BIOSYNTHESIS OF VITAMIN B12 AND ANAEROBIC METABOLISM

沙门氏菌将其基因组的1%用于合成维生素B12（钴胺素），维生素B12是沙门氏菌从头合成的一种大辅因子（分子量1570），而大肠杆菌则不是。杆菌 另外1%的沙门氏菌基因组编码需要B12的代谢途径（包括乙醇胺和丙二醇的降解）。 对B12的合成、进口和应用的几个方面进行了研究，包括B12的低级配体（二甲基苯并咪唑）的来源。 我们有一个周质钴胺素结合蛋白参与运输的遗传证据，并有突变体，表明运输蛋白可能有助于抑制的cob（生物合成）操纵子超越仅仅是重要的效应。 对B12依赖性降解途径的分析揭示了几个意想不到的特征。(1)B12的生理重要性很可能是厌氧的（B12产生的唯一条件）。 在丙二醇和乙醇胺上的钴胺素依赖性厌氧生长发生在电子受体连四硫酸盐上，但不发生在富马酸盐或硝酸盐上。 我们将继续研究连四硫酸盐还原的遗传学、生物化学和调控，并试图了解为什么这种受体是独特的，以及它在自然界中出现的地方。 (2)非常大的操纵子编码使用乙醇胺（eut; 17个基因）和丙二醇（pdu;大于20个基因）的酶。 (3)这些操纵子的大多数基因在所使用的条件下没有突变表型，这表明它们在新的条件下起作用，或者对其他代谢功能是多余的。 (4)这两个操纵子编码多个蛋白质同源的壳蛋白的羧基体，细胞器，这是我们最近可视化。 这表明eut和pdu途径可能涉及CO2固定。 在高CO2条件下生长的细胞有第二种乙醇胺使用途径，它只需要一种由17-基因（eut）操纵子产生的酶（EutE）; CO2途径依赖于B12，但不需要B12依赖性酶乙醇胺氨裂解酶（EutBC）。 我们正在描述这些路径。 我们认为B12代谢是沙门氏菌（一种病原体）和它的姐妹种大肠杆菌之间的主要差异的基础。杆菌 虽然这些生物在实验室中看起来非常相似，但它们很容易在分类学上加以区分。 B12（cob、cbi）的合成、丙二醇（pdu）的使用和多硫化物（ttr、phs、asr）的还原都是用于区分沙门氏菌和大肠杆菌的特征。杆菌 了解这种代谢可能有助于了解鲑鱼的自然生活方式。

Salmonella dedicates 1 percent of its genome to synthesis of vitamin B12 (cobalamin) a large cofactor (MW 1570) synthesized de novo by Salmonella, but not E. coli. Another 1 percent of the Salmonella genome encodes metabolic pathways (including degradation of ethanolamine and propanediol) that require the B12. We propose work on several aspects of B12 synthesis import and use, including the source of the lower ligand of B12 (dimethylbenzimidazole). We have genetic evidence for a periplasmic cobalamin binding protein involved in transport and have mutants that suggest the transport proteins may contribute to repression of the cob (biosynthetic) operon beyond mere important of the effector. Analysis of the B12-dependent degradative pathways has revealed several unexpected features. (1) Physiological importance of B12 is likely to be anaerobic (the only conditions under which B12 is made). Cobalamin- dependent anaerobic growth on propanediol and ethanolamine occurs with the electron acceptor tetrathionate, but not with fumarate or nitrate. We will pursue the genetics, biochemistry and regulation of tetrathionate reduction and will try to learn why this acceptor is unique and where it appears in nature. (2) Very large operons encode enzymes for use of ethanolamine (eut; 17 genes) and propanediol (pdu; greater than 20 genes). (3) Most of the genes of these operons have no mutant phenotype under the conditions used, suggesting that they function under novel conditions or are redundant to other metabolic functions. (4) Both operons encode multiple proteins homologous to the shell proteins of carboxysomes, organelles, which we have recently visualized. This suggests that the eut and pdu pathways could involve CO2 fixation. Cells grown with high CO2 have a second pathway for ethanolamine use which requires only one (EutE) enzyme produced by the 17-gene (eut) operon; the CO2-pathway depends on B12 but does not require the B12-dependent enzyme ethanolamine ammonia lyase(EutBC). We are characterizing these pathways. We feel that B12 metabolism underlies major differences between Salmonella (a pathogen) and its sister- species E. coli. While these organisms appear very similar in the lab, they are easily distinguished taxonomically. Synthesis of B12 (cob, cbi), use of propanediol (pdu) and reduction of polysulfide (ttr, phs, asr) are all properties used to distinguish Salmonella from E. coli. Understanding this metabolism may contribute to understanding the natural lifestyle of Salmonellae.