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Revision as of 21:04, 11 August 2015

IGEM Bordeaux 2015

Our Solution

What is Curdlan?

To start with, let's talk about glucans. Glucan molecules are polysaccharides of D-glucose monomers linked by glycosidic bonds. One of them is called Curdlan, a (1→3)-β-D-glucan. This molecule is a linear homopolymer which may have as many as 12,000 glucose units. It is naturally produced by Agrobacterium sp. ATCC31749 which uses it as an Extracellular PolySaccharides (EPS) in it's capsule (REF). The capsule formation is correlated with cell aggregation (floc formation) and it is suggested that the capsule and floc formation together function as protective structures in cases of Nitrogen-starvation of the post-stationary phase. The protective effects are due to the fact that Curdlan forms a capsule that completely surrounds the outer cell surface of bacteria.

« Ok and how will Curdlan be useful to you? »


« Let me explain our purpose. »


Figure 4: Representation of the Curdlan molecule showing the beta 1,3 links between the glucose units



Curdlan belongs to the class of biological response modifiers that enhance or restore normal immune defenses including antitumor, anti-infective, anti-inflammatory, and anticoagulant activities. (REF) As a matter of fact, this β1,3 glucan can stimulate the plant's immune system.

More precisely, applied to grapevine plants, sulfated Curdlan induces the accumulation of phytoalexins (organic antimicrobial substances) and the expression of a set of Pathogenesis-Related proteins .

However, non-sulfated Curdlan doesn't trigger the hypersensitive response characterized by the rapid death of cells in the local region surrounding an infection, avoiding a complete contamination of the plant. This response has been studied in Arabidopsis thaliana through a mutant gene: pmr4 . This mutant is resistant to mildew infections but is unable to induce Pathogenesis-Related proteins expression .

Also, activation of a Pathogenesis-Related protein called PR1 in grapevine is regulated by the salicylic acid signaling pathway . The lack of PR1 expression in non-sulfated Curdlan-treated grapevine could be explained by a negative feedback of glucan. This is demonstrated by the study of a double mutant of pmr4 which restore the susceptibility to mildew. It suggests that linear β-1,3 glucan negatively regulates the salicylic acid pathway. So, sulfation of the glucan would counteract the negative feedback effect.

To conclude, activation of the innate immune system before the invasion of pathogens is a way to improve the resistance of plant against infection and to reduce the use of chemicals products.


« Ok i understand the value of producing Curdlan. How will you proceed? »


« This is a good question i'll answer. »


Before starting the project, we took a few weeks to decide which host organism we would use and how they could be useful. To begin with we looked at three different organisms: Escherichia coli , Bacillus subtilis and Saccharomyces cerevisiae and compared their glucan metabolic pathways. We rapidly eliminated Bacillus subtilis from our possible hosts due to it's lack of enzymes involved in the metabolic pathway of beta 1,3 glucans. However, we found that yeast naturally produces Curdlan in it's cell wall, like Agrobacterium . Furthermore, Escherichia coli is only missing one enzyme (the Beta glucan synthase) to synthethize Curdlan. We therefore concluded that we could keep these two organisms: one where we would overexpress the beta 1,3 glucan synthase using a constititive promoter and one where we would insert the ability to create curdlan by adding the enzyme that is needed.






Figure 5: Schematic representation of the beta 1,3 glucan synthetic pathway (REF)



In Agrobacterium , three genes (crdA, crdS and crdC) are required for Curdlan production. The putative operon crdASC contains crdS, encoding β-(1,3)-glucan synthase catalytic subunit, flanked by two additional genes : crdA and crdC. The first assists translocation of the nascent polymer across the cytoplasmic membrane and the second assists the passage of the nascent polymer across the periplasm. However, all Curdlan biosynthesis is dependent of nitrogen starvation and various parameters. We want to simplify all of this.


