FBA

In order to choose the best Yarrowia lipolytica strain to produce rare fatty acids such as jacaric acid and punicic acid, we have performed metabolic simulations, also called Flux Balance Analysis (FBA), on several known strains of Y. lipolytica.

First, we have got an improved version of iYali4 [1], a metabolic model of Yarrowia lipolytica W29, on GitHub.

We have used the Python package COBRApy [2] to add a simplified version of the reaction transforming linoleate into jacaric acid. By clicking below, you can download the original model and the code we have created to add the reaction.

• Code
• Model

Then, we have performed an FBA on this model, which correspond to the JMY195 [3] strain, with the software OptFlux [4]. This simulation gave us a biomass value of 29.5 and a jacaric acid production of 226.54. These values are specific to the software and cannot be linked to a real yield. However, these values allow us to compare different strains. We have constructed models of the strains JMY1877 [5], JMY1233 [6], JMY2159 [7], JMY3325 [8] and JMY3820 [9] and performed FBA on them with the OptFlux tool “Gene Under-Over Expression Simulation”. In such models, the normal value for gene expression in set to 1. To knockout genes in the models, we have set this value to 0, and we have set it to 10 to simulate an over-expression. The FBA results are presented in Table 1.

Y. lipolytica strain	Mutations	Biomass		Jacaric acid
JMY195	Po1d (Ura3-, Leu2-, Xpr2-)	29.50	100.0%	226.54	100.0%
JMY1233	Po1d, pox1-6Δ	29.50	100.0%	226.54	100.0%
JMY1877	Po1d, dga1Δ dga2Δ lro1Δ are1Δ	0.00	0.0%	0.00	0.0%
JMY2159	Po1d, pox1-6Δ dga1Δ dga2Δ lro1Δ fad2Δ	0.00	0.0%	0.00	0.0%
JMY3325	Po1d, pox1-6Δ dga1Δ dga2Δ lro1Δ fad2Δ, pTEF-FAD2-LEU2	0.00	0.0%	0.00	0.0%
JMY3820	Po1d, pox1-6Δ tgl4Δ, pTEF-DGA2, pTEF-GPD1	29.50	100.0%	386.41	170.6%

For JMY1877, JMY2159 and JMY3325, the results are not relevant since their models were unable to show any growth. This is certainly due to a lack of genes in the original model, which make essential some genes that we have deleted. Indeed, when we have determined critical genes with OptFlux, are1 appeared to be essential, even if it is not in real life. About JMY2159 and JMY3325, it is likely that dga1, dga2 and lro1 are essential when deleted together, which again is not the case.

For JMY1233, we see that both the biomass and the jacaric acid production have the same value than for the control JMY195.

Finally, the FBA indicates that JMY3820 is able to produce 70% more jacaric acid than JMY195, without changing the biomass production.

These results suppose that JMY3820 is the best strain to produce jacaric acid and punicic acid, which is also made from linoleate, and we kept that in mind during the wet lab part. Happily, this has been confirmed by the experiments.

To go further, we have also tried to determine new genes that could be deleted to optimize the production of jacaric acid with Yarrowia lipolytica. To do so, we have used the evolutionary optimization tool of OtpFlux on JMY195 and set the minimum biomass production to 95% of the control. As results, this tool gave us genes or sets of genes of which deletion increases the jacaric acid production to 1,000, which is the maximum potential value of the reaction. These genes are presented in Table 2. The solutions proposed by this tool should always be manually verified since a model does not represent the exact reality and could propose absurd deletions.

Gene	Occurrence in solution
YALI0D00583g	10
YALI0F15631g	7
YALI0E33517g	4
YALI0F24475g	4
YALI0E25740g	4
YALI0F01210g	4
YALI0F26323g	3
YALI0E18568g	3
YALI0A05379g	2
YALI0B09647g	2
YALI0B22682g	2
YALI0E33099g	2
YALI0C05258g	1
YALI0D04741g	1
YALI0D10131g	1
YALI0E31009g	1
YALI0F14025g	1
YALI0E31471g	1
YALI0D24750g	1
YALI0E24013g	1
YALI0C11407g	1

YALI0D00583g, YALI0F15631g and YALI0F24475g are coding for a proton-transporting protein responsible for the vacuolar acidification; YALI0E33517g is coding for an oxoglutarate dehydrogenase involved in the Krebs cycle; YALI0E25740g is coding for a guanine deaminase involved in the reaction EC:3.5.4.3; YALI0F01210g is coding for a membrane water transporter; etc.

Interestingly, YALI0F14025g is the only gene that was not proposed in a set a gene. The only deletion of this gene is able to increase the production of jacaric acid to its maximum. This gene is coding for a kinase that is involved in the inositol-phosphate biosynthesis.

None of the deletions that have been performed in tested strains, including tgl4 which is deleted in JMY3820, was proposed by OptFlux.

We hope that this work would help future studies working on optimizing Yarrowia lipolytica for rare fatty acids production.

References

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