具土霉味的萜類化合物2-MIB（2-甲基異茨醇）和Geosmin(土腥素)被認(rèn)為是引起各種淡水水體異味問題的主要原因,。自然界中多種微生物（主要是藍藻、鏈霉菌和真菌等）可以通過次生代謝合成和釋放這類化合物,。隨著全球富營養(yǎng)化的加重,，藍藻水華頻繁爆發(fā),，由此引起的異味問題和毒素問題極大地降低了自然和飲用水體的安全性。作為富營養(yǎng)化水體中重要的生物類群,，藍藻和放線菌通常被認(rèn)為是異味物質(zhì)合成和釋放的主要生物類群,。近年來，放線菌和藍藻的Geosmin生物合成機制,、放線菌2-MIB的合成機制已經(jīng)相繼被闡明,，控制其合成的基因也獲得鑒定，而藍藻的2-MIB合成基因尚未克隆鑒定,。在此背景下,，藍藻合成2-MIB的分子基礎(chǔ)引起了研究者的關(guān)注,。
在中科院水生生物研究所李仁輝研究員的指導(dǎo)下，博士研究生王中杰等利用PCR技術(shù)成功從兩株產(chǎn)2-MIB的藍藻（Pseudanabaena sp.和Planktothricoids raciborskii）中克隆了2-MIB生物合成相關(guān)基因,。一個SAM依賴性甲基轉(zhuǎn)移酶基因和一個單萜環(huán)化酶基因通過連續(xù)的兩步催化將前體物GPP（geranyl pyrophosohate）轉(zhuǎn)化為2-MIB：GPP的甲基化和隨后的環(huán)化反應(yīng),。這兩個基因位于兩個預(yù)測的環(huán)化核苷酸結(jié)合蛋白基因（Crp-Fnr調(diào)控家族）之間并與這兩個基因形成了一個操縱子結(jié)構(gòu)。序列相似性分析表明,，藍藻和放線菌的2-MIB相關(guān)基因具有相對較高的相似度和同源性,。不同生物類群單萜環(huán)化酶基因motif位點的比較和基因的排列順序比較的結(jié)果顯示，2-MIB相關(guān)基因在進化過程中可能發(fā)生了基因間的重組,。系統(tǒng)分析表明,，藍藻的2-MIB基因可能來源于放線菌，但是在進化的過程中形成了不同的進化路線,?；虮磉_結(jié)果顯示，光照強度是2-MIB轉(zhuǎn)錄調(diào)節(jié)的一個關(guān)鍵的環(huán)境因子,，低光強促進基因轉(zhuǎn)錄而高光強抑制轉(zhuǎn)錄,。在藍藻中克隆2-MIB合成基因?qū)⒂欣谄洚愇段镔|(zhì)形成的研究和分子監(jiān)測、預(yù)警方法的建立,。
該研究得到了國家基礎(chǔ)研究973計劃,，國家重大科技水專項和中科院“百人計劃”等項目的資助。研究論文4月7日在線發(fā)表于PLoS ONE,。（生物谷Bioon.com）
生物谷推薦原文出處：
PLoS ONE 6(4): e18665. doi:10.1371/journal.pone.0018665
Genes Associated with 2-Methylisoborneol Biosynthesis in Cyanobacteria: Isolation, Characterization, and Expression in Response to Light
Zhongjie Wang1,2, Yao Xu3, Jihai Shao4, Jie Wang5, Renhui Li1*
1 Key Laboratory of Aquatic Biodiversity and Conservation, Institute of Hydrobiology, Chinese Academy of Sciences, Wuhan, People's Republic of China, 2 Graduate University of Chinese Academy of Sciences, Beijing, People's Republic of China, 3 College of Geography Science, Nanjing Normal University, Nanjing, People's Republic of China, 4 Resources and Environment College, Hunan Agricultural University, Changsha, People's Republic of China, 5 College of Life Science, Shanxi University, Taiyuan, People's Republic of China
The volatile microbial metabolite 2-methylisoborneol (2-MIB) is a root cause of taste and odor issues in freshwater. Although current evidence suggests that 2-MIB is not toxic, this compound degrades water quality and presents problems for water treatment. To address these issues, cyanobacteria and actinomycetes, the major producers of 2-MIB, have been investigated extensively. In this study, two 2-MIB producing strains, coded as Pseudanabaena sp. and Planktothricoids raciborskii, were used in order to elucidate the genetic background, light regulation, and biochemical mechanisms of 2-MIB biosynthesis in cyanobacteria. Genome walking and PCR methods revealed that two adjacent genes, SAM-dependent methyltransferanse gene and monoterpene cyclase gene, are responsible for GPP methylation and subsequent cyclization to 2-MIB in cyanobacteria. These two genes are located in between two homologous cyclic nucleotide-binding protein genes that may be members of the Crp-Fnr regulator family. Together, this sequence of genes forms a putative operon. The synthesis of 2-MIB is similar in cyanobacteria and actinomycetes. Comparison of the gene arrangement and functional sites between cyanobacteria and other organisms revealed that gene recombination and gene transfer probably occurred during the evolution of 2-MIB-associated genes. All the microorganisms examined have a common origin of 2-MIB biosynthesis capacity, but cyanobacteria represent a unique evolutionary lineage. Gene expression analysis suggested that light is a crucial, but not the only, active regulatory factor for the transcription of 2-MIB synthesis genes. This light-regulated process is immediate and transient. This study is the first to identify the genetic background and evolution of 2-MIB biosynthesis in cyanobacteria, thus enhancing current knowledge on 2-MIB contamination of freshwater.
