BIOSYNTHESIS OF VITAMIN B12 & ANAEROBIC METABOLISM

钴胺素（B-12）是人体必需的营养素， 由微生物和一些原生生物合成。 B-12的生物合成是 相当不了解。 中间体的复杂性和不稳定性 使得生物化学方法变得困难，大多数研究都集中在 基因系统非常差的细菌上。 我们发现， 一种微生物，鼠伤寒沙门氏菌，能够从头合成B-12， 在厌氧条件下生长。 我们已经启动了一个主要的基因 合成B-12的方法以及该合成是如何调节的。 我们 我希望能找出所有合成酶的结构基因， 阐明这些基因响应于 最终产物（可能是腺苷钴胺素），cAMP和氧化还原状态的 cell. B-12的合成、运输和再循环似乎需要约1%的 沙门氏菌基因组 尽管这种基因投资在B的规模- 12，只有四种酶是已知的沙门氏菌，需要B-12作为一个 辅助因子;这些功能似乎都不是根本重要的。 B-12依赖性功能包括一种（两种）甲硫氨酸途径， 合成非必需的tRNA修饰，以及使用 乙醇胺和丙二醇作为碳源。 这些中的哪一个，或者什么 未被发现的功能，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？ （许多 已知专性需氧菌能够合成B-12。 什么 生理因素占B-12在自然界中的分布？ （它 是植物中不存在的，是动物和一些原生生物所需要的， 主要由微生物合成）。 为了探讨B-12的生物学意义，我们将 研究沙门氏菌的厌氧代谢。 尽管大 兴趣一直集中在沙门氏菌的遗传学和代谢， 其相对分子量E.大肠杆菌，厌氧行为已经收到相当少， 关注 我们必须为更好地理解无氧代谢做出贡献， 其调节，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.