DMSP SYNTHESIS VIA A NOVEL ENZYME IN CYANOBACTERIA AND DIVERSE BACTERIA

二甲基磺基丙酸酯（DMSP）是一种重要的高丰度有机硫化合物。它由许多藻类、细菌和一些高等植物合成，其中它被认为涉及趋化性、食草动物威慑、植物保护、低温保护、静水压力保护和/或抗氧化应激。DMSP及其气态分解产物二甲基硫（DMS）和甲烷硫醇（MeSH）是硫从海洋向大气转移的主要生物分解物。大气DMS和MeSH是气候活性气体（CAG），形成气溶胶和云凝结核，减少全球辐射收支并“冷却”当地气候。以前认为，只有蓝细菌中的束丝藻属物种和低比例的海洋细菌才能产生显著水平的DMSP。然而，我们的试点工作表明，许多蓝藻，例如高度丰富的海洋聚球藻和盐沼物种，产生DMSP，以及其他重要和丰富的细菌门（例如γ-变形菌）。这些微生物含有一个基因，我们称之为dsyC，我们显示它编码关键的S-甲基转移酶，催化DMSP合成的承诺和限速步骤。这项工作很重要，因为dsyC基因存在于高达5%的海洋细菌中，并且在地球的透光沃茨和表面沉积物中高度转录，这是根据我们对塔拉海洋和当地数据集的分析建立的。相比之下，我们和其他人发现的其他DMSP合成基因（dsyB，DSYB，mmtN和TpMMT），编码交替细菌和藻类DMSP生物合成途径的关键S-甲基转移酶，总体上远不如dsyC丰富和转录。因此，蓝细菌和其他不同的细菌（氰基/细菌）与DsyC可能是非常重要的贡献者全球生产的DMSP，因此，从它衍生的CAG。这将是一个范式转变的发现，表明氰基/细菌，几乎忽略了重要的DMSP生产商，是大规模的贡献者全球生产。然而，如果没有这里计划的多学科工作，我们无法做出这样的声明。第一个主要目标将是建立DMSP在具有DsyC同系物的氰基/细菌中产生的环境条件。然后，我们将在海洋聚球藻属物种和一系列具有dsyC的其他细菌中产生dsyC敲除。然后将在一系列环境或胁迫条件下比较野生型和突变菌株的生长，以评估DMSP在这些细菌中的作用，以及重要的是DsyC依赖性DMSP合成的主要环境驱动因素。同时，我们将确定野生型和突变体样品中每个细胞的DMSP量和合成速率。下一个目标将是确定DsyC酶的关键特征。为了解决这个问题，我们将检查来自一系列氰基/细菌的DsyC的酶活性和底物亲和力，以鉴定可以确定它们是高活性还是低活性类型的关键氨基酸残基。最后，通过分析的氰基/细菌样品从广泛的贫营养和沿海沃茨，盐沼蓝藻垫的季节性研究，我们将量化DMSP和CAG水平和合成率，并比较这些实验室培养的模式生物。这将使我们能够估计每年，全球生产率的DMSP的海洋聚球藻和潜在的原绿球藻物种，本地化生产的所有物种在西太平洋和东印度洋，本地化盐沼和潜在的生产DMS，因此蓝藻DMSP生产的环境影响。

Dimethylsulfoniopropionate (DMSP) is an important and highly abundant organo-sulfur compound. It is synthesised by many algae, bacteria and some higher plants, where it is thought to be involved in chemotaxis, grazer deterrence, osmoprotection, cryoprotection, hydrostatic pressure protection and/or resistance to oxidative stress. DMSP, and its gaseous breakdown products, dimethyl sulfide (DMS) and methanethiol (MeSH) are the major biosources of sulfur transferred from the oceans to the atmosphere. Atmospheric DMS and MeSH are climate active gases (CAGs) that form aerosols and cloud condensation nuclei, which reduce the global radiation budget and 'cool' the local climate.It was previously thought that only Trichodesmium species of cyanobacteria and low proportions of marine bacteria produce DMSP at significant levels. However, our pilot work shows that many cyanobacteria, e.g. highly abundant marine Synechococcus and saltmarsh species, produce DMSP, as well as other important and abundant bacterial phyla (e.g. gammaproteobacteria). These microbes contain a gene that we term dsyC, which we show encodes the key S-methyltransferase enzyme that catalyses the committed and rate limiting step of DMSP synthesis. This work is important because dsyC genes occur in up to 5% of marine bacteria and are highly transcribed in Earth's photic waters and surface sediment, established from our analysis of the Tara Oceans and local datasets. In comparison, the other DMSP synthesis genes that we and others discovered (dsyB, DSYB, mmtN and TpMMT), encoding the key S-methyltransferase of alternate bacterial and algal DMSP biosynthesis pathways, are collectively far less abundant and transcribed than dsyC. Therefore, cyanobacteria and other diverse bacteria (cyano/bacteria) with DsyC could be very significant contributors to global production of DMSP, and, thus, of CAGs derived from it. This would be a paradigm-shifting finding, showing that cyano/bacteria, pretty much ignored as significant DMSP producers, are large-scale contributors to global production. However, without the multidisciplinary work planned here we are unable to make such statements.The first major goal will be to establish the environmental conditions under which DMSP is produced in cyano/bacteria that have DsyC homologues. We will then generate dsyC knockouts in marine Synechococcus species and a selection of other bacteria with dsyC. Growth of wild-type and mutant strains will then be compared under a range of environmental or stress conditions to assess the role of DMSP in these bacteria and importantly the major environmental drivers of DsyC-dependent DMSP synthesis. In conjunction, we will determine the amount of DMSP per cell and synthesis rates in wild-type and mutant samples. The next goal will be to determine the key features of the DsyC enzyme. To address this, we will examine the enzymatic activity and substrate affinity of DsyC from a range of cyano/bacteria to identify the key amino acid residues that could determine whether they are of a high activity or low activity type. Finally, via analysis of cyano/bacterial samples from a broad range of oligotrophic and coastal waters, and a seasonal study of saltmarsh cyanobacterial mats, we will quantify DMSP and CAG levels and synthesis rates and compare these to laboratory cultured model organisms. This will allow us to estimate annual, global production rates of DMSP by marine Synechococcus and potentially Prochlorococcus species, localised production by all species in Western Pacific and Eastern Indian oceans, and localised saltmarshes and potential production of DMS and therefore the environmental impact of cyanobacterial DMSP production.