Flashcards in lecture 11 Deck (28)
Why is studying the pathogen of an organism helpful to studying the biology of that organism?
- pathogens, particularly those that are really well adapted to an organism, understand the organisms biology exquisitely
- e.g. "alleyway muggers" vs "white collar criminals" such as Agrobacterium
- studying human biology by studying its pathogens/plant biology by its pathogens, is a great way to learn how an organism functions
- it is the pathogens that know the organism better than anything else
What/how many GM plants are different countries growing?
2012: over 5
- Australia: <1 million hectares; cotton, canola
- US: 70 million hectares; papaya (hawaii)
- Canada: 10.8 million hectares
- South america (e.g. Brazil and Argentina): soy beans
- Some european countries e.g. portugal and spain
What is one of the fastest adopted agricultural technologies in history?
- initially industrial countries were the first to adopt it while developing countries lagged
- 2012 developing countries overtook industrial in terms of GM crops being grown
- a lot of that by small farmers (couple of hectares or less)
- china and india etc
- 1%/small changes in agricultural production feed millions of people — don't need to make huge changes to make a difference as in other areas of medical research
What are the current crops that are being transformed and grown at a large scale?
Are transgenic animals as widely used?
- GM crops very important while GM animals generally only used in research
What is a plant cancer?
- called 'crown gall disease' because tumours typically form at the junction between root and stem (the crown)
- huge mass of undifferentiated cells
- major problem for grapes, (almonds, cherries and so on) and fruit trees; galls can form on a wide range of dicots and some monocots (e.g., asparagus)
What is a gall?
- large mass of undifferentiated cells
- can also have ones where there is organisation
- 'shooty' gall - leaf-like identity of gall cells
- 'rooty' gall
- all caused by the same thing but different manifestations
Why do we get different manifestations of gall?
- different strains of the bacteria that cause the disease
What did Erwin Smith show?
- Erwin Smith showed that a bacterium 'Bacterium tumefaciens' caused galls on many plants
- "when minced galls are buried in the earth near the roots of sound trees, the latter develops galls. The disease is therefore a communicable one"
What is Bacterium tumefaciens?
gram negative bacterium found commonly in the soil
What has since been shown about crown gall disease?
- galls can be removed from the plant and grown in culture without added hormones (auxin and cytokinin)
- different Agrobacterium strains produce galls that look different (rooty vs shooty)
- galls make low molecular weight compounds — opines. Different strains produce galls that make different opines.
- Each Agrobacterium strain can grow on its own opine
- Galls do not need bacteria after initial infection ('tumour inducing principle')
What are opines?
- there are many different opines
- the two most studied are:
-- octopine — carbonyl compound is pyruvic acid
-- nopaline — carbonyl compound is alpha-keto glutaric acid
- combination of an amino acid (typically arginine) and a carbonyl compound (or in some cases a sugar)
- carbonyl compound typically keto acids
What happened in 1971?
- US president Richard Nixon declared war on cancer
- stimulated research on crown gall as a model for human cancer
Was crown gall a good model for studying human cancer?
What was the 'tumour inducing principle'?
- transfer of the DNA sitting in the agrobacterium into a plant cell
- that DNA got called the transfer DNA/T-DNA
- bacterium that had worked out how to transform a plant by introducing DNA into the plant
What is the biology of crown gall?
- two basic types of agrobacterium sitting in the soil: those which are infectious and those which aren't
- the only difference between them is whether they have the Ti Plasmid
- within that plasmid is a region of DNA called the T-DNA
- senses wounded plants and migrates to hunker very close to them
- closely related to E. coli
- transfer T-DNA region from the bacterial cell to the plant cell - becomes incorporated in the plants DNA
- causes a dysregulation of hormone production
- starts to produce hormones cytokines and auxin which are necessary for cell division
- cells reenter the cell cycle
- also synthesising opines in large amounts
- this is used as a food by agrobacterium (can only live on its own type of opine)
- Ti Plasmid is the site where opines are metabolised back into food
- also conjugation of bacteria - transfer of plasmid
How does Agrobacterium infection occur?
1. sensing and chemotaxis: wound signals, flagellated bacteria
2. attachment, intimate contact
3. T-DNA transfer and integration
How was the infection process of Agrobacterium investigated?
