生物工程学报  2023, Vol. 39 Issue (6): 2265-2283
http://dx.doi.org/10.13345/j.cjb.230063
中国科学院微生物研究所、中国微生物学会主办
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文章信息

程亚田, 汤皓, 孙丽丽, 胡雅婷, 马莹, 郭娟, 黄璐琦
CHENG Yatian, TANG Hao, SUN Lili, HU Yating, MA Ying, GUO Juan, HUANG Luqi
植物源二萜类化合物微生物合成研究进展
Advances on the microbial synthesis of plant-derived diterpenoids
生物工程学报, 2023, 39(6): 2265-2283
Chinese Journal of Biotechnology, 2023, 39(6): 2265-2283
10.13345/j.cjb.230063

文章历史

Received: January 31, 2023
Accepted: March 21, 2023
植物源二萜类化合物微生物合成研究进展
程亚田1 , 汤皓1,2 , 孙丽丽1,3 , 胡雅婷4 , 马莹1 , 郭娟1 , 黄璐琦1     
1. 中国中医科学院中药资源中心 道地药材国家重点实验室, 北京 100700;
2. 南京中医药大学药学院, 江苏 南京 210023;
3. 浙江中医药大学药学院, 浙江 杭州 310053;
4. 首都医科大学中医药学院, 北京 100069
摘要:植物源二萜类天然产物结构复杂且功能多样,具有抗癌、抗炎和抗菌等多种药理活性,在药品、化妆品和食品添加剂等方面广泛应用。近年来,基于植物源二萜类化合物(diterpenoids)生物合成途径中功能基因的逐步揭示和合成生物技术的发展,科研人员采用代谢工程技术构建了多种二萜类化合物的微生物细胞工厂,且多个化合物达到克级产量。本文对植物源二萜类化合物微生物细胞工厂的构建情况进行综述,介绍并探讨植物源二萜类化合物微生物合成的研究进展和改造策略,为高产二萜类化合物细胞工厂构建和工业化生产提供参考。
关键词植物源二萜类化合物    合成生物学    代谢工程    微生物合成    
Advances on the microbial synthesis of plant-derived diterpenoids
CHENG Yatian1 , TANG Hao1,2 , SUN Lili1,3 , HU Yating4 , MA Ying1 , GUO Juan1 , HUANG Luqi1     
1. State Key Laboratory of Dao-di Herbs, National Resource Center for Chinese Materia Medica, China Academy of Chinese Medical Sciences, Beijing 100700, China;
2. School of Pharmacy, Nanjing University of Chinese Medicine, Nanjing 210023, Jiangsu, China;
3. School of Pharmacy, Zhejiang University of Chinese Medicine, Hangzhou 310053, Zhejiang, China;
4. School of Traditional Chinese Medicine, Capital Medical University, Beijing 100069, China
Abstract: Natural plant-derived diterpenoids are a class of compounds with diverse structures and functions. These compounds are widely used in pharmaceuticals, cosmetics and food additives industries because of their pharmacological properties such as anticancer, anti-inflammatory and antibacterial activities. In recent years, with the gradual discovery of functional genes in the biosynthetic pathway of plant-derived diterpenoids and the development of synthetic biotechnology, great efforts have been made to construct a variety of diterpenoid microbial cell factories through metabolic engineering and synthetic biology, resulting in gram-level production of many compounds. This article summarizes the construction of plant-derived diterpenoid microbial cell factories through synthetic biotechnology, followed by introducing the metabolic engineering strategies applied to improve plant-derived diterpenoids production, with the aim to provide a reference for the construction of high-yield plant-derived diterpenoid microbial cell factories and the industrial production of diterpenoids.
Keywords: plant-derived diterpenoids    synthetic biology    metabolic engineering    microbial synthesis    

二萜类化合物(diterpenoids)是指分子骨架由4个异戊二烯单位构成的、含20个碳原子的一类化合物。天然存在的二萜化合物骨架有126种,通过天然产物辞典(dictionary of natural products, http://dnp.chemnetbase.com )数据库检索发现有18 000余种二萜类化合物[1]。目前,植物中二萜类化合物逐渐被分离出来,在药物研发、食品添加剂、化妆品等行业得到了广泛的应用。值得注意的是植物源二萜类化合物因其具有多种药理活性在药物研发中广受关注,并且许多含植物源二萜类成分的药物已应用于临床疾病的治疗,如含丹参酮IIA (tanshinone IIA)成分的复方丹参滴丸和复方丹参片可作为冠心病、中风和关节炎的预防或治疗药物[2];含紫杉醇(taxol)成分的紫杉醇注射液可作为卵巢癌、胰腺癌和乳腺癌等疾病的治疗药物[3];含银杏内酯B成分(ginkgolide B)的银杏内酯注射液可作为治疗缺血性脑卒中的药物[4];含雷公藤甲素(triptolide)成分的的雷公藤片可作为治疗类风湿性关节炎的药物[5];这些药物显示出植物源二萜类化合物较高的药用价值。

目前,植物源二萜类天然产物主要通过植物提取和化学合成方式获得,因植物生长周期长、次生代谢产物含量低等原因,往往不能大量获得。如紫杉醇在红豆杉(Taxus cuspidate)树皮中含量约为0.001%–0.05% (质量分数),即每千克红豆杉树皮经提取分离最多可得500 mg紫杉醇[6-7];且红豆杉属植物资源稀缺,远不能满足市场需求,供求矛盾十分突出。雷公藤甲素只能从药用植物雷公藤(Tripterygium wilfordii)中提取,得率更低,为0.000 1%–0.002% (质量分数)[8-9],且雷公藤花粉有毒,不适宜大规模种植以供提取[5];银杏内酯主要从银杏(Ginkgo biloba)的叶片中提取,且银杏内酯含量较低[10]。此外,二萜成分在原植物中结构类似物较多,增加了提取分离难度,而且其复杂的化学结构也限制了化学合成的得率。因此,合成生物学生产这一绿色可持续获取方式引起了研究者的广泛关注。

