BIOSYNTHESIS OF VITAMIN B12 AND ANAEROBIC METABOLISM

钴胺(B-12)是人类的一种基本营养素，只有 由微生物和一些原生生物合成。B-12的生物合成是 人们对此知之甚少。中间体的复杂性和不稳定性 使生化方法变得困难，大多数研究都集中在 基因系统非常差的细菌。我们已经发现，基因 生物，鼠伤寒沙门氏菌，能够合成B-12从头开始当 在厌氧条件下生长的。我们已经启动了一种主要的基因 B-12的合成方法以及该合成是如何调控的。我们 希望确定所有合成酶的结构基因，并 阐明这些基因受调控的机制 最终产物(可能是腺苷钴胺)、cAMP和氧化还原状态 手机。 B-12的合成、运输和回收似乎需要大约1%的 沙门氏菌基因组。尽管对B-基因的投资规模很大- 12，目前已知沙门氏菌中只有四种酶需要B-12作为 辅因；这些功能似乎都不是根本重要的。 B-12依赖的功能包括蛋氨酸的(两条)路线， 一种非必需tRNA修饰的合成及其使用能力 乙醇胺和丙二醇作为碳源。这些中的哪一个，或者什么 未知的功能，解释了B-12对沙门氏菌的重要性？ 为什么沙门氏菌只能在厌氧条件下合成B-12？(很多 已知专性好氧菌能够合成B-12。)什么 生理因素是B-12在自然界中分布的原因吗？(它 不存在于植物中，这是动物和一些原生生物所必需的 主要由微生物合成。) 为了探讨B-12的生物学意义问题，我们将 调查沙门氏菌的厌氧代谢。尽管有很大的 对沙门氏菌和沙门氏菌的遗传和代谢的兴趣 其相对的大肠杆菌，厌氧行为收到的相当少 请注意。 我们必须为更好地理解无氧代谢和 它的监管，特别注意B-12的作用。

Cobalamin (B-12) is an essential nutrient for humans that is only synthesized by microbes and some protists. The biosynthesis of B-12 is rather poorly understood. The complexity and instability of intermediates has made biochemical approaches difficult and most study has been focused on bacteria with very poor genetic systems. We have found that the genetic organism, Salmonella typhimurium, is able to synthesize B-12 de novo when grown under anaerobic conditions. We have initiated a primarily genetic approach to the synthesis of B-12 and how that synthesis is regulated. We hope to identify the structural genes for all of the synthetic enzymes and elucidate the mechanisms whereby these genes are regulated in response to the end product (probably adenosylcobalamin), cAMP, and redox state of the cell. Synthesis, transport, and recycling of B-12 appears to require about 1% of the Salmonella genome. Despite the size of this genetic investment in B- 12, only four enzymes are known in Salmonella that require B-12 as a cofactor; none of these functions appears to be fundamentally important. The B-12 dependent functions include on (of two) routes to methionine, synthesis of a nonessential tRNA modification, and ability to use ethanolamine and propanediol as carbon sources. Which of these, or what undiscovered function, accounts for the importance of B-12 to Salmonella? Why does Salmonella synthesize B-12 only under anaerobic conditions? (Many obligate aerobes are known to be able to synthesize B-12.) What physiological factors account for the distribution of B-12 in nature? (It is absent from plants, required by animals and some protists, and synthesized primarily by microbes.) To approach questions of biological significance of B-12, we will investigate the anaerobic metabolism of Salmonella. Despite the great interest that has been focused on genetics and metabolism of Salmonella and its relative E. coli, anaerobic behavior has received rather little attention. We have to contribute to a better understanding of anaerobic metabolism and its regulation, with special attention to the role of B-12.