Cloning the smell of the seaside - molecular genetics of dimethyl sulphide production by bacteria

我们都去过海边，我们都被知心的父母告知要“呼吸臭氧”，因为它“对你有好处”。首先，它不是臭氧，其次，它对你不是很好。这种独特的香气实际上是另一种叫做二甲基硫化物（DMS）的气体，自1971年以来，人们就知道它非常重要，全世界每年约有3000万吨二甲基硫化物被释放到空气中。一旦进入大气层，它就会产生其他主要影响，成为引发海洋云层形成的“种子”。事实上，有人提出，这种分子的生产规模如此之大，以至于对世界气候产生了重大影响。然而，尽管如此，我们完全不知道，在分子水平上，这个过程是如何发生的。更令人惊讶的是，我们早就知道，许多海洋细菌，其中一些很容易在实验室中生长，如果提供关键的前体分子，可以释放DMS，称为二甲基磺胺丙酸酯（简称DMSP）。我们每天读到的化合物不是一种，但在世界的海洋和海岸中有超过20亿吨的化合物。这差不多是另一个海边标志——黑潭塔的重量——7万倍。不可思议。这种DMSP分子被大量的海洋植物——海藻和微小的浮游生物——用作缓冲剂或渗透保护剂，抵御海水的咸味。当这些植物死亡时，从它们身上逸出的一些DMSP被一些海洋细菌用作食物，当它们这样做时，它们在这个过程中将其中一些转化为DMS气体。我们最近从诺福克海岸分离出了一种消耗dmsp的细菌，并使用了各种分子技术来获取与之相关的一些基因。通过观察它们的序列，我们可以猜测这些基因可能在做什么，到目前为止，看起来这些机制与之前提出的假设非常不同。我们还发现，非常相似的基因也存在于其他一些意想不到的细菌中，比如那些在陆地植物根部共生的细菌。因此，细菌产生DMS的范围可能比我们想象的要广泛和多样得多。我们现在希望对这一过程有更深入的了解，至少是在“我们的”菌株中。我们想要识别和描述所有涉及的酶，我们想知道途径是如何调节的——我们已经知道这些细菌并不愚蠢，因为它们只有在化合物存在于它们的环境中时才会打开它们的系统来降解DMSP。一旦我们知道诺福克菌株发生了什么，就应该很容易发现其他种类的海洋细菌是否也以同样的方式吃DMSP。所以，这是第一次，我们接近于真正了解这条路径的分子细节，让我们能够娱乐，吸引和教育我们的朋友下次我们去大雅茅斯，有人问起从金色沙滩上飘上来的腐烂海藻的微妙气味。

We've all been to the seaside and we've all been told by a knowing parent to 'breathe in that ozone', because it's 'good for you'. Well, firstly, it's not ozone and second, it's not terribly good for you. That distinctive aroma is, in fact another gas, called dimethyl sulphide (DMS) and it has been known since 1971 that it is hugely important, with some 30 million tons of it being liberated into the air, world wide, every year. And once in the atmosphere it has other major effects, being the 'seed' that sets off cloud formation over the oceans. Indeed, it has been proposed that the production of this molecule is on such a scale that it has major effects on the world's climate. Yet, despite all this, we have absolutely no idea of how, at a molecular level, this process occurs. This is all the more surprising since we have known for some time that many marine bacteria, some of which are easy to grow in the laboratory, can liberate DMS if supplied with the key precursor molecule, called Dimethylsulphiopropionate - DMSP for short. Not a compound one reads about every day, yet there are over two billion tonnes of it in the world's oceans, seas and seashores. That's the weight, give or take, of another seaside symbol, the Blackpool Tower - 70,000 times over. Amazing. This DMSP molecule is used by the great masses of marine plant life - seaweeds and microscopic plankton - as a buffer, or osmo-protectant, against the saltiness of the sea. When these plants die, some of the DMSP that escapes from them is used as food by some marine bacteria and, when they do so, they convert some of it to the DMS gas in the process. We recently isolated one such DMSP-consuming bacterium from the Norfolk coast and used various molecular techniques to get our hands on some of the genes that are involved. By looking at their sequences, we can guess what the genes might be doing and, so far, it looks as if the mechanisms are very different from those hypothetical ones that had been proposed before. We also saw that very similar genes exist in some other, very unexpected, types of bacteria, such as those that live, symbiotically, on the roots of land plants. So the extent of DMS production by bacteria may be far wider and varied than we had thought. We now hope to get a much deeper understanding on this process, at least in 'our' strain. We want to identify and characterize all the enzymes that are involved and we want to know how the pathway is regulated - we already know that these bacteria are not stupid, since they only switch on their systems for degrading the DMSP if the compound is present in their environment. Once we know what is happening with this Norfolk strain, it should be fairly straightforward to find out if other types of marine bacteria that eat DMSP do so in the same way. So, for the first time, we are close to getting a real insight into the molecular details of this pathway, allowing us to amuse, fascinate and educate our friends the next time we go to Great Yarmouth and somebody asks about the delicate scent of rotting seaweed that drifts up from the golden sands.