以合成生物学技术生产植物源天然产物具有不依赖野生和栽培资源,节约土地,不受环境、气候和场地限制等优势,为植物源天然产物的资源保护供给提供了新的策略。近年来,合成生物学技术在生产植物天然产物方面取得了突破性进展,2013年Keasling团队[11]以酿酒酵母(Saccharomyces cerevisiae)为底盘细胞实现了抗疟良药青蒿素直接前体青蒿酸的从头合成,结合光转化反应,实现了63 m2发酵车间约等于3千多hm2土地青蒿素产量的代表性突破。此外,生物碱活性分子如托品烷类生物碱莨菪碱、东莨菪碱[12]和抗癌药物长春花碱[13]也实现了在酿酒酵母中的从头合成。这一系列明星分子的合成生物学研究不仅扩展了合成生物学的应用,而且为其他天然活性成分细胞工厂的构建和生产提供了有效策略。

近年来,植物源二萜类化合物的合成生物学研究也取得了一些进展[14-15],部分植物源二萜类化合物及其相关中间产物实现了从头全合成,其中少数化合物产量达到克级产量。由于二萜类化合物作为植物次生代谢产物,在植物体内生物合成途径较为复杂,由多种结构修饰酶如细胞色素P450s (cytochrome P450s, CYP450s)、糖基转移酶(glycosyltransferases, GTs)、2-酮戊二酸依赖性双加氧酶(2-oxoglutarate-dependent dioxygenases, 2OGDs)和酰基转移酶(acyltransferases, ACTs)等参与经过多步催化合成,因此,大部分植物源二萜类化合物的生物合成网络并未被完全解析。此外,由于参与植物源二萜类化合物生物合成的修饰酶在微生物底盘细胞中表达量和催化效率低、二萜类化合物对微生物底盘细胞具有一定毒性等问题,大多数植物源二萜类化合物产量未达到工业化水平;因此,植物源二萜类化合物的微生物合成仍存在巨大挑战。本文将介绍植物源二萜类化合物以微生物为底盘细胞的生物合成概况和合成生物学研究进展,包括其在微生物中的合成的上游、中游和下游途径模块,并详细介绍二萜类化合物生物合成此3个途径模块的设计改造策略和底盘细胞调控研究,以期为构建高产二萜类化合物细胞工厂和实现天然二萜类化合物工业化生产提供参考。

1 二萜类化合物微生物合成途径

在微生物底盘细胞中重构二萜类化合物生物合成途径常被分为3个途径模块,即上游、中游和下游途径模块。上游途径模块的构建主要针对微生物内源性途径即4-磷酸甲基赤藓糖醇(methylerythritol 4-phosphate, MEP)途径和甲羟戊酸(mevalonate, MVA)途径[16-19]进行改造。这2个途径以葡萄糖为碳源经过多步酶催化生成异戊烯焦磷酸酯(isopentenyl pyrophosphate, IPP)和二甲基丙烯焦磷酸酯(dimethylallyl diphosphate, DMAPP);随后,IPP和DMAPP缩合形成二萜类化合物前体香叶基香叶基二磷酸酯[(E, E, E)‐geranylgeranyl diphosphate, GGPP]。中游途径模块主要是GGPP在二萜合酶(diterpene synthases, TPS)催化下生成结构多样的二萜化合物骨架的过程,如次丹参酮二烯(miltiradiene)、左旋海松二烯(levopimaradiene)、对-贝壳杉烯(ent-kaurene)和紫杉二烯[taxadiene/taxa-4(5), 11(12)-diene]等二萜化合物骨架。在下游途径模块中,以各种二萜类化合物骨架为前体在CYP450s、GTs、2OGDs和ACTs等多种修饰酶催化下生成结构和功能多样的二萜类化合物(图 1)。

图 1 植物源二萜类化合物微生物生物合成途径[5, 8, 10, 15-16, 20-21] Fig. 1 Biosynthetic pathway of plant-derived diterpenoids in microbes[5, 8, 10, 15-16, 20-21]. nor-CPP:去甲-焦磷酸古巴酯;ent-CPP:对-焦磷酸古巴酯;SmCPS1:丹参焦磷酸古巴酯合酶1基因;SmKSL1:丹参贝壳杉烯合酶基因;TwTPS7v2/9v2/27v2:雷公藤二萜合酶7v2/9v2/27v2基因;GbLPS:银杏左旋海松二烯二萜合酶基因;CfTPS2:毛喉鞘蕊花二萜合酶2基因;SrCPS:甜叶菊焦磷酸古巴酯合酶基因;SsLPPS:鼠尾草焦磷酸赖百当烯二醇酯合酶基因;SsTPS:鼠尾草二萜合酶基因;SrKS:甜叶菊对-贝壳杉烯合酶基因;SrKO:甜叶菊对-贝壳杉烯氧化酶基因;SrKAH:甜叶菊贝壳烯酸-13α-羟化酶基因;UGTs:尿苷二磷酸糖基转移酶基因;TAT:紫杉烯醇5α-乙酰氧化基转移酶基因;T5αOH:紫杉烯-5α-醇羟化酶基因;DBAT:10β-去乙酰巴卡亭Ⅲ乙酰氧化基转移酶基因 nor-CPP: nor-copalyl diphosphate; ent-CPP: ent-copalyl diphosphate; SmCPS1: Salvia miltiorrhiza copalyl diphosphate synthases 1 gene; SmKSL1: Salvia miltiorrhiza kaurene synthase like 1 gene; TwTPS7v2/9v2/27v2: Tripterygium wilfordii diterpene synthases7v2/9v2/27v2 genes; GbLPS: Ginkgo biloba levopimaradiene diterpene synthase gene; CfTPS2: Coleus forskohlii diterpene synthase 2 gene; SrCPS: Stevia rebaudiana copalyl diphosphate synthase gene; SsLPPS: Salvia sclarea labdenediol diphosphate synthase gene; SsTPS: Salvia sclarea terpene synthase gene; SrKS: Stevia rebaudiana kaurene synthase gene; SrKO: Stevia rebaudiana ent-kaurene oxidase gene; SrKAH: Stevia rebaudiana kaurenoic acid 13α-hydroxylase gene; UGTs: Uridine diphosphate glycosyltransferase genes; TAT: Taxadine-5α-ol O-acetyltransferase gene; T5αOH: Taxadien-5α-ol hydroxylase gene; DBAT: 10-deacetylbaccatin Ⅲ-10β-O-acetyltransferase gene.
2 二萜类化合物上游途径改造

MEP途径存在于大多数原核生物而MVA途径存在于大多数真核生物和部分原核生物。以常用底盘细胞大肠杆菌(Escherichia coli)和S. cerevisiae为例(图 1),分别介绍参与MEP途径和MVA途径的酶和代谢产物。