Using Bacteria: Escherichia coli

Firstly, we decided to produce curdlan with Escherichia coli , because Agrobacterium and it are Gram negative bacteria and have a lot of membrane similarity. Moreover Escherichia coli is a simple pretty good bacteria, it can be grown and cultured easily and inexpensively in a laboratory setting unlike Agrobacterium .

Secondly, we are going to put these three genes into Escherichia coli under an easier control than N-starvation by using a constitutive promoter.

Thirdly, when we produce curdlan with Escherichia coli , we will sulfate it with a chemical method. In fact, sulfated Curdlan is a better form of Curdlan to activate the plant's immune system.

To sum it up, we would like to produce Curdlan in Escherichia coli and then sulfate it to use it as a preventive treatment for the vine against the mildew infection and continue to produce good wine and make everyone happy.

Figure 6: strains of agrobacterium producing curdlan (green) observed with a microscope using flurescence (???) SOURCE

Using Yeast: Saccharomyces cerevisiae

Yeast cell walls are made up of various layers which are represented in the following diagram. First there is a layer of chitin, then a layer of beta glucans and finally a mixed layer of proteins and mannan. Commonly, the yeast cell wall is made of 5-10% of beta 1,6 glucans and 50-55% of beta 1,3 glucans and beta 1,6 glucans.

Figure 5: Schematic representation of the yeast's cell wall

Since the layer of mannan and proteins as well as chitin is insoluble in alkali solutions, beta glucans are easily separated from the rest of the yeast cell wall. Therefore, the only alkali soluble components are a mix of beta 1,6 and beta 1,3 glucans. (aimanianda et al 2009) In order to separate the two we plan on using beta 1,6 glucanases in order to obtain a solution of beta 1,3 glucans and therefore our curdlan molecule.

We therefore decided to over-express the curdlan metabolic pathway by inserting into yeast an inducible promoter (gal1) for the glucan synthase gene (Fks1) hoping that this would allow the cell to produce curdlan in greater quantities. This would allow us to compare our curdlan production in E. coli to the natural production in an organism and the enhanced production through the addition of a promoter.
To do this , we will extract the FKS1 gene from yeast DNA and amplify it by PCR. We will then insert FKS1 in one hand, into plasmid pYES2 to integrate the modified plasmid in Saccharomyces cerevisiae and boost production of curdlan. On the other hand, we will integrate the FKS1 gene into the iGEM plasmid to get our famous BioBrick that we'll send to Boston. However, site-directed mutagenesis may be necessary when integrating the gene into the plasmid because there are restriction sites ( EX and SP) unwanted within the gene.



« It's very clear now. It looks like cool. But, i have a last question: why did you choose this subject? »


« Because, as explained on the problem page, downy mildew is a real problem for the Aquitaine region. This is why we wanted to bring an ecological solution to this problem. And also, we are SWAG (Secretly We Adore Glucan) »


Other useful properties of Curdlan

Curdlan belongs to the class of biological response modifiers that enhance or restore normal immune defenses, including antitumor, anti-infective, anti-inflammatory, and anticoagulant activities. CrdS is an integral inner membrane protein with seven transmembrane (TM) helices, one non-membrane-spanning amphipathic helix and a Nout–Cin disposition

Sulfation of Curdlan. Acquired immunodeficiency syndrome (AIDS) caused by human immunodeficiency virus (HIV) is a severe disease that can destroy the body’s immune system, so the discovery of methods to prevent AIDS infection is of great importance. All the other curdlan clinical applications in cancer, diabetes, hypertension, hypertriglyceridemia etc. are listed here.

Curdlan is also neutral and insoluble in water. If it is heated in aqueous suspension , it adopts simple helical conformations ( 55-80 ° C) or triple helical connected ( 80-130 ° C). It then acts as a gelling and form two types of gels (low-set gel or high-set gel) . This property is widely used in the food industry since Indeed, curdlan is a food additive ( E424 ).







Reference : www.link.springer.com


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