Funabashi Masanori, Funa Nobutaka, Horinouchi Sueharu
Department of Biotechnology, Graduate School of Agriculture and Life Sciences, The University of Tokyo,Tokyo 113-8657, Japan.
J Biol Chem. 2008 May 16;283(20):13983-91. doi: 10.1074/jbc.M710461200. Epub 2008 Mar 24.
Type III polyketide synthases (PKSs) found in plants, fungi, and bacteria synthesize a variety of aromatic polyketides. A Gram-positive, filamentous bacterium Streptomyces griseus contained an srs operon, in which srsA encoded a type III PKS, srsB encoded a methyltransferase, and srsC encoded a flavoprotein hydroxylase. Consistent with this annotation, overexpression of the srs genes in a heterologous host, Streptomyces lividans, showed that SrsA was a type III PKS responsible for synthesis of phenolic lipids, alkylresorcinols, and alkylpyrones, SrsB was a methyltransferase acting on the phenolic lipids to yield alkylresorcinol methyl ethers, and SrsC was a hydroxylase acting on the alkylresorcinol methyl ethers. In vitro SrsA reaction showed that SrsA synthesized alkylresorcinols from acyl-CoAs of various chain lengths as a starter substrate, one molecule of methylmalonyl-CoA, and two molecules of malonyl-CoA. SrsA was thus unique in that it incorporated the extender substrates in a strictly controlled order of malonyl-CoA, malonyl-CoA, and methylmalonyl-CoA to produce alkylresorcinols. An srsA mutant, which produced no phenolic lipids, was highly sensitive to beta-lactam antibiotics, such as penicillin G and cephalexin. Together with the fact that the alkylresorcinols were fractionated mainly in the cell wall fraction, this observation suggests that the phenolic lipids, perhaps associated with the cytoplasmic membrane because of their amphiphilic property, affect the characteristic and rigidity of the cytoplasmic membrane/peptidoglycan of a variety of bacteria. An srs-like operon is found widely among Gram-positive and -negative bacteria, indicating wide distribution of the phenolic lipids.
在植物、真菌和细菌中发现的III型聚酮合酶（PKSs）能合成多种芳香族聚酮化合物。革兰氏阳性丝状细菌灰色链霉菌含有一个srs操纵子，其中srsA编码一种III型PKS，srsB编码一种甲基转移酶，srsC编码一种黄素蛋白羟化酶。与该注释一致，在异源宿主变铅青链霉菌中过表达srs基因表明，SrsA是一种III型PKS，负责合成酚类脂质、烷基间苯二酚和烷基吡喃酮，SrsB是一种作用于酚类脂质以产生烷基间苯二酚甲基醚的甲基转移酶，SrsC是一种作用于烷基间苯二酚甲基醚的羟化酶。体外SrsA反应表明，SrsA以各种链长的酰基辅酶A作为起始底物、一分子甲基丙二酰辅酶A和两分子丙二酰辅酶A合成烷基间苯二酚。因此，SrsA的独特之处在于它以丙二酰辅酶A、丙二酰辅酶A和甲基丙二酰辅酶A严格控制的顺序掺入延伸底物以产生烷基间苯二酚。一个不产生酚类脂质的srsA突变体对β-内酰胺类抗生素（如青霉素G和头孢氨苄）高度敏感。再加上烷基间苯二酚主要在细胞壁部分被分离出来这一事实，该观察结果表明，酚类脂质可能因其两亲性与细胞质膜相关，会影响多种细菌细胞质膜/肽聚糖的特性和刚性。在革兰氏阳性和阴性细菌中广泛发现了一个类似srs的操纵子，这表明酚类脂质分布广泛。