MEP途径以丙酮酸(pyruvic acid)和3-磷酸甘油醛(glyceraldehyde 3-phosphate, G3P)为前体在5-磷酸脱氧木酮糖合成酶(1-deoxy-d-xylulose 5-phosphate synthase, DXS)和5-磷酸脱氧木酮糖还原异构酶(1-deoxy-d-xylulose 5-phosphate reductoisomerase, DXR)催化下生成关键中间体MEP;MEP依次在4-二磷酸胞嘧啶-2-甲基赤藓糖醇合酶(2-C-methyl-d-erythritol-4-phosphate cytidylyhransferase, ispD)、4-二磷酸胞嘧啶-2-甲基赤藓糖醇激酶(2-C-methyl-d-erythritol-4- phosphate kinase, ispE)、甲基赤藓醇-2, 4-环焦磷酸合酶(2-C-methyl-d-erythritol-2, 4-cyclodiphosphate synthase, ispF)、甲基赤藓醇-2, 4-环焦磷酸还原酶(2-C-methyl-d-erythritol-2, 4-cyclodiphosphate reductase, ispG)和羟甲基-丁烯-4-焦磷酸还原酶(hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate reductase, ispH) 5个酶的催化下生成IPP和DMAPP[20-21]。MVA途径则是以第一个关键中间体乙酰辅酶A (acetyl-coenzyme A, acetyl-CoA)起始,在乙酰乙酰辅酶A硫解酶(acetoacetyl-CoA thiolase, ERG10)、羟甲基戊二烯辅酶A还原酶(hydroxymethylglutaryl-CoA synthase, ERG13)的催化下生成羟甲基戊二酰辅酶A (hydroxymethylglutaryl-CoA, HMG-CoA),随后HMG-CoA在羟甲基戊二酰辅酶A还原酶(hydroxymethylglutaryl-CoA reductase, HMGR, 酿酒酵母中的同工酶为HMG1和HMG2)催化下还原生成关键中间体MVA。MVA在甲羟戊酸激酶(mevalonate kinase, ERG12)、磷酸甲羟戊酸激酶(phosphomevalonate kinase, ERG8)、甲羟戊酸焦磷酸脱羧酶(mevalonate pyrophosphate decarboxylase, MVD1/ERG9)以及异戊烯焦磷酸异构酶(isopentenyl diphosphate isomerase, IDI)的催化作用下生成IPP和DMAPP。随后IPP和DMAPP在法尼基焦磷酸合酶(farnesyl pyrophosphate synthase, FPS/ERG20)催化下生成法尼基二磷酸(farnesyl diphosphate, FPP),FPP在香叶基香叶基二磷酸合酶(geranylgeranyl diphosphate synthase, GGPPS)催化下生成GGPP[16-17, 20, 22] (图 1)。

上游途径改造策略主要集中在提高微生物GGPP产量。由于微生物自身合成的GGPP产量较低,仅靠其自身提供的GGPP无法满足植物源二萜类化合物的高效生产,因此提升微生物内源性GGPP产量是实现植物源二萜类化合物高效生产的重要基础。目前,以大肠杆菌为底盘细胞构建二萜类化合物细胞工厂时,采用的主要策略为强化上游途径基因的表达;多数植物源二萜类化合物以酵母为底盘细胞构建细胞工厂,主要策略除了强化上游途径基因表达外,抑制或下调竞争途径基因表达及提高关键中间体乙酰辅酶A的产量同样至关重要。

强化GGPP途径基因的表达是提高微生物体内GGPP产量的有效策略,该策略主要是对GGPP合成途径中的关键基因和限速基因进行过表达。DXSDXRIDI被证明是MEP途径中的限速酶基因[23]。Ajikumar等[24]在大肠杆菌中采用多元模块代谢工程构建产紫杉二烯细胞工厂,过表达了MEP途径中DXSIDIispDispF 4个关键基因,在MEP途径高效生产GGPP的基础上,继续增强下游途径基因,使得紫杉醇前体紫杉二烯的滴度高达1.0 g/L,较未改造菌株提高了约15 000倍。此外,HMG1是GGPP合成关键限速基因,研究表明过表达截短的HMG1tHMG1可以有效提高MVA途径的通量,目前该策略已成为提高萜类化合物工程菌株前体通量的通用手段[25-27]。左旋海松酸(levopimaric acid)是一种植物来源的二萜树脂酸,是银杏内酯类化合物的前体骨架[10],具有抗癌药理活性[28]。Liu等[29]在构建左旋海松酸酿酒酵母细胞工厂过程中,除了过表达MVA途径基因外,特别强化了tHMG1基因的表达,提升了GGPP通量。研究表明,在酿酒酵母中过表达来自柚木硅杆菌(Silicibacter pomeroyi)中的SpHMGR[30]和内源性HMG2突变体HMG2K6R基因[31]可提高MVA途径代谢通量;Cao等[32]在以酿酒酵母为底盘细胞构建香紫苏醇细胞工厂时,过表达了ERG10tHMG1HMG2K6RSpHMGR基因提高了胞内MVA水平,进而提高GGPP产量。此外,Nowrouzi等[33]在构建紫杉二烯酿酒酵母细胞工厂时,除了过表达酿酒酵母MVA途径基因外,又过表达了一拷贝来自粪肠球菌(Enterococcus faecalis)的MvaSMvaE以增加乙酰辅酶A向HMG-CoA的代谢流,并且过表达tHMG1和来自红发夫酵母(Xanthophyllomyces dendrorhous)中的GGPPS基因CrtE,提高了工程菌中前体GGPP通量。

在酿酒酵母GGPP合成过程中存在多个竞争途径,因此,弱化或下调竞争途径基因的表达也是提高GGPP的有效方法(图 2)。角鲨烯(squalene)是GGPP的竞争途径主要产物之一,与GGPP竞争其前体FPP,FPP在角鲨烯合成酶(squalene synthase, SQS) ERG9的催化下产生角鲨烯,弱化或下调ERG9基因表达能促使FPP流向二萜前体GGPP。Asadollahi等[34]通过将天然ERG9基因的启动子替换为MET3,下调了ERG9基因的表达,削弱了FPP向麦角甾醇(ergosterol)的碳代谢流,使其更多地流向GGPP。如Hu等[35]在构建次丹参酮二烯工程菌时,通过敲除ERG9启动子的上游部分激活序列,使GGPP去磷酸化产物香叶基香叶醇(geranylgeraniol, GGOH)产量从40.3 mg/L提高至196.4 mg/L。Engels等[36]在构建紫杉二烯酿酒酵母工程菌时,通过过表达摄取控制转录因子(uptake control transcriptional regulator, UPC2)突变体UPC2.1 (G888D)基因,促进酿酒酵母细胞在有氧生长条件下吸收利用麦角甾醇,降低麦角甾醇与紫杉二烯竞争碳代谢流,从而使前体GGPP更多地流向紫杉二烯。