- using the tools of bacterial mutagenesis e.g. transposon mutagenesis
- integration into the bacterial genome of a transposon that affects one of the three steps of infection
- Tn5 inserted into a DNA region of a plasmid inserted into E. Coli
- Plasmid carries a certain antibiotic resistance (kanamycin)
- agrobacterium has rifamycin resistance
- try to get Tn5 to migrate from E. coli to Agrobacterium
- at the end of the experiment select for cells that have Kanamycin resistance (meaning they have Tn5) and Rif resistance (meaning they have the agrobacterium chromosomes)
- take this bacterium and see if it can infect a plant
- if not it means one of these processes has been affected
- test all Kan^r/Rif^r bacteria for ability to form a crown gall
- select all that are avirulent
- test for site of Tn5 insertion — most are in the bacterial chromosome and affect attachment to plant surface
-- the cel locus codes for a gene involved in cellulose synthesis (coat made of cellulose that enables close attachment)
-- the att locus encodes cell surface proteins involved in attachment
How is the Ti plasmid involved in tumour formation?
- 200-250 kb in size
- two regions are associated with tumour formation
-- the T-DNA region (tumour inducing principle)
-- the Vir region (in opine synthesis region)
- approximately half the plasmid is involved in opine synthesis and half opine degradation
What is the role of the Vir region in the agrobacterium Ti Plasmid?
- A wounded plant releases chemicals: sugars from damaged cells and a compound called acetosyringone (phenolic)
- trying to block the wound site
- these sorts of compounds are sensed by a protein on the bacterial membrane: a chemosensor
- also a kinase on the inner surface
- substrate for the kinase is a protein called VirG (a gene regulator/transcription factor)
- activated once phosphorylated
- now capable of going and binding to promoter regions for all the other genes in the Vir region and activating them
What are the different genes and their functions in the Vir region?
Locus/no. of genes/function:
- A / 1 / environmental sensory
- G / 1 / transcription factor
- C / 2 / T-DNA processing
- D / 5 / T-DNA processing
- E / 2 / T-DNA processing
- B / 11 / transmembrane pore
- F / 1 / F-box protein
- overall focus is to get T-DNA into a plant cell
- produce only one T-DNA strand
What is the T-DNA region?
- T-DNA has no genes involved with excision, transfer or integration into host DNA
- left and right borders - imperfect 25 bp repeats that define the transferred segment
- relies entirely upon the Vir region for its own transfer
- integration into the host DNA is done by proteins/genes found in the host cell
What basically happens in the transfer of a T-DNA strand?
- single-stranded nicking of T-DNA borders
- endogenous repair mechanisms will come along and try to repair the strand causes displacement of T-strand — unwinding of T-strand, DNA synthesis
- T-strand is ss-DNA
- Formation of a pore in the bacterial membrane
- Unknown how the T-strand gets across the plant cell wall/membrane/nuclear membrane
- also transfers a protein across called VirF
What are the major proteins involved in the process that produces a T-strand?
- VirD1 binds to borders and recruits VirD2 'relaxase'
- VirD2 'nicks at borders and covalently attaches to 5' end
- other bacterial enzymes responsible for DNA synthesis etc
- VirE2 is a ssDNA binding protein and coats/protects the ss T-strand
- pore formed by VirB
- nuclear localisation signals on VirB target T-strand to nuclear membrane of plant cell/take it into the nucleus
What is involved in the transfer of the T-complex?
- T-strand covered with VirE2 and with VirD2 covalently attached to its 5' end
- Transfer by a type IV secretion pathway that translocates T-DNA and associated proteins across the cell envelope
- Requires at least one host protein (particularly for attachment)
- Type IV secretion is often associated with pathogenesis (e.g. Helicobacter pylori)
- pore complex formed from VirB1-VirB11
- VirD4 involved in recruiting DNA-protein complexes for translocation
- either acting as a syringe or as a pore (unsure)
How do we get T-complex integration?
T-strand covered with VirE2 and VirD2 covalently attached to its 5' end
- protection and transport through cytoplasm to nucleus (nuclear localisation signals - NLS)
- subsequent steps largely depend on host cell proteins and processes
- integration into genome shows a preference for insertion into transcribed regions
What does VirF do?
- T-strand along VirF injected into plant cytoplasm
- T-strand and VirF move into nucleus
- Inside nucleus VirF associates with host cell proteins to produce an E3 ligase that selectively adds ubiquitin to VirE2
- this step is necessary for chromosomal integration of the T-strand
- i.e. gives the proteins a means for getting rid of the proteins which are now unnecessary/inhibitory