Adopting a Theophylline-Responsive Riboswitch for Flexible Regulation and Understanding of Glycogen Metabolism in Synechococcus elongatus PCC7942.

Shanshan Zhang,Guodong Luan,Xuefeng Lu,Cuncun Qiao,Xintong Chi,Yangkai Duan,Huili Sun

doi:10.3389/fmicb.2019.00551

Abstract

Cyanobacteria are supposed to be promising photosynthetic microbial platforms that recycle carbon dioxide driven into biomass and bioproducts by solar energy. Glycogen synthesis serves as an essential natural carbon sink mechanism, storing a large portion of energy and organic carbon source of photosynthesis. Engineering glycogen metabolism to harness and rewire carbon flow is an important strategy to optimize efficacy of cyanobacteria platforms. ADP-glucose pyrophosphorylase (GlgC) catalyzes the rate-limiting step for glycogen synthesis. However, knockout of glgC fails to promote cell growth or photosynthetic production in cyanobacteria, on the contrary, glgC deficiency impairs cellular fitness and robustness. In this work, we adopted a theophylline-responsive riboswitch to engineer and control glgC expression in Synechococcus elongatus PCC7942 and achieved flexible regulation of intracellular GlgC abundance and glycogen storage. With this approach, glycogen synthesis and glycogen contents in PCC7942 cells could be regulated in a range from about 40 to 300% of wild type levels. In addition, the results supported a positive role of glycogen metabolism in cyanobacteria cellular robustness. When glycogen storage was reduced, cellular physiology and growth under standard conditions was not impaired, while cellular tolerance toward environmental stresses was weakened. While when glycogen synthesis was enhanced, cells of PCC7942 displayed optimized cellular robustness. Our findings emphasize the significance of glycogen metabolism for cyanobacterial physiology and the importance of flexible approaches for engineering and understanding cellular physiology and metabolism.

Highlights

Cyanobacteria are photoautotrophic prokaryotes, that are widespread in diverse ecosystems, including the ocean, fresh water, and terrestrial environments (Waterbury et al, 1979)
The PCC7942 mutant carrying the KmRPtrc-ENYC4-glgC cassette on the chromosome was termed as PCC7942-XC1 (XC1), while the wild type strain of PCC7942 was termed as PCC7942-WT (WT) as a control
We reported that, as the main compatible solute of PCC7942 to resist osmotic stress, synthesis of sucrose is closely linked to glycogen accumulations (Qiao et al, 2018), which might be a reason for the advantages of the XC1 strain with increased glycogen synthesis

Summary

Introduction

Cyanobacteria are photoautotrophic prokaryotes, that are widespread in diverse ecosystems, including the ocean, fresh water, and terrestrial environments (Waterbury et al, 1979). They evolved oxygenic photosynthesis, an efficient system converting solar energy and CO2 into organic compounds (HohmannMarriott and Blankenship, 2011). During the night or in the absence of a carbon source, carbon and energy stored in the glycogen is mobilized and used for the central metabolism (Stal and Moezelaar, 1997; Guerra et al, 2013). There are hints that glycogen metabolism protects cyanobacteria against unfavorable environmental conditions (Suzuki et al, 2010; Grundel et al, 2012; Hickman et al, 2013)

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