图 2 代谢改造提升酿酒酵母中GGPP生物合成[16-17, 21, 32, 35] Fig. 2 Metabolic modifications to improve GGPP biosynthesis in Saccharomyces cerevisiae[16-17, 21, 32, 35]. 红色标注基因为过表达MVA途径相关基因,蓝色箭头代表下调途径基因表达;虚箭头表示经过两步或多步反应;PAN6:泛酸合酶基因;MVAP:甲羟戊酸三磷酸;MVAPP:甲羟戊酸二磷酸 The red-labeled ones are overexpressed MVA pathway genes; The blue arrow represents the down-regulated gene expression, and the dotted arrow indicates the multi-step reaction; PAN6: Pantothenate synthase gene; MVAP: Mevalonate-3-phosphate; MVAPP: Mevalonate-diphosphate.

此外,为了减少副产物和竞争途径代谢流,改造异戊二烯类催化酶从而强化GGPP产能也是行之有效的手段(图 2)。例如来自嗜酸热硫化叶菌(Sulfolobus acidocaldarius)双功能GGPPS基因SaGGPPS能催化IPP和DMAPP直接生成GGPP,该酶被用于产次丹参酮二烯酿酒酵母工程菌中,进一步提升了次丹参酮二烯的产量[37]。ERG20F96C突变体能够连续催化异戊二烯类化合物生成FPP,减少向GPP竞争途径的代谢流,酿酒酵母中过表达ERG20F96C成为提高GGPP产量常用策略[38];如在次丹参酮二烯工程菌构建中,将ERG20F96C与酿酒酵母内源性GGPPS基因BTS1融合表达后,使得次丹参酮二烯产量提升了15倍[39]。FPS突变体FPSF112A也可催化IPP和DMAPP直接生成GGPP,该突变体基因被应用于甜茶苷酵母细胞工厂的构建中[40]。融合蛋白构建策略可以将连续2步反应的催化距离缩短,使前一步反应产物能快速被下一步蛋白抓取,减少副产物生成,提高催化效率,如Dai等[37]在次丹参酮二烯酿酒酵母中将ERG20BTS1基因融合表达,提高了GGPP的产量。此外,在香紫苏醇工程菌中将BTS1和来自桃枝枯病菌(Phomopsis amygdali)中的PaGGPPS基因融合表达后可直接将IPP和DMAPP转化为GGPP,减少了途径中副产物的产生,进一步提高GGPP产量[32]

研究表明,萜类化合物在工程酿酒酵母中产量低可能与前体乙酰辅酶A不足有关[41-42]。在酿酒酵母中,乙酰辅酶A参与酿酒酵母中心碳代谢,并在多种代谢途径中扮演着重要角色[43]。在酿酒酵母中葡萄糖经过糖酵解转化为丙酮酸后,丙酮酸进入胞质或细胞器中生成乙酰辅酶A (图 2)。二萜类化合物前体GGPP合成场所是胞质,提高胞质中的乙酰辅酶A通量对提高GGPP至关重要[44]。由于真核生物不同细胞器合成的乙酰辅酶A不能透过细胞器膜,因此提高胞质中乙酰辅酶A的主要策略为强化胞质中丙酮酸-乙酰辅酶A途径及过表达上游及支路合成途径相关基因等方面。

酿酒酵母胞质中的乙酰辅酶A通过丙酮酸脱氢酶旁路(pyruvate dehydrogenase bypass, PDH旁路)产生。胞质的丙酮酸通过丙酮酸脱羧酶(pyruvate decarboxylase, PDC)转化为乙醛,乙醛进一步通过乙醇脱氢酶(alcohol dehydrogenase, ADH)转化为乙醇,或通过乙醛脱氢酶(aldehyde dehydrogenase, ALD)转化为乙酸。胞质中的乙酸通过乙酰辅酶A合成酶(acetyl-CoA synthetase, ACS)转化为乙酰辅酶A[44]。此外,酵母含有2个ACS基因,即ACS1ACS2,两者都可以在胞质发挥催化活性,但其编码蛋白的定位取决于碳源的类型[43]。以葡萄糖为碳源时,ACS1受葡萄糖抑制;而ACS2不受葡萄糖抑制,可在葡萄糖存在下表达为胞浆提供乙酰辅酶A[44]。综上所述,强化胞质中丙酮酸-乙酰辅酶A途径主要策略为过表达PDCALDACS以及设计PDH旁路以减少能源和碳源消耗。Shiba等[45]在酿酒酵母中设计了PDH旁路途径,过表达ALD和来自肠沙门氏菌(Salmonella enterica)对乙酰辅酶A反馈不敏感的乙酰辅酶A合成酶基因突变体SeACSL641P,以增加乙酰辅酶A途径的碳通量,从而增加了异戊二烯的产量。自此SeACSL641P作为乙酰辅酶A途径中的关键靶点广泛应用于酿酒酵母工程菌的设计和构建中[46]。此外,de Jong等[47]将乙醇降解途径碳流重新引导至乙酰辅酶A,分别过表达编码乙醇脱氢酶基因ADH2和乙醛脱氢酶基因ALD6SeACSL641P,增加了乙酰辅酶A产量。

