Minus Stranded RNA Viruses

Question 1

Introduciton to (-)ssRNA viruses

Answer 1

• The most important member of the (-) ssRNAviruses are the rhabdoviruses.
• The name (rhabdovirus) was coined from the Greek word Rhabdos= a Rod. (Rod-like viruses = Rhabdovirus)
• Rhabdoviruseshave minus (-) strand RNA genomes whose size range between 11-15 kb.
• Minus-strand RNA viruses belongs to group (V) based on Baltimore genetic classification of viruses

.•Common hosts are:

• Plants (rod-like with rounded ends)
• Fishes (mostly bullet shaped)
• Insects (mostly bullet shaped)
• Mammals (bullet shaped

Question 2

General characteristics of the ssRNA-viruses

classification

Answer 2

Rhabdovirusesbelong to the virus family Rhabdoviridaewith many genera

Question 3

General characteristics of the (-)ssRNAviruses

shape

Answer 3

3. Shape: Rhabdovirusvirionis rodlikewith the following features:

I.Envelop
II.Helical nucleocapsid
III.Genome: single-stranded RNA minus polarity
IV.Size of the genome ranged between 11 –15 kb.

Question 4

General characteristics of the (-)ssRNAviruses

replication

Answer 4

4. Replication: Rhabdovirusescan replicate in a variety of host types especially the plant rhabdovirusescapable of replicating both in their insect vectors and plant hosts (circulative viruses

Question 5

General characteristics of the (-)ssRNAviruses

disease

Answer 5

5. Disease: Rhabdovirusescause diseases such as:

• Rabies,
• Vesicular stomatitis
• Yellow dwarf of potat

Question 6

Important rhabdovirues

rabies

Answer 6

1.Rabies virus

•Are capable of infecting a variety of mammalian hosts in the wild

.•Lab strains have been found to infect birds as well as cell cultures containing birds, insects and mammals.

•Examples of mammalian hosts include: humans, dogs, fox, racoon, bat, cat.

Question 7

Transmission of rabies virus to host

Answer 7

• Rabies virus is mainly transmitted through saliva gaining access to the neurons of a host cell through an injured skin.
• The virus spread to other neurons in the central nervous system (CNS), then to the cells of the salivary glands where they can be transmitted to a new host through saliva

Figure 2: Transmission of rabies virus.The rabies virus gains access through the damaged skin of the host where it get into the neurons through the nerve endings. The virus replicate in the cell body of the neuron (site containing nucleus and lysosomes where synthesis and degradation of neuronal proteins and membranes occur). It spread to other neurons , then to the CNS and the salivary gland where it is transmitted to a new hos

Question 8

Epidemiology of rabies virus to man

dog bite

Answer 8

: Most human infection by rabies virus are caused by dog bites from rabid dog

Question 9

Epidemiology of rabies virus to man

bats

Answer 9

: rare cases of rabies in humans have been recorded from the bite of bats which could be fata

Question 10

Epidemiology of rabies virus to man

organ transplant

Answer 10

: However, a few cases of rabies in humans arise from organ transplant from an infected person

Question 11

Epidemiology of rabies virus to man

Answer 11

• Each year rabies virus result in the death of a good number of humans and other mammals (dogs, cattle etc).
• Rabies virus is very fatal in humans with high mortality rate
• The virus is estimated to kill about 55,000 humans annua

Question 12

Control of rabies virus (vaccines)

Answer 12

• Several vaccines have been developed against rabies virus to prevent human and animal infection.
• Attenuated vaccine: One of the earliest vaccine to be developed against rabies.
• Recombinant vaccine: Incorporate the recombinant vacciniavirus that expresses the rabies virus G protein

Humans and domestic dogs receive shots of the vaccine while for wild animals, the vaccine is incorporated into food baits and dropped from air-craft in the wild

Question 13

rhabdoviruses

(2. Vesicular stomatitis virus

Answer 13

2. Vesicular Stomatitis Virus (VSV):•Infection with the VSV caused lesions on the foot and mouth of infected animals, similar to foot and mouth virus.

• Commonly infect animals such as sheep, cattle pigsand horses.
• Immunological evidence reveal that the VSV virus could be present in some wild animals such as monkeys, bats and deer.
• Though the lifecycle of the VSV is not fully known, it has also been recovered in some insects such as, mosquitoes, sand fly and blackfly.
• Recent findings suggests the virus is transmitted by the phlebotominesand fly.
• Infection in domestic animals results in loss of meat and milk products.
• The VSV virus is common in the tropics but present in the temperate regions of the world as well.
• The VSV can be cultured in cell cultures from insects, mammals, fish, birds, which has enabled studies on the structures of the rhabdoviruses

Question 14

Rhabdovirus virion genome organization

Answer 14

•Rhabdovirusesare enveloped virionswith rod or bullet-shaped structures whose genome encodes five proteins (N, P, M, G, L). Some may encode one or more additional proteins

Question 15

Rhabdovirus virion genome organization

N

Answer 15

•The N protein (nucleoprotein): raps the RNA in the ratio of one N-protein to nine nucleotides giving rise to a nucleocapsidwith helical symmetry

Question 16

Rhabdovirus virion genome organization

P

Answer 16

•The P protein (Phosphoprotein): Is a minor virionprotein associated with ribonucleoprotein. P protein is a non enzymatic cofactor that interacts with protein N and L for virus replicatio

Question 17

Rhabdovirus virion genome organization

L

Answer 17

•The L protein (Large): It is a large protein associated with the ribonucleoproteinoccupying about 50% of the virus genome. Multifunctional protein (encoding virionpolymerase).

