Viriods, Virophages, and Prions Flashcards
What is a Viroid
•Viriodsare very tiny particles with covalently closed, single stranded circular RNA.
do viroids have a capsid
•The RNA of viriodsare not protected by a protein coat (capsid
Viroid nomenclature
- The name viriodswas coined from their similarity with viruses;•Viriods= Vir(Virus) + iod(like
- Thus, viriodsare regarded as subviruse
Viroid genome
introduction
•Viriodgenome do not encode proteins, but are capable of independent replication in hosts leading to disease conditions.
What do viroids infect
•Viriodsare so far known to infect mainly plants, but one species have been found to cause Hepatitis D in humans
Viroid RNA length
•The length of the viriodRNA varies between 246 -434 nucleotide
Discovery of viroids
first discovered by theodor Diener from infected potatos in 1976
thought to be a virus, bout would not pellet after centrigation
the virus-like agent was found to be sensitive to RNA, but insensitie to DNA, chloroform, ethanol and phenol
Viroids as the potential origin of life
- Recently, a group of scientist proposed that viriods may have been the simpliest life form since they exist as RNA without a protein coat (Moelling and Broecker 2021).
- They are the smallest of life forms capable of replication which have undergone Darwinian evolution.
- Thus, since early life forms existed as simple forms, viriods are likely the first form of life on earth since they aresimple both in structure and function and do not code for protein
What are the two families of viroids
- Pospiviriodae
2. Avsunvirioda
Pospiviriodae
Pospiviriodae:
Viroid
The members of this group are named after the potato spindle tuber viriod.
Members have rod-like shape, a small single stranded region and a central conserved region.
They replicate in the nucleus, where their RNA is replicated by plant RNA polymerase II.
The ends of the synthesized RNA are ligated to form a circular RNA
- Avsunviriodae
•2. Avsunviriodae:
Viroid
Members are rod-like and named after the avocado sunblotchviriod.
- Members replicate their RNA in the chloroplast as theylack a conserved region.
- ViriodRNA are transported to the chloroplast where concatemericRNAs are synthesized using the chloroplast-dependent RNA polymersase.
- Cleavage of the synthesized viriodRNA is achieved by the viriodencoded ribozymes
Viroids as plant pathogens
- Generally, viriodsare pathogen of plants with great economic importance in agriculture industry.
- More than 30 viriodshave been identified; 28 were found to infect the dicotyledons(plants with two seed leaf), while 2 infect the monocotyledons (plants with one seedleaf).
examples of viroids as plant pathogens
- Coconut cadang-cadangviriod: causes a deadly infection in coconut trees
- Apple scar skin viriod: Infections in apple trees results in unappealing fruits.
- Avocado sunblotchviriod(ASBV): Infects avocado plants
- Chrysanthemum stunt viriod(CSV): causes chrysanthemum stu
Viriodattachment and entry
•Viriodattachment and entry is mediated by viriodbinding proteins (Virp1
viroid RNA and proteins
•viriodRNA isn’t known to encode any protein: the AUG initiation codon is absent in some specie
Viroid enzymes (for replication)
•Enzymes involved in viriodreplication are; RNA polymerase, RNA cleavage and RNA ligase
Viroid autonomous replication
•Viriodsundergo autonomous replication in host cells, involving RNA polymerase II, an enzyme which play a role in transcription into mRNA in most cellular organisms.
Viroid RNA replication
•RNA replication mechanism involves a “rolling circle”, resulting in the synthesis of a concatemeric, linear (+) strand RNA
Viroid production of progeny RNA
•Progeny RNA is produced through cleavage into unit length monomers that are ligated into circle
Transmission fo viriods
. Cell to cell transmission of progeny viriodin plant cells occur through;
I) Plasmodesmata(channel between two adjacent cells through the cell wall).
II) Phloem: Progeny viriodscan be transported through the phloem to infect new cells.
From these cells, viriodspenetrate the pollen, ovule and seed. Young plant arising from an infected seed becomes infected with the viriod.
New infection mechanisms of Viroids
New infection can occur through:
Viriodcontaminated seeds, cutting and tubers
Viriodcontaminated equipment and implements
Insect vectors (aphids
Signs of plants infected with viroids
- The most common symptoms of viriod infected plants are:
- stunted growth,
- deformation of leaves and fruits,
- stem necrosis,
- death of plant (final resort)
Prevention and Control of Viriods
- Use of chemical disinfectants in the field to spray infected plants such as:
- 1 -5% sodium hypochloride
- 2% formaldehyde
- 6% hydrogen peroxide
- Also, these chemicals can inactivated viriods. Thus, can be used to disinfect farm equipment and implements to avoid contaminating plan
Introduction to virophages
- Virophagesare viruses capable of infecting other larger (giant) viruses-infecting host for the derivation of some its needed replication machinery as well as parasitizes the helper virus.
- Thus, virophagesare obligate parasites.
- Similar to satellite viruses, but virophagesmostly are parasitic to the helper virus (large virus) resulting in death in most case