具土霉味的萜类化合物2-MIB（2-甲基异茨醇）和Geosmin(土腥素)被认为是引起各种淡水水体异味问题的主要原因。自然界中多种微生物（主要是蓝藻、链霉菌和真菌等）可以通过次生代谢合成和释放这类化合物。随着全球富营养化的加重，蓝藻水华频繁爆发，由此引起的异味问题和毒素问题极大地降低了自然和饮用水体的安全性。作为富营养化水体中重要的生物类群，蓝藻和放线菌通常被认为是异味物质合成和释放的主要生物类群。近年来，放线菌和蓝藻的Geosmin生物合成机制、放线菌2-MIB的合成机制已经相继被阐明，控制其合成的基因也获得鉴定，而蓝藻的2-MIB合成基因尚未克隆鉴定。在此背景下，蓝藻合成2-MIB的分子基础引起了研究者的关注。
在中科院水生生物研究所李仁辉研究员的指导下，博士研究生王中杰等利用PCR技术成功从两株产2-MIB的蓝藻（Pseudanabaena sp.和Planktothricoids raciborskii）中克隆了2-MIB生物合成相关基因。一个SAM依赖性甲基转移酶基因和一个单萜环化酶基因通过连续的两步催化将前体物GPP（geranyl pyrophosohate）转化为2-MIB：GPP的甲基化和随后的环化反应。这两个基因位于两个预测的环化核苷酸结合蛋白基因（Crp-Fnr调控家族）之间并与这两个基因形成了一个操纵子结构。序列相似性分析表明，蓝藻和放线菌的2-MIB相关基因具有相对较高的相似度和同源性。不同生物类群单萜环化酶基因motif位点的比较和基因的排列顺序比较的结果显示，2-MIB相关基因在进化过程中可能发生了基因间的重组。系统分析表明，蓝藻的2-MIB基因可能来源于放线菌，但是在进化的过程中形成了不同的进化路线。基因表达结果显示，光照强度是2-MIB转录调节的一个关键的环境因子，低光强促进基因转录而高光强抑制转录。在蓝藻中克隆2-MIB合成基因将有利于其异味物质形成的研究和分子监测、预警方法的建立。
该研究得到了国家基础研究973计划，国家重大科技水专项和中科院“百人计划”等项目的资助。研究论文4月7日在线发表于PLoS ONE。（生物谷Bioon.com）
生物谷推荐原文出处：
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.
