Engineering synthetic pathways to bio-ethylene production in Cyanobacteria sp.

背景：目前，乙烯是由乙烷的裂化过程产生的，这一过程会产生大量的二氧化碳，导致全球变暖。2000年，蒸汽裂解的一次能源使用量为30亿千兆焦耳，二氧化碳排放量约为2亿吨。乙烯是最常见的塑料——聚乙烯的单体，全球年产量约为8000万吨。寻找一种可持续的或碳中性的替代乙烯生产是势在必行的。蓝藻是革兰氏阴性的光合原核生物。此前，研究人员曾试图通过异源表达efe基因，在蓝藻、长聚球菌（Synechococcus elongates sp. PCC 7942）和聚胞球菌（Synechocystis sp. PCC 6803）中从二氧化碳中产生乙烯，但结果不一。目的：这个项目的目的是设计蓝藻作为生产碳氢化合物产品的平台，如乙烯。丁香假单胞菌乙烯生成酶基因efe的表达。synnechocystis sp. pcc6803中的Phaseolicola导致连续乙烯生产。我们现在正处于通过定向进化和代谢工程提高产量的过程中。作为这个过程的一部分，我们希望设计一个合成途径，利用植物的Yang途径来生产乙烯。这提供了一个令人兴奋的机会，在蓝藻中实现一种新的途径，并将乙烯生产与增长联系起来。乙烯是由1-氨基环丙烷-1-羧酸（ACC）高效生物合成的（Zhou et al., 2002），它本身来源于蛋氨酸，是Yang循环的一个分支（Wang et al., 2002）。这个过程的能量效率很高，因为它保留了高能量的蛋氨酸硫醚键。该途径利用SAM合成酶、ACC合成酶和ACC氧化酶。1-氨基-环丙烷-1-羧酸（ACC）转化为乙烯释放氰甲酸，氰甲酸自发脱羧释放氰化氢，主要通过CAS途径解毒（Machingura et al., 2016）。该途径的实施将为蓝藻解毒氰化物提供一种机制。培训：该项目将允许在独特的多学科环境中进行培训，包括基因组工程、气体发酵、合成生物学、前沿分子生物学和系统生物学建模。该项目将为学生提供大量可转移的技能，这些技能在不断发展的生物经济中受到雇主的高度重视。该项目还将提供一些高影响力的出版物。这个转化项目将在诺丁汉的BBSRC/EPSRC合成生物学研究中心（SBRC）内进行，该中心由70多名研究生和博士后研究人员组成（www.clostron.com/people.php），目前的预算为2700万英镑。

Background: Ethylene is currently produced from the stream cracking of ethane which produces large quantities of CO2, contributing to global warming. In 2000, steam cracking had a primary energy use of 3 billion Gigajoules and accounted for approximately 200 million tons of CO2 emissions. Ethylene is the monomer for the most common plastic, polyethylene, and annual global production is approximately 80 million tons. Finding a sustainable or carbon neutral alternative to ethylene production is imperative. Cyanobacteria are Gram-negative photosynthetic prokaryotes. Researchers have previously attempted to produce ethylene from CO2 through heterologous expression of the efe gene in the cyanobacteria, Synechococcus elongates sp. PCC 7942 and Synechocystis sp. PCC 6803, with mixed results. Aim: The aim of this project is to engineer cyanobacteria as a platform for the production of hydrocarbon-based products such as ethylene. Expression of the efe gene encoding the ethylene-forming enzyme from Pseudomonas syringae pv. Phaseolicola in Synechocystis sp. PCC 6803, led to continuous ethylene production. We are now in the process of improving production through directed evolution and metabolic engineering. As part of this process we would like to engineer a synthetic pathway for ethylene production utilising the Yang pathway from plants. This provides an exciting opportunity to implement a novel pathway in cyanobacteria and link ethylene production to growth. Ethylene is efficiently biosynthesized from 1-aminocyclopropane-1-carboxylic acid (ACC) (Zhou et al., 2002), which is itself derived from methionine as a branch of the Yang cycle (Wang et al., 2002). This process is energetically efficient as it preserves the high-energy methionine thioether bond. This pathway utilises SAM synthetase, ACC synthase and ACC oxidase. The conversion of 1-amino-cyclopropane-1-carboxylic acid (ACC) to ethylene releases cyanoformic acid, which spontaneously decarboxylates to release hydrogen cyanide, which is principally detoxified by the CAS pathway (Machingura et al., 2016). The implementation of this pathway will provide a mechanism for detoxifying cyanide in cyanobacteria. Training: The project will allow for training in a unique multidisciplinary environment, incorporating genomic engineering, gas fermentation, synthetic biology, cutting edge molecular biology and systems biology modelling. The project will providing the student with a vast array of transferable skills, highly prized by employers in the growing bio-economy. The project will also provide several high impact publications. This translational project will be carried out within the BBSRC/EPSRC Synthetic Biology Research Centre (SBRC) at Nottingham which comprises 70+ graduate and postdoctoral researchers (www.clostron.com/people.php) and a current budget of £27M.