Team:Toulouse/project/attract

iGEM Toulouse 2015

Attract

Content

About varroasis

Varroasis occurs with the varroa entrance in the hive, carried by infected bees: the mite can begin its parasitism and attack the brood. When the queen gives birth to new larvaes in honeycombs, the fertilized adult female varroa mite will come into it before capping, and lay her eggs. The larvaes will develop, increasing the overall infection that affects bee population [1]. To tackle this issue, it is necessary to attract varroa carried by honeybees before they come into the hive.

Figure 1 : Varroa destructor life cycle, adapted from B. Alexander

How to attract varroa

Just before capping, bee larvaes produce a wide range of molecules, those molecules warn the mite about the upcoming capping and motivate it to enter the cell [2]. Of all these molecule, scientific studies have shown that one can significantly attract varroa: butyrate [3].

Butyrate is a volatile acid which is non-toxic for honeybees nor the human being, because it is already present at physiologic concentrations in the digestive tract. Moreover this molecule is naturally produced by some bacterial strains like Clostridium, which is an asset for this synthetic biology project [4].

Therefore we decided to modify Apicoli so it will synthesize butyrate in order to attract varroa.

Figure 2: Results of butyrate attraction test with quadrants method

Butyrate attraction test

Figure 3: Butyrate attraction test using T tube, with varroa mite in the middle

To check adequacy and relevance of this study (Figure 2), an experiment using a T-tube has been developed (Figure 3). In the first branch, there is a cotton soaked with 50 µL of water, in the second a cotton with 50 µL of butyrate at 4%, and finally the last one contains the varroa.

The butyrate being very volatile, our system used a pump to renew air, producing a concentration gradient.

How to produce butyrate with E.coli?

In this project, an Escherichia coli strain is used for its known simplicity of genetic manipulation and its adequacy with butyrate synthesis. Indeed, among the five enzymes of the butyrate pathway, two enzymes are naturally produced by the bacteria. The following engineered butyrate pathway has been designed:

Figure 4: Engineered butyrate pathway

The initial substrate is glucose which is decomposed into acetyl-CoA during glycolysis. Finally, butyrate pathway begin with acetyl-CoA: five genes are required with two homologous and three heterologous genes.

atoB present in E.coli, coding for acetyl-CoA acetyltransferase, an acetyltransferase catalyzing the combination of two acetyl-CoA.

Figure 5: Reaction catalyzed by acetyl-CoA acetyltransferase
hbd present in Clostridium acetobutylicum coding for 3-hydroxybutyryl-CoA dehydrogenase, an oxidoreductase catalyzing the formation of an alcohol function.

Figure 6: Reaction catalyzed by 3-hydroxybutyryl-CoA dehydrogenase
crt present in C.acetobutylicum coding for 3-hydroxybutyryl-CoA dehydratase, a lyase cleaving carbon-oxygen bond.

Figure 7: Reaction catalyzed by 3-hydroxybutyryl-CoA deshydratase
ccr present in Streptomyces collinus coding for crotonyl-CoA reductase, an oxidoreductase acting on CH=CH double bond. This enzyme is also in C.acetobutylicum with bcd gene coding for butyryl-CoA dehydrogenase, with the disadvantage to run with Electron Transfer Flavoprotein (ETF) which complicates the reaction [6].

Figure 8: Reaction catalyzed by crotonyl-CoA reductase
tesB present in E.coli coding for acyl-CoA transferase 2, a thiolase which enables coenzyme A transfer.

Figure 9: Reaction catalyzed by acyl-CoA transferase 2

References

[1] Boecking O, Genersch E. 2008. Varroosis – the Ongoing Crisis in Bee Keeping. J. Verbr. Lebensm. 3:221–228.
[2] Le Conte Y, Arnold G, Trouiller J, Masson C, Chappe B, Ourisson G. 1989. Attraction of the parasitic mite varroa to the drone larvae of honey bees by simple aliphatic esters. Science 245:638–639.
[3] Methods for attracting honey bee parasitic mites. [accessed 2015 Jul 24].
[4] Louis P, Flint HJ. 2009. Diversity, metabolism and microbial ecology of butyrate-producing bacteria from the human large intestine. FEMS Microbiol. Lett. 294:1–8.
[6] Wallace KK, Bao Z-Y, Dai H, Digate R, Schuler G, Speedie MK, Reynolds KA. 1995. Purification of Crotonyl-CoA Reductase from Streptomyces collinus and Cloning, Sequencing and Expression of the Corresponding Gene in Escherichia coli. European Journal of Biochemistry 233:954–962.