此外,除了提升酿酒酵母内源乙酰辅酶A代谢通量外,寻找高产乙酰辅酶A菌种,在底盘菌中引入异源乙酰辅酶A途径或相关基因也是提高胞质中乙酰辅酶A的有效策略。Kozak等[48]在酿酒酵母胞浆中过表达和组装了来自粪肠球菌(Enterococcus faecalis)的腺嘌呤核苷三磷酸柠檬酸裂解酶(adenosine triphosphate, ATP)非依赖性丙酮酸脱氢酶复合体(pyruvate dehydrogenase complex),结果表明其可以取代胞浆乙酰辅酶A合成通路,最大限度地降低乙酰辅酶A前体供应的能量成本。研究表明ATP柠檬酸裂解酶(ATP-citrate lyase, ACL)存在于产油酵母中,产油酵母在低氮源条件下,抑制异柠檬酸脱氢酶(isocitrate dehydrogenase, ICDH)基因的表达会导致线粒体柠檬酸积累并向胞质转运,胞质中的柠檬酸在ACL催化下生成乙酰辅酶A[49-50]。基于此原理,Rodriguez等[51]过表达了来自结节曲霉(Aspergillus nidulans)中的AnACL基因并敲除了酿酒酵母NAD+依赖性异柠檬酸脱氢酶基因IDH1 (NAD+-dependent isocitrate dehydrogenase, IDH1)以阻断柠檬酸向2-酮戊二酸(2-oxoglutarate)的代谢流,提高了胞浆中乙酰辅酶A的通量。调控酿酒酵母中心代谢和辅因子供给是提高目标化合物产量的有效方法之一,Nielsen课题组[52]通过过表达mpc1mpc3、两种异源ACL基因[AnACLMmACL (Mus musculus ACL)]、YHM2IDP1CIT1和来自圆红冬孢酵母(Rhodosporidium toruloides)中的RtCIT1提高了酿酒酵母中乙酰辅酶A通量,随后作者又过表达ZWF1GND1TKL1TAL1提高磷酸戊糖途径中的NADPH再生,从而提高了目标化合物脂肪酸的产量。Cao等[32]在构建香紫苏醇酿酒酵母细胞工厂时,将上述构建的调控中心代谢和辅因子改造的菌株作为生产香紫苏醇细胞工厂,与未进行中心代谢调控的对照菌株相比香紫苏醇产量提高了22倍。

此外,辅酶A (coenzyme A, CoA)及其衍生物是乙酰辅酶A等生物合成反应的必需前体,在酿酒酵母中,CoA生物合成由5种泛酸激酶(必需基因CAB1-CAB5编码)和两种底物(泛酸、半胱氨酸)参与。乙酰辅酶A对泛酸激酶(CAB1基因编码的PANK蛋白)具有反馈抑制作用,因此,在不断提升乙酰辅酶A产量的同时,应提升CAB1基因的表达或蛋白催化效率来保证CoA的供给。Wegner等[46]通过过表达泛酸激酶基因CAB1和培养基中补充泛酸,提高了胞内CoA的产量。Olzhausen等[53]过表达CAB1突变体基因CAB1W331R与参与CoA生物合成的CAB2CAB3HAL3CAB4CAB5基因,使胞内CoA水平提高了15倍。提高胞内CoA策略有望进一步应用于植物源二萜类化合物酿酒酵母细胞工厂的构建研究中。

此外,E. coli可以葡萄糖、乙酸盐和脂肪酸为底物合成乙酰辅酶A,Zhang等[54]E. coli中将葡萄糖利用途径中的葡萄糖转运蛋白基因ptsG (phosphotransferase system)和调控半乳糖代谢galR基因敲除并分别替换为葡萄糖促进因子glk (glucokinase)和来自运动发酵单胞菌(Zymomonas mobilis)的半乳糖氢离子转运基因zglf,同时将E. coli的丙酮酸氧化酶基因poxB (pyruvate oxidase)替换为乙酰辅酶A合成酶基因ACS,以阻断丙酮酸的旁路途径并增加丙酮酸通量,从而提高E. coli中的乙酰辅酶A产量。Sun等[55]在构建甜菊醇工程大肠杆菌中,发现在工程菌中引入异源MVA途径能够增强其本身的MEP途径从而提高了甜菊醇的产量。因此,在工程E. coli中有望将提高乙酰辅酶A和引入异源MVA途径2种策略结合并进行深入研究,以期提高E. coli中前体GGPP通量,为以E. coli为底盘细胞生产植物源二萜类化合物提供新的思路。

3 二萜类化合物中游途径改造

以高产GGPP底盘菌为出发菌,在二萜合酶催化下,如何在体内高效地合成二萜化合物的骨架是中游途径改造面临的主要问题。目前,除了采用对二萜合酶进行密码子优化和增加其拷贝数等策略外,结合二萜合酶的结构特征,通过对N端信号肽进行截短和构建融合蛋白等方法,可提高二萜合酶的可溶性表达和催化效率,使碳代谢流更多地流向二萜化合物骨架。

次丹参酮二烯是松香烷型二萜类化合物丹参酮的前体,GGPP在丹参(Salvia miltiorrhiza)二萜合酶SmCPS1 (copalyl diphosphate synthases 1, CPS1)和SmKSL1 (kaurene synthase like 1, KSL1)催化下生成次丹参酮二烯;在其工程菌构建过程中,研究者们针对二萜合酶采用了多种改造方式。Zhou等[56]采用途径模块化策略,快速将次丹参酮二烯合成途径组装至酿酒酵母中,并对SmCPS1SmKSL1进行融合表达,最后在15 L发酵罐发酵获得次丹参酮二烯产量提高到了365.0 mg/L。Dai等[37]SmCPS1SmKSL1基因整合至酿酒酵母多拷贝位点delta位点,增加了该基因拷贝数,在充足前体GGPP供给下,补料发酵使次丹参酮二烯产量达488.0 mg/L。Hu等[35]为了筛选催化效率最高的二萜合酶催化元件,在酿酒酵母中评估了来自不同物种的二萜合酶合成次丹参酮二烯的能力,结果表明来源于毛喉鞘蕊花(Coleus forskohlii)的Ⅱ类二萜合酶基因CfTPS1和来自丹参的I类二萜合酶基因SmKSL1为组合时,将GGPP催化生成次丹参酮二烯的效率最高;将SmKSL1的N端截短并与CfTPS1构建融合蛋白后进一步提高了次丹参酮二烯的产量,摇瓶发酵最终产量为550.0 mg/L,5 L生物反应器实现克级产量为3.5 g/L。Wei等[39]SmCPS1SmKSL1基因N端截短并以融合蛋白进行构建,结果表明N端截短的tSmCPS1tSmKSL1并以Gstssgssg为连接肽构建融合蛋白时,次丹参酮二烯产量最高,较未截短和未融合版本菌株产量提高了8倍。

