Yu T W, Shen Y, Doi-Katayama Y, Tang L, Park C, Moore B S, Richard Hutchinson C, Floss H G
Department of Chemistry, Box 351700, University of Washington, Seattle, WA 98195-1700, USA.
Proc Natl Acad Sci U S A. 1999 Aug 3;96(16):9051-6. doi: 10.1073/pnas.96.16.9051.
The assembly of the polyketide backbone of rifamycin B on the type I rifamycin polyketide synthase (PKS), encoded by the rifA-rifE genes, is terminated by the product of the rifF gene, an amide synthase that releases the completed undecaketide as its macrocyclic lactam. Inactivation of rifF gives a rifamycin B nonproducing mutant that still accumulates a series of linear polyketides ranging from the tetra- to a decaketide, also detected in the wild type, demonstrating that the PKS operates in a processive manner. Disruptions of the rifD module 8 and rifE module 9 and module 10 genes also result in accumulation of such linear polyketides as a consequence of premature termination of polyketide assembly. Whereas the tetraketide carries an unmodified aromatic chromophore, the penta- through decaketides have undergone oxidative cyclization to the naphthoquinone, suggesting that this modification occurs during, not after, PKS assembly. The structure of one of the accumulated compounds together with (18)O experiments suggests that this oxidative cyclization produces an 8-hydroxy-7, 8-dihydronaphthoquinone structure that, after the stage of proansamycin X, is dehydrogenated to an 8-hydroxynaphthoquinone.
利福霉素B的聚酮骨架在由rifA - rifE基因编码的I型利福霉素聚酮合酶（PKS）上的组装，由rifF基因的产物终止，rifF基因是一种酰胺合酶，它将完整的十一酮肽作为其大环内酰胺释放出来。rifF的失活产生了一个不产生利福霉素B的突变体，该突变体仍然积累了一系列从四酮肽到十酮肽的线性聚酮化合物，在野生型中也能检测到，这表明PKS以连续的方式运行。rifD模块8、rifE模块9和模块10基因的破坏也会导致此类线性聚酮化合物的积累，这是聚酮组装过早终止的结果。四酮肽带有未修饰的芳香发色团，而五酮肽到十酮肽已经经历氧化环化形成萘醌，这表明这种修饰发生在PKS组装过程中，而不是之后。其中一种积累化合物的结构以及（18）O实验表明，这种氧化环化产生了一种8 - 羟基 - 7,8 - 二氢萘醌结构，在原安莎霉素X阶段之后，脱氢形成8 - 羟基萘醌。
