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

doi:10.1186/s13068-016-0506-1

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

Biotechnol Biofuels. 2016 Apr 23:9:91. doi: 10.1186/s13068-016-0506-1. eCollection 2016.

Authors

Akihiro Kato¹, Kazuhide Use¹, Nobuyuki Takatani², Kazutaka Ikeda³, Miyuki Matsuura¹, Kouji Kojima², Makiko Aichi⁴, Shin-Ichi Maeda², Tatsuo Omata⁵

Affiliations

¹ Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, 464-8601 Japan.
² Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, 464-8601 Japan ; Japan Science and Technology Agency, CREST, Tokyo, Japan.
³ Institute for Advanced Biosciences, Keio University, Tsuruoka, Yamagata 997-0052 Japan ; Japan Science and Technology Agency, CREST, Tokyo, Japan ; Laboratory for Metabolomics, RIKEN Center for Integrative Medical Sciences, Yokohama, 230-0045 Japan.
⁴ Department of Biological Chemistry, Chubu University, Kasugai, 487-8501 Japan ; Japan Science and Technology Agency, CREST, Tokyo, Japan.
⁵ Graduate School of Bioagricultural Sciences, Nagoya University, Nagoya, 464-8601 Japan ; Japan Science and Technology Agency, CREST, Tokyo, Japan ; Laboratory of Molecular Plant Physiology, Graduate School of Bioagricultural Sciences, Nagoya University, Furo, Chikusa, Nagoya, 464-8601 Japan.

Abstract

Background: 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. 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.

Results: By 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.

Conclusion: Synechococcus 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.

Keywords: Biofuel production; Cyanobacteria; Free fatty acids; O-antigen.