Ti-Plasmid

Introduction

 Agrobacterium tumefaciens is a rod shaped, gram negative Bacteria, soil born motile bacterium. This bacterium infects the parts of the plant which are in contact with soil. The genus Agrobacterium inclides four species of bacteria. They are A.tumefaciens, A.radiobacter, A.rubi, A.rhizogens. They  are pathogenic in nature and cause cancerous growth in the infected plant tissues. They infect the plants and cause tumberous growth in the infected portion. The induction of tumerous growth is due to the presence of a large sized plasmid named Ti Plasmid. Accourding to Zaenen et al. (1974) almost all the strain of A.tumefaciens contain Ti plasmid.

Agrobacterium is a bacterium which is used to produce transgenic plants because it contain tow kinds of vectors which are used in the transfer of genes of one plant to the genome of another plant:-

 1. Ti plasmid, 

 2. Ri plasmid,

                                                              Structure  

T-DNA carry genes for phytohormones, AND opine

Auxin:- The main function of auixn is to help plant grow. Auxin stimulates plant cell to elongate, and the apical meristem of a plant is one of the main places where auxin is produced.

Cytokinin:- Increased cell division by stimulating the process of mitosis. Increased formation of shoot and buds, as well as development of fruits and seeds.

Opin:- Nutritional source for agrobacteria and fall into different chemical grouping such as nopaline, Octopine, Mannopine, agrocinopine etc.

Fig:- Crown gall disease

Vir genes:-

Vir Gene / Protein	Function / Role
Vir A	Kinase protein present in the bacterial membrane; acts as a receptor for phenolic compounds released by wounded plant cells
Vir A & Vir G	Constitutively expressed; together sense and respond to phenolic compounds from wounded plants
Vir G	Acts as a transcriptional activator; activates expression of other vir genes
Vir E / Vir E2	Protects T-DNA from nucleases; targets T-DNA to plant cell; acts as SSB (single-stranded binding protein)
Vir C	Stimulates T-DNA transfer; promotes high-efficiency T-strand synthesis
Vir D (overall)	Responsible for virulence activity of the bacterium
Vir D1	Topoisomerase-like protein; helps VirD2 recognize and cleave the 25 bp Left Border (LB) sequence
Vir D2	Endonuclease; cuts T-DNA at Right Border (RB); helps in integration of T-DNA into host genome
Vir D1 (additional role)	Prevents exonuclease attack at the 5′ end of T-DNA; important for phosphodiester bond cleavage
Vir B / Vir D4	ATP-dependent protein complex; forms Type IV secretion system spanning inner and outer bacterial membranes for T-DNA transfer
Vir D2 / Vir E2	Contain Nuclear Localization Signals (NLS) for transport of T-DNA into plant nucleus
Vir F	Expressed in plant cell cytosol; interacts with VBF1 protein (involved in uncoating T-DNA complex)

 

Fig:- Ti Plasmid Used in Recombinant DNA technology

                                                               Advantage

• Occure naturally in Bacteria.
• Have different restriction sites.
• Replication completely independent of Bacteria.
• Genes are easily inserted into plasmids.
• Easily transformed into bacteria.
  
  
  
  
  

Vir Genes MCQ Quiz

1. VirA protein functions mainly as:

Transcription factor Kinase receptor for phenolic compounds Endonuclease SSB protein

2. VirG protein acts as:

Receptor protein DNA ligase Transcriptional activator ATPase

3. Which Vir protein protects T-DNA from nucleases?

VirC VirB VirE2 VirA

4. VirC gene is involved in:

T-strand synthesis enhancement Nuclear localization Phenolic recognition Protein degradation

5. Which Vir protein cleaves T-DNA at Right Border?

VirD1 VirD2 VirE2 VirF

6. VirD1 mainly assists VirD2 by acting as:

Endonuclease Topoisomerase Helicase Polymerase

7. VirB and VirD4 together form:

Ribosome complex Type IV secretion system DNA replication fork Spliceosome

8. Nuclear localization signals are present in:

VirA and VirG VirC only VirD2 and VirE2 VirB proteins

9. VirF protein functions inside:

Bacterial cytoplasm Plant cell cytosol Bacterial membrane Plant nucleus

10. Vir genes are activated in response to:

UV radiation Antibiotics Phenolic compounds from wounded plants Heat shock