

VP1 and VP3 are enzymes associated with the genome with each made up of 12 copies.

VP1 has the RNA-dependent RNA polymerase activity

VP3 has guanylyltransferaseand methyl transferaseactivities

Question 9

Rotavirus genome

Answer 9

• The genome of the rotavirus has 11 dsRNAsegments.
• Each of the 11 segments encode one protein except for one which encodes two proteins, making a total of 12 proteins.

`•Of the 12 encoded proteins, 6 codes for structural proteins while 6 codes for non-structural proteins

Question 10

what cells do rotavirus replicate within?

Answer 10

•Rotaviruses infect and replicate in the enterocytes: These are cells at the ends of the villi (finger-like projections) in the small intestine.

Question 11

Which cells do rotaviruses affect

Answer 11

• Rotaviruses infect and replicate in the enterocytes: These are cells at the ends of the villi (finger-like projections) in the small intestine.
• Attachment and Entry•Transcription•Translation•Genome replication and secondary transcription•Assembly•Exi

Question 12

Rotavirus attachment and entry

Answer 12

• The mechanism of cell entry by rotaviruses are complex and remains relatively uncertain.
• First, the virionspike protein (VP4) is cleaved (cut) to smaller proteins VP5 and VP8, facilitating the entry of the virioninto the host cell

Question 13

Mechanism of entry by rotaviruses

Answer 13

•Rotaviruses may enter the cells through two possible mechanisms

:1.Direct penetration via the plasma membrane

2.Endocytosis

Question 14

Early events of rotavirus transcription

activation of transcription

Answer 14

• Activation of transcription: Once the virionenters a cell, the outer of the 3 layers is shed leaving the middle and inner layers in which transcription is activated.
• The 11 genome segments are seemingly linked with VP1 in the synthesis of new plus strand (+)RNA and a molecule of VP3 caps the 5’ end of the new RNAs.
• Nucleotides for RNA synthesis enter the particles via channels in the protein layers and are released via same rout

Question 15

Figure 8: Early events in rotavirus replicatio

Answer 15

Figure 8: Early events in rotavirus replication:(+)

RNA is transcribed from each ofthe genome segments in the double-layered particle derived from the cell invading virion.

Initially this RNA functions as mRNA (green) and the 12 virus proteins are translated.

NSP2 and NSP5 play major roles in the assembly of viroplasm, and other virus proteins accumulate in these structures.

VP1 and VP3 bind to (+) RNA molecules (dark blue).

VP2 forms roughly spherical structuresaround the
(+)RNA (inner layer).

(–) RNA (light blue) is synthesized in the viroplasm

.VP6 is added to form progeny double-layeredParticles (middle layer

Question 16

Early events in rotavirus replication

myristylation

Answer 16

Early events in rotavirus replication continued•Some virus proteins such as NSP5 is phosphorylated while VP2 and VP3 are myristylated

Question 17

Early events in rotavirus replicatio

Viroplasms

Answer 17

•Viroplasms: are structures where synthesized virionproteins are populated or accumulate. NSP2 and NSP5 play major role in the formation of a viroplasm

Question 18

Early events in rotavirus replication

partial assembly

Answer 18

• Partial assembly: Within the viroplasm, newly synthesized (+)RNA bind VP1 and VP3 (enzymes), and to VP2 which forms the inner capsid layer which is spherical.
• A rigorous selection process ensures each newly synthesized virionstructure gets a full complement of viral genome (e.g. the full-11 RNA species

Question 19

Early events in rotavirus replication

•VP1 which encodes the RNA polymerase is activated via interaction with VP

Answer 19

• (+)RNA strand serves as templates for producing the complementary (-)RNA, producing the 11 dsRNAmolecules of the genome in each structure.
• Thus, the dsRNAof the infecting virionremains intact as the mode of replication is conservative.
• VP6 is added to the core, forming the middle (second layer) of the capsi

Question 20

Late events in rotavirus replication

Secondary transcription

Answer 20

Secondary transcription: A second phase of transcription (late phase) occurs in the doubled-layer virionformed at the early phase.

• Transcripts aren’t capped in late phase but capped in the early phase.
• Translation machinery undergo changes which will enable the selection of uncapped transcripts in preference to the capped. Thus, NSP3 plays a role in linking the 5’ end of a mRNA to its 3’ end.
• This shuts down translation of cell proteins and continues the replication of virus proteins

Question 21

Late events in rotavirus replication

•Rotaviruses do not replicate its 12 proteins in equal amount.

Answer 21

• Rotaviruses do not replicate its 12 proteins in equal amount.
• While the 11 dsRNAis generated in equimolaramount, the mRNA species are not generated in equimolaramount.
• For example: VP6 (middle layer of capsid protein) is produced in large quantities.
• VP1 is produced in small numbe

Question 22

Figure 9: Late happenings in rotavirus replication in the endoplasmic reticulum

Answer 22

Late/secondary transcription occurs in the progeny virionsresulting in synthesis of uncapped mRNA.

The 12 virus proteins are synthesized in the cytoplasm, but only NSP4 and VP4 translation occur in the endoplasmic reticulum.VP4 and VP7 form the outer layers of the capsi

Question 23

Rotavirus Assembl

Answer 23

• Concrete scientific evidence regarding the assembly of virus remains uncertain. However, the following points should be noted:
• The final stage of virionassembly involved the addition of the outer layer of the capsid (VP7) and spikes (VP4)

.•VP7 and NSP4 are synthesized and N-glycosylated in the rough endoplasmic reticulum.

• NSP4 has binding sites for VP4 and double layered virionparticles.
• After binding of the components, the particle buds off through the membrane into a vesicle within the rough endoplasm reticulum.
• Temporary envelop: this is formed from the vesicle membrane and is made up of VP

Question 24

Rotavirus assembly of spikes and exit

Answer 24

• VP4 is added to the doubled-layered particle to form virionspikes.
• VP7 molecule is cleaved to release them from the cell membrane to form the outer capsid layer and lock the spikes in place.
• Virionsare released from the cell either by endocytosis or cell lysis

Question 25

Rotavirus disease

Answer 25

* Most rotavirus infection occurs in the gut, but the virus may occasionally infect other tissues. * Rotavirus infect and destroy the enterocytesof the villi leading to reduced absorption of water sugars, and salt from the gut. * The virus spike protein (VP4) causes the leakage of chloridefrom cells into the gut lumen. * Dehydrationresults from diarrhea due to loss of water and solute by secretary cells. * Treatment is by rehydrating the patient with solution of salt and sugar. * Infection could result in death if not treate