紫杉二烯是紫杉醇的重要前体化合物,由于紫杉醇生物合成途径冗长复杂,高产的紫杉二烯底盘细胞是构建紫杉醇工程菌的重要基础。Nowrouzi等[33]在酿酒酵母细胞中过表达密码子优化的来自红豆杉(Taxus cuspidate)的紫杉二烯合酶基因TASY (taxadiene synthase, TASY),并对该基因的启动子强度、辅因子浓度及截短形式进行考察,结果表明该基因N截短60个或79个氨基酸残基可以改善紫杉二烯合酶的溶解性;随后,作者又将紫杉二烯合酶基因N端融合MBP (maltose-binding protein, MBP)助溶标签后再与ERG20融合,将该融合蛋白整合2拷贝至染色体上,最后在20 ℃发酵条件下紫杉二烯滴度提高了22倍,摇瓶发酵得到最高产量为129.0 mg/L。Ajikumar等[24]以IPP和DMAPP为节点,将紫杉二烯生物合成途径分为上游模块(产GGPP前体的内源性MEP途径模块)和下游模块(合成异源萜类化合物模块),利用改变质粒拷贝数和替换强启动子的方法调节下游模块GGPPS和二萜合酶的表达强度,通过上下游模块的平衡使整个代谢途径达到最优化,使紫杉二烯滴度达到近1.0 g/L。

香紫苏醇生物合成途径简单,底盘细胞成功合成GGPP后,仅需2个二萜合酶催化后即可获得香紫苏醇。杨薇等[57]在酿酒酵母过表达了来自鼠尾草(Salvia sclarea)的Ⅱ类二萜合酶焦磷酸赖百当烯二醇酯合酶基因(Salvia sclarea labdenediol diphosphate synthases, SsLPPS)和I类二萜合酶香紫苏醇合酶基因(Salvia sclarea terpene synthases, SsTPS)构建了产香紫苏醇工程菌,随后采用蛋白质融合和N端截短信号肽策略对SsLPPSSsTPS进行改造,结果表明2种改造策略均可提高香紫苏醇产量,但SsLPPSSsTPS融合后菌株的香紫苏醇产量最高,在摇瓶培养条件下,组合优化得到的香紫苏醇工程菌株产量近9.0 mg/L。Cao等[32]在构建香紫苏醇细胞工厂时,除了采用将SsLPPSSsTPS融合表达的策略外[58],又在该融合基因的N端添加了MBP标签[59],发酵后使得香紫苏醇工程菌产量提升了43%。

4 二萜类化合物下游途径改造

二萜类化合物下游途径由多种结构修饰酶参与,如CYP450s、GTs、2OGDs和ACTs等。植物源结构修饰酶的跨膜域、信号肽、定位表达和辅因子依赖等特征限制了其在微生物底盘细胞中的高效表达,因此,下游途径的改造策略主要集中在对结构修饰酶及辅因子等方面,如增加目的基因拷贝数、N端截短信号肽和融合蛋白构建、提升辅因子供给和蛋白定向进化等,以提高修饰酶在底盘细胞中的表达量、溶解度和催化效率。

鼠尾草酸生物合成途径包含3个CYP450s基因,是鼠尾草酸酿酒酵母工程菌的关键改造目标。Wei等[39]根据CYP450s的表达特性,通过促进电子链传递、增加目的基因拷贝数、提升表达区域和辅因子供给等策略进行改造。首先,将丹参中的P450还原酶(cytochrome P450 reductase, CPR) SmCPR1Salvia pomifera中的细胞色素b5 (cytochrome b5, Cytb5) SpCytb5基因进行N端截短和融合蛋白构建,结果表明SmCPR1t28SpCytb5基因融合后产物铁锈醇(ferruginol)产量提升最高,并将SmCPR1t28SpCytb5基因融合后与CYP76AH1整合至酵母的多拷贝位点delta位点以增加基因的拷贝数。由于CYP450s定位于内质网膜,作者通过过表达内质网大小调节因子INO2促使内质网膜面积增大,增加CYP450s定位表达区域。最后,过表达了血红素(heme)合成基因Heme3,提升酵母中CYP450s催化过程中所需辅因子血红素的供给,最终构建的鼠尾草酸工程菌的摇瓶发酵产量提升至25.0 mg/L[39]

针对于CYP450s膜结构溶解性差造成的催化效率低的问题,Xu等[40]将来自拟南芥(Arabidopsis thaliana)的贝壳烯酸-13α-羟化酶(kaurenoic acid 13α-hydroxylase, KAH) AtCYP714A2和来自甜叶菊(Stevia rebaudiana)中的SrCPR的跨膜域结构截掉以改善关键中间体甜菊醇(steviol)合成效率,使甜菊醇滴度增加了231.2%。此外,为了提高甜茶苷生物合成中多步反应中的底物运输效率,作者将对-贝壳杉烯氧化酶基因(ent-kaurene oxidase, KO)、KAHSrCPR以不同连接肽进行融合,发酵测试甜菊醇产量均有提高;接下来作者将INO2的内源性启动子替换为更强的PGK1启动子以增加内质网膜表面积;最后,作者阐明了UDP-葡萄糖(uridine diphosphate glucose, UDP-Glu)是甜茶苷合成的限制因素,并采用基因组规模代谢模型(genome- scale metabolic model, GSMM)和OptKnock模型[64]结合预测潜在靶点,其中敲除GAL7基因使得产量提高了19.4%,产量为250.0 mg/L;为了进一步提高胞内UDP-Glu通量,作者在敲除GAL7基因菌株基础上分别过表达了参与UDP-Glu合成相关基因PGM1 (phosphoglucomutase)、PGM2 (phosphoglucomutase)和UGP1 (uridine triphosphate glucose-1-phosphate uridylyltransferase),结果表明过表达PGM2基因菌株甜茶苷产量最高,为302.0 mg/L。

采用蛋白定向进化提高修饰酶在底盘细胞中的催化效率和产物专一性也是提高下游途径目标产物产量的有效方法。Sun等[55]从2种植物来源的KOKAH和7种Cytb5基因中筛选出最优组合,即来自黄花蒿(Artemisia annua) AkKOAtCYP714A2及甜叶悬钩子(Rubus suavissimus)中的RsCytb5,并将其构建于工程菌中,在此基础上作者对AtCYP714A2进行蛋白突变研究,进一步获得提升甜菊醇产量效果最佳的突变体AtCYP714A2I403L,多策略组合构建的大肠杆菌工程菌株在5 L发酵罐中实现了克级规模的甜菊醇从头合成,产量达1.1 g/L。

