description of caulimo viruses and genome of CAMV and vectors of CAMV and its advantages and disadvantages

1. CAULIMO VIRUSES
AS A
VECTOR
D.INDRAJA

2. Plant Viruses:
• Plant viruses are considered as efficient gene transfer agents as they infect
the intact plants and amplify the transferred genes through viral genome
replication
• They are non integrative vectors Eg, Pepper mint mottle virus, Leaf curl
virus
Cauliflower Mosaic Virus (Caulimovirus)
• Cauliflower mosaic virus (CamV) belongs to the group caulimovirus, can be
used as potential candidate to deliver foreign gene into the plant.
• It is perhaps the best studied viruses among plant virus, which infects several
members belonging to Cruciferae family.
• As an infective agent, can cause disease in wide range of commercially
important cultivated crops.
• Cauliflower mosaic DNA has been subjected to a wide range of
manipulation.
• This was the only and first virus to be manipulated and used as a favourable
choice for genetic engineering work.

3. CaulimoViruses
• Caulimoviruses contain circular ds DNA and are spherical in shape
• This group includes 15 viruses of which CaMV is the most important for
gene transfer
• The other caulimoviruses includes carnation etched virus, dahlia mosaic
virus mirabilis mosaic virus and strawberry vein banding virus
• CaMV is a plant virus that infects mostly brassicaceae family( cauliflower
and turnips) and Solanaceae species
• CaMV is transmitted in a non circulatory manner by aphid species Mysus
• It is a icosahedron structure with a diameter of 52nm built from 420 cap
protein subunits

4. Genome of CaMV
• It has circular ds DNA containing 8kbp
• The promoter 35S RNA is a very strong constitutive promoter responsible
for the transcription of the whole CaMV genome.
• It is well known for its use in plant transformation.
• It causes high levels of gene expression in dicot plants
• The 35S RNA is particularly complex, containing a highly structured 600
nucleotide long leader sequence .
• This leader is followed by seven tightly arranged, longer ORFs that encode
all the viral proteins.
• The mechanism of expression of these proteins is unique, in that the ORF VI
protein (encoded by the 19S RNA) controls translation reinitiation of major
open reading frames on the polycistronic 35S RNA, a process that normally
only happens on bacterial mRNAs

5. Criteria needed for vector:
• The virus must be capable of spreading from cell to cell through
plasmodesmata
• The viral genome should be able to replicate in the absence of viral coat
protein and spreads from cell to cell
•Elicit little or no disease symptoms
• Should have broad range of host

6. CamV Vector:
• Cauliflower mosaic virus can be used as a potential vector due to the
infective nature of its genetic material.
• This could be proved by applying viruses on the leaf rubbed with abrasive
material. The CamV cannot accommodate foreign DNA, if the size exceeds
its normal size.
• The inserted DNA may destabilize infectious nature of the virus.
• Other constraints are the packaging of genome and limitation of the insertion
of foreign DNA.
• Despite the marginal constraints, CamV genome can be packed in
nucleosome and is able to undergo transcription by plant RNA polymerase
II.
• For effective transmission of CaMV the foreign DNA must be encapsulated
in viral protein and the inserted gene should not interfere with native
assembly of virus
• CaMV does not contain any non coding region wherein foreign DNA can be
inserted

7. • The genome of CamV consists of six major and two minor open reading
frames (ORF), in tightly packed arrangements.
• The two ORF regions, one (ORFII) codes for insect transmission factor and
other (ORF VII) with unknown functions can be replaced with gene of
interest.
• So the gene of interest can be replaced in the place of these two genes
• Use of CaMV as a vector pertains to the bacterial dhfr (dihydroxy folate
reductase) gene inserted in the place of gene II and successfully expressed in
plants
• This dhfr gene is needed for providing resistance to methotrexate( inhibitor
of dhfr and extremely toxic to plants)
Limitation of CaMV:
• Limited capacity for insertion
• Infective capacity is lost if few hundreds of nucleotides are introduced
• Because of its exceeding natural genome size they are not effectively
suitable for gene transfer