Modulation of the balance of fatty acid production and secretion is crucial for enhancement of growth and productivity of the engineered mutant of the cyanobacterium Synechococcus elongatus.

Akihiro Kato,Tatsuo Omata,Makiko Aichi,Kazuhide Use,Nobuyuki Takatani,Kouji Kojima,Kazutaka Ikeda,Miyuki Matsuura,Shin-Ichi Maeda

doi:10.1186/s13068-016-0506-1

Abstract

BackgroundAmong the three model cyanobacterial species that have been used for engineering a system for photosynthetic production of free fatty acids (FFAs), Synechococcus elongatus PCC7942 has been the least successful; the FFA-excreting mutants constructed from this strain could attain lower rates of FFA excretion and lower final FFA concentrations than the mutants constructed from Synechocystis sp. PCC6803 and Synechococcus sp. PCC7002. It has been suggested that S. elongatus PCC7942 cells suffer from toxicity of FFA, but the cause of the low productivity has remained to be determined.ResultsBy modulating the expression level of the acyl–acyl carrier protein thioesterase and raising the light intensity during cultivation, FFA secretion rates comparable to those obtained with the other cyanobacterial species were attained with an engineered Synechococcus elongatus mutant (dAS1T). The final FFA concentration in the external medium was also higher than previously reported for other S. elongatus mutants. However, about 85 % of the total FFA in the culture was found to remain in the cells, causing severe photoinhibition. Targeted inactivation of the wzt gene in dAS1T, which gene manipulation was previously shown to result in loss of the hydrophilic O-antigen layer on the cell surface, increased FFA secretion, alleviated photoinhibition, and lead to 50 and 45 % increase in the final cell density and the total amount of FFA in the culture (i.e., the sum of the cellular and extracellular FFA), respectively. The average rate of production of total FFA by the culture of the ∆wzt strain was 2.7 mg L−1 h−1, being five times higher than those reported for Synechocystis sp. PCC 6803 and comparable to the rates of triacylglycerol production in green algae.ConclusionSynechococcus elongatus PCC7942 has larger capacity of FFA production than Synechocystis sp. PCC6803 but accumulates most of the product in the cell because of the imbalance of the rates of FFA production and secretion. This causes severe photoinhibition and exerts adverse effects on cell growth and FFA productivity. Enhancement of FFA secretion would be required to fully exploiting the capacity of FFA production for the purpose of biofuel production.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0506-1) contains supplementary material, which is available to authorized users.

Highlights

Among the three model cyanobacterial species that have been used for engineering a system for photosynthetic production of free fatty acids (FFAs), Synechococcus elongatus PCC7942 has been the least successful; the FFA-excreting mutants constructed from this strain could attain lower rates of FFA excretion and lower final FFA concentrations than the mutants constructed from Synechocystis sp
Production of free fatty acids (FFAs) using genetically engineered cyanobacteria is thought to be a promising method of production of renewable biofuels, because FFAs are excreted into the medium and as a result, the two energy-intensive steps in microbial biofuel production, i.e., recovery of the cells from the growth medium and extraction from the cells of the photosynthetic product, can be omitted [1,2,3]
Improvement of FFA productivity by attenuation of ‘tesA expression In the FFA-producing strain dAS1T, the ‘tesA coding sequence was transcriptionally fused to the nitrogenregulated promoter (PnirA) of the nitrate assimilation operon and introduced into the aas locus (Fig. 1a, b)

Summary

Introduction

Among the three model cyanobacterial species that have been used for engineering a system for photosynthetic production of free fatty acids (FFAs), Synechococcus elongatus PCC7942 has been the least successful; the FFA-excreting mutants constructed from this strain could attain lower rates of FFA excretion and lower final FFA concentrations than the mutants constructed from Synechocystis sp. PCC 7002 and Synechococcus elongatus PCC 7942, the ‘tesA-expressing aas-deficient mutants showed relatively low FFA productivity, attaining extracellular FFA concentrations of 40 and 49.3 mg L−1 with average excretion rates of 0.083 and 0.103 mg L−1 h−1, respectively [2, 3]. The molecular basis of FFA toxicity remains to be determined, these results indicated that different factors limit FFA production in different cyanobacterial species [2]

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Results

Discussion

Conclusion