DMS SYNTHESIS

Introduction

Dimethyl Sulfide (DMS) is a simple volatile material formed through multi-step reactions by some marine organisms. After formed in the ocean, it is volatilized, decomposed by being exposed to ultraviolet rays in the sky. Then it is converted to sulfate aerosol. In nature this sulfate aerosol plays a role as one of Cloud Condensation Nuclei (CCN), which are tiny particles around which water vapor condenses to form cloud (Fig. 1) [1].

Fig. 1 The marine organisms produce DMS and DMSP. DMS becomes sulfate aerosol, and then plays a role of CCN.

There exists Methionine (Met) - DimethylSulfidePropionate (DMSP) - DMS route as one of DMS biosynthetic pathway. This route consists of Met-DMSP synthetic pathway of certain diatoms and corals and DMSP-DMS metabolic pathway of marine bacteria [2] (Fig.1).

Fig. 2 The synthetic pathway of Met to DMS

The structure of each intermediate chemical had been already clarified.(Fig. 2)

They are 4-Methylthio-2-oxobutyrate(MTOB), 4-Methylthio-2-hydroxybutyrate(MTHB) and 4-Dimethylsulfonio-2-hydroxy-butyrate(DMSHB). However, genes responsible for each intermediate reaction of Met-DMSP pathway are still unknown (Fig. 2). And only candidates in Fragilariopsis cylindrus are suggested by Barbara R. Lyon et al [5]. This organism is a model sea-ice diatom and can produce DMS in Met-DMSP synthetic pathway.

Fig. 3-1 F. cylindrus uses DMSP in order to control their osmotic pressure.

Fig. 3-2 Barbara R. Lyon et al. compared the expression level under two different conditions (high-salinity and general-salinity) and chose the 5 candidate genes of Met-DMSP synthetic pathway.

Their osmotic pressure is controlled by DMSP density (Fig. 3-1). So, the authors hypnotized that proteins whose amounts increase under a condition when F. cylindrus produces a lot of DMSP seem to be enzymes that catalyze the DMSP biosynthetic pathway. They compared the proteome of F. cylindrus cultured under high-salinity with under general salinity by using 2-dimensional electrophoresis and found that the amounts of some proteins increased under high salinity condition compared to general condition (Fig. 3-2). Taking advantage of the mass spectrometry to these proteins, chemical reaction types in the Met-DMSP biosynthesis pathway and knowledge on substances of known enzymes, they speculated 5 candidate genes (AT, REDOX, SAMmt, DECARB, DiDECARB ) might assigned to each step.

On the other hand, a gene concerned with DMSP-DMS pathway is identified. The gene is called dddD. The enzyme which catalyzes the one-step reaction is encoded by dddD gene. dddD gene of Marinomonas sp., one of marine bacteria, was functional in E. coli. [3]

Depending on these previous works, we speculate introducing the 5 genes of F. cylindrus and dddD could enable E. coli to produce DMS. But it is still unclear whether each of these genes finely express in E. coli because these genes are just candidates. So we tried to introduce separately into E. coli and verify its expression. In order to know whether the candidate proteins properly catalyze each reaction step, we used High Performance Liquid Chromatography (HPLC) to detect each reaction product of Met-DMSP synthetic route. And to detect DMS, we used DMS detecting tube. aa