Question 18

Rhabdovirus virion genome organization

M

Answer 18

•The M protein (matrix): it coats the helical ribonucleoproteinforming a layer between the nucleocapsidand envelope.

Question 19

Rhabdovirus virion genome organization

G

Answer 19

•The G protein (glycoprotein): Is a trimerthat form spikes that protrude from the envelop. Plays important role in binding to host cell receptors

Question 20

Rhabdovirusattachment and entry:

Answer 20

• Rhabdovirusesgain entrance into cell by attachment and endocytosis.
• They attach to host cell surface via specific receptors on host cells leading to an invagination of the cell plasma membrane.
• The invagination first form a clathrin-coated pit which later replaced by actin as the pit deepens.
• The virus uses protein G spike proteinson it envelop to bind to cell receptors, leading to a conformational changefollowed by fusion of the virionwith the endosome membrane.
• Fusion of the virionwith the endosome membrane is followed by the release of the virionnucleocapsidinto the cell cytoplasm.
• Virus matrix protein (M) is associated with the endosome, some are released into the nucleus where it serves to inhibit the expression of cell genes.

Question 21

Transcription in rhabdoviruses

Answer 21

• Transcription begins once the virus RNA and its associated proteins (N, P, L) are released into the cytoplasm.
• Transcription is mediated by a RNA-dependent RNA polymerase activity present with four other enzymatic activity in the L-protein.
• The polymerase activity is activated when L protein form complex with P-proteins in the ratio (1L:3P).
• Rhabdovirusesencode these enzymes as they are absent in the cytoplasm

Question 22

Products of rhabdovirustranscription

Answer 22

• The first RNA synthesized is the leader RNA and the others are mRNAs.
• Prior to release, each mRNA is polyadenylatedby the Poly(A) polymerase activity of L.
• Rhabdovirusesdo not need all gene products in equal amount: They require large number of N protein to coat daughter RNAs and lesser amount of L protein.
• They regulate the synthesis of each gene by controlling the relative quantities of transcripts to be synthesized

Question 23

Translation in rhabdoviruses

where, what on, and post-translational modifications

Answer 23

• All rhabdovirusproteins are translated on free ribosomes except for G proteins which is translated on rough endoplasmic reticulum.
• G proteins (glycoprotein) and P proteins (phosphoprotein) undergo co-and post-translational modifications to incorporate glycan and phosphate groups respectivel

Question 24

Genome replication in rhabdoviruses

Answer 24

• As protein synthesis proceed, accumulated N, P, L proteins form inclusions in which RNA synthesis can occur.
• Minus-strand RNA virus genome is replicated by synthesis of it complimentary strand (+RNA).
• (-) RNA viruses produces 4-10 folds more copies of (-) RNA than genome length (+) RNA.
• Some (-)RNA are used as templates for the synthesize more (+)RNA, some as templates for more transcription while some supply the genome for progeny virions.

Question 25

Rhabdovirusassembly and cell exit

role of M and G proteins

Answer 25

• The M (matrix) protein serves several roles in virionassembly.
• M protein condenses the nucleocapsidinto a tightly coiled helix and links it with a region of the plasma membrane where copies of G proteins have been incorporated.
• Virionsare released through buddingfrom the region of plasma membrane with inserted G proteins, each acquiring an envelope in the process.

Question 26

Rhabdovirusinhibition of host gene expression

Answer 26

• Virus M protein plays important role in inhibiting host gene expression.
• Reasons are:

1. M protein have been observed to inhibit transcription of all three RNA polymerases.
2. M protein have been observed to inhibit translation of cell proteins. E.g. M protein have been reported to inhibit the synthesis of interferon.

Question 27

Rhabdovirusesand other minus-strand RNA viruseswith ambisensegenome

Answer 27

Minus-strand RNA viruses could have segmented or non-segmented genome(-)RNA viruses with segmented genome are rabies, measles, Ebola.Segmented genome could be ambisense(Lassa fever virus) or non-ambiscense(influenza virus).Ambiscenceimplies that one or more segment of the RNA virus genome could encode two genes (one in the Plus-sense and the second in the minus sens

Question 28

Measles virus

Answer 28

• Are (-)ssRNAvirus in the family Paramyxoviridae.
• Enveloped virus with icosahedral shape.
• The virus resides in the throat mucous and nose of an infected person.
• Virus released through coughing, sneezing can last for up to two hours in the open air.
• Transmission is through airborne and fomite and contamination of mouth, nose and eyes.
• Highly contagious virus which can infect up to 90% of non-immune persons in contaminated area.
• Transmission can occur four days before and after a rash appears.
• Symptoms include, high fever, cough runny nose, red, watery eyes and rashes around the neck, arms, trunk, legs and feet

.•The virus can be prevented through vaccine (MMR

Question 29

Influenza (flu) virus

family

types

epidemology

Answer 29

• Are (-)ssRNAvirus in the family Orthomyxoviridae
• Has an icosahedral-shaped capsid.
• There are two main types (A,B,C and D).
• Causes infection of the respiratory system such as nose, throat and lungs.
• Causes the death of about 290,000-650,000 persons annually.
• Several types:

1. Avian (bird) flu: Caused by influenza type A virus infecting birds and animal species and rarely infect humans
2. Canine (dog) flu: Caused by influenza type A virus infecting dogs. No human infection has been recorded.
3. Swine flu: Caused by influenza type A virus infecting pigs. sporadic human infection has been recorded.
4. Human Influenza virus: Type A and B are the major cause of human illness