The volatile microbial metabolite such as 2-methylisoborneol (2-MIB) and geosmin is a root cause of taste and odor issues in freshwaters. These compounds degrade water quality and present problems for water treatment. 2-MIB and geosmin are synthesized and usually secreted as secondary metabolites by microorganisms, such as fungi, myxobacteria, actinomycetes, and cyanobacteria. In eutrophied waters, cyanobacteria and actinomycetes are considered as the main producer of 2-MIB. Recent years, the genetic background of geosmin was elucidated and genes controlling 2-MIB synthesis in actinomycetes have been isolated. It is interesting to know that which genes are involved in 2-MIB synthesis in cyanobacteria. 

Under the guidance of Prof. Li Renhui, principal investigator of the Research Group of Biology of Harmful Algae at Institute of Hydrobiology, Chinese Academy of Sciences (IHB), the 2-MIB associated genes were isolated and cloned successfully from MIB-producing cyanobacteria Pseudanabaena sp. and Planktothricoids raciborskii by IHB doctoral students Wang Zhongjie et al. 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. 

A paper entitled “Genes associated with 2-methylisoborneol biosynthesis in cyanobacteria: Isolation, characterization, and expression in response to light” was published in the journal PLoS ONE on April 7. 

Related link: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0018665