5 其他策略

大多数植物源二萜类化合物会对底盘细胞的生长产生不利影响,导致底盘细胞生产效率和细胞密度降低。除以上策略外,针对底盘细胞和其他相关靶点改造以增强其鲁棒性,对于实现植物源二萜类化合物工业化生产至关重要。目前,对底盘细胞的改造策略主要集中在提高底盘细胞对二萜类化合物耐受性、减少二萜类化合物对底盘细胞的应激反应、促进产物外排以降低对底盘细胞毒性及调控途径中相关调控因子以提高产量等方面。此外,天然产物生物合成途径较长且相对复杂,而微生物共培养(co-culture)将生物合成途径分成2个或多个模块并构建在两种或多种宿主中,从而减少宿主代谢负担,并充分利用每个宿主的先天代谢潜力[65],为植物源二萜类化合物高效生产提供新方向。

为了提升底盘细胞对二萜类化合物的耐受性,研究人员通过查阅文献对底盘细胞中的相关靶点进行挖掘和测试。如Wei等[39]在构建鼠尾草酸酿酒酵母工程菌时,过表达了酿酒酵母中不同定位的过氧化氢酶基因ScCTA1[66] (定位于过氧化物酶体和线粒体)和ScCTT1[67] (定位于细胞质),以减轻由CYP450s过量表达导致的H2O2积累对细胞造成的氧化应激[68],结果表明单独过表达2个基因均可以提高鼠尾草酸的产量。在真菌中,细胞应激反应调节系统可以增强细胞在苛刻发酵条件下的适应能力[69],Xu等[40]构建甜茶苷酿酒酵母细胞工厂时,敲除了6个压力响应因子基因即WAR1MSN4MOT3PDR3ARO80YRR1[70-75]。结果表明敲除这6个压力响应因子基因后均可以不同程度提升细胞对甜茶苷的耐受能力,其中WAR1基因敲除产量提升最高为63.6%。

酿酒酵母中存在3种外排蛋白家族,即ABC转运蛋白家族(ATP-binding cassette transporter family, ABC transporter family)、多药和有毒化合物输出蛋白家族(multidrug and toxic compound extrusion protein family)和促进因子超家族(major facilitator superfamily)[69, 76-78]。Xu等[40]将甜茶苷和不同类型的转运蛋白进行分子对接,发现其与ABC转运蛋白亲和力最高,推测此类蛋白在甜茶苷细胞膜转运中发挥重要作用。通过工程菌的转录组测序和分析,作者筛选得到了YOR1PDR11PDR12共3个转运蛋白基因,强化其表达后,甜茶苷产量分别提升了34.0%、129.8%和10.1%。

除了以上述常用的改造策略外,改造底盘细胞中的调控因子也可提升工程菌中二萜类化合物产量。常见的调控因子靶点有ROX1YPL062WYJL064WDOS2YER134CVBA5YNR063WYGR259C等。ROX1是一种通过下调麦角甾醇相关基因的表达从而减少GGPP积累的转录调节因子[79-80]。在次丹参酮二烯酵母工程菌中,仅敲除ROX1表达框,能够使GGOH产量提升近2倍[35]。据报道,敲除酿酒酵母中YPL062WYJL064W可以维持质粒稳定性和高细胞密度[81],Hu等[35]敲除了上述2个靶点,进一步提高了GGOH产量。此外,代谢组和转录组等多组学分析技术也是筛选工程菌株中潜在调控因子常用手段。Trikka等[81]将通过多组学分析筛选的提高胡萝卜素产量的靶点用于香紫苏醇工程酿酒酵母的构建,结果表明筛选的100个调控因子中的6个靶点基因ROX1YPL062WYJL064WDOS2YER134CVBA5YNR063WYGR259C的组合敲除能够使香紫苏醇产量提高了12倍,摇瓶发酵产量达750.0 mg/L。Cao等[32]也将以上6个调控因子靶点应用于香紫苏醇酿酒酵母工程菌的构建,并且通过转录组数据筛选出了多个调控因子,其中鞘氨醇N-酰基转移酶基因LAC1和NADPH脱氢酶基因OYE3 (old yellow enzyme) 2个靶点基因在工程菌中过表达后使得香紫苏醇产量提升了20%;最后,作者采用多种策略包括上游途径强化、调控中心碳代谢和辅因子供给、萜类合酶基因融合和截短、敲除相关调控因子等使工程菌香紫苏醇的产量达11.0 g/L,是目前报道二萜类化合物最高产量的工程菌(表 1)。

表 1 二萜类成分微生物生物合成策略及产量举例 Table 1 Some examples of diterpenoids biosynthesis by engineered microbes
Diterpenoid Host Carbon source Main relevant modifications Titer References
Geranylgeraniol S. cerevisiae Glucose
Ethanol
BTS1–DPP1HMG1
BTS1–ERG20
3.3 g/L
(10 L bioreactor)
[60]
Geranylgeraniol
S. cerevisiae Glucose
ERG9↓ ∆YJL064W ∆ROX1 ∆YPL062W 2.1 g/L
(5 L bioreactor)
[35]
Gibberellic acid
Yarrowia lipolytica Glucose
tHMG1GGPPSSQS↓ tAtCPStAtKStAtKO 12.8 mg/L
(shake flask)
[61]
Levopimaradiene
E. coli Glycerol
DXS↑ IDI ↑ ispD↑ ispF↑
GGPPSCD↑ LPSIF
700.0 mg/L
(3 L bioreactor)
[62]
Levopimaric acid S. cerevisiae Glucose tHMG1IDI1BTS1–ERG20
tLPSIF↑ CYP720B1↑
400.3 mg/L
(5 L bioreactor)
[29]
Taxadiene
E. coli Glycerol
DXS↑ IDI ↑ ispD↑ ispF↑ 1.0 g/L
(3 L bioreactor)
[24]
Taxadiene
S. cerevisiae Glucose ERG8↑ ERG12↑ ERG19↑ IDI1↑
MBP–TASY–ERG20F96C
129.0 mg/L
(shake flask)
[33]
Ferruginol S. cerevisiae Glucose BTS1–ERG20tHMG1
SmKSL–SmCPSSmCPRCYP76AH1
10.5 mg/L
(shake flask)
[63]
Miltiradiene
S. cerevisiae Glucose BTS1–ERG20tHMG1
SmKSL–SmCPS
365.0 mg/L
(15 L bioreactor)
[56]
Miltiradiene S. cerevisiae Glucose tHMGRUPC2.1SaGGPPS
BTS1–ERG20
488.0 mg/L
(5 L bioreactor)
[37]
Miltiradiene
S. cerevisiae Glucose ERG9↓ ∆YJL064W ∆ROX1 ∆YPL062W tSmKSL–CfTPS1 3.5 g/L
(3 L bioreactor)
[35]
Rubusoside
S. cerevisiae Glucose tHMG1IDI1FPSF112A
KAH–trCPR1↑ INO2↑ PDR11
PGM2∆WAR1 ∆GAL7
1.4 g/L
(15 L bioreactor)
[40]
Steviol E. coli Glycerol
MVA/MEP pathway↑
RsCytb5SrCPR1
17αtag–tAkKOAtCYP714A2I403L
1.1 g/L
(5 L bioreactor)
[55]
Sclareol
E. coli Glycerol
CrtEtSsLPSMVAK1↑ MVAK2↑
MVAD↑ FNI ↑ ERG20↑
1.5 g/L
(3.7 L bioreactor)
[58]
Sclareol
S. cerevisiae Glucose ∆ROX1 ∆DOX2 ∆VBA5∆YER134C
∆YNR063W ∆YGR259C
750.0 mg/L
(shake flask)
[38]
Sclareol
S. cerevisiae Glucose Global rewiring of cellular metabolism tHMG1↑ SpHMGR↑ HMG2K6R↑ ERG20F96C↑ ERG9↓
MBP–SsLPPS–SsTPSOYE3
BTS1–PaGGPPS↑ LAC1
Regulator factors as above line

11.4 g/L
(1 L bioreactor)

[32]
Carnosic acid S. cerevisiae Glucose BTS1–ERG20 F96CINO2Heme3
tSmCPS1–tSmKSL1
SmCPR–t28SpCytb5
75.2 mg/L
(5 L bioreactor)
[39]
DPP1: Diacylglycerol diphosphate phosphatase gene; GGPPSCD: GGPPS S239C/G295D; LPSIF: LPSM593I/Y700F; : Knockout; ↓: Down-regulation; ↑: Overexpression; : Connecting peptide; t: Truncated; PGM2: Phosphoglucomutase gene; MVAK1: Mevalonate kinase gene; MVAK2: Phosphomevalonate kinase gene; FNI: Isopentenyl diphosphate isomerase; MVAD: Phosphomevalonate decarboxylase gene.

目前,也有采用微生物共培养(co-culture)技术来生产植物源二萜类化合物的相关研究。Zhou等[82]将紫杉醇前体含氧紫杉烷(oxygenated taxanes)的合成途径分为紫杉二烯合成模块和含氧紫杉烷合成模块,并分别构建在大肠杆菌和酿酒酵母中,在优化共培养条件后,发酵获得33.0 mg/L的含氧紫杉烷。此外,作者还将该系统应用于丹参酮合成途径中中间体铁锈醇(ferruginol)的生产,发酵获得18.0 mg/L的铁锈醇[82]

6 总结与展望

本文系统总结了近年来植物源二萜类化合物微生物合成研究进展和代谢改造策略,包括二萜类化合物在微生物合成上游途径、中游途径、下游途径改造和其他策略4个方面,为高产二萜类化合物细胞工厂构建和实现工业化生产提供参考。目前,大部分植物源二萜类化合物生物合成途径未被完全解析,且存在途径长、蛋白种类复杂、蛋白表达和催化难等问题,因此很少二萜类化合物实现了从头合成。植物源二萜类化合物完整生物合成的解析和表征是实现微生物合成的关键和前提。随着基因组学、转录组学、代谢组学技术的发展,结合AlphaFold蛋白结构预测和蛋白质晶体结构解析技术,将推动植物源二萜类化合物下游途径关键基因挖掘和表征。此外,植物源二萜类成分如紫杉醇、雷公藤甲素(triptolide)等具有较长的生物合成途径,而长异源途径的组装和调控是构建高产工程菌株的关键。

针对植物源蛋白在宿主细胞中表达量低、催化效率低和异源长途径组装及调控等问题,研究人员尝试采用微生物共培养(co-culture)技术和外源酶区室化调控等策略来最大程度获得目标成分。微生物共培养(co-culture)将生物合成途径以模块化形式分解并构建在具有生产优势的宿主中以期最大程度减少代谢负担,该技术已应用于多种天然产物的生物合成[65, 82]。区室化策略能够防止设计的目标化合物的生物合成途径与其他途径的不利相互作用,并提供有利于特殊定位酶活性的局部环境。除利用酵母本身的细胞器和其中的代谢流外,人工设计病毒样颗粒构建酿酒酵母中的蛋白表达细胞区室也已被实现[83]。因此,根据植物源二萜类化合物生物合成途径中外源酶的亚细胞定位,采用区室化调控策略精细调控外源酶的表达位置,从而实现外源基因的高效表达。此外,采用基因回路(genetic circuit)技术根据调控元件和被调控的基因构成的特定逻辑关系设计和构建遗传装置,从而对目的基因进行调控,将菌株发育过程中代谢负担降至最低[84]。植物源二萜类化合物对底盘细胞有一定的毒性,驯化具有高耐受性、鲁棒性的工程菌株也是构建高产二萜类化合物工程菌的关键。另外,无细胞体系(cell-free system)不依赖活细胞,具有操作简单、便于控制等优势,能够将系统物质能量集中在目标蛋白质和活性化合物的生产上,是合成生物学发展的新趋势,也为植物源二萜类化合物生物合成提供新方向[85]。此外,人工元件设计、生物传感器实现基因线路动态智能调控等技术有待被应用来解决二萜类化合物长途径重建和适配性问题。

除此之外,通过设计计算工作平台,能够系统地筛选生物合成途径的优势元件并设计合成路径,生物碱及其衍生物从头生物合成平台的构建,证明了化学信息工具在合成生物学和代谢工程中预测反应、途径和酶的价值。人工智能与合成生物学交叉,不仅为底盘细胞的自动化转化、培养、筛选等操作提供设施,还能对代谢物进行快速质谱表征,解析底盘细胞的潜在调控靶点,从而实现二萜类化合物合成生物体系的高通量工程构建与优化[86]

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