Jones G H
Department of Biology, Emory University, Atlanta, Georgia 30322.
J Biol Chem. 1993 Apr 5;268(10):6831-4.
Actinomycin synthetase I, which activates the precursor of the chromophore of actinomycin, has been purified nearly 1000-fold from extracts of Streptomyces antibioticus. The enzyme has an M(r) of 45,000 and appears to function as a single polypeptide chain. The formation of the enzyme is repressed when S. antibioticus mycelium is grown on glucose as a carbon source. Several benzoic acid derivatives, including intermediates in the putative pathway for actinomycin biosynthesis, were capable of supporting ATP-pyrophosphate exchange catalyzed by actinomycin synthetase I. Interestingly, three synthetic phenoxaziones also functioned as substrates in the exchange reaction. It proved possible to purify a second enzyme involved in actinomycin biosynthesis, 3-hydroxyanthranilic acid 4-methyltransferase, from the same extracts prepared for the purification of actinomycin synthetase I and by similar procedures. Streptomyces lividans, previously shown to contain a silent gene for the enzyme phenoxazione synthase, apparently does not contain a silent pathway for the complete synthesis of actinomycin since neither actinomycin synthetase I nor the methyltransferase could be detected in strains containing a cloned sequence that activates phenoxazione synthase expression.
放线菌素合成酶I可激活放线菌素发色团的前体，已从抗生链霉菌提取物中纯化了近1000倍。该酶的相对分子质量为45,000，似乎作为一条单多肽链发挥作用。当抗生链霉菌菌丝体以葡萄糖作为碳源生长时，该酶的形成受到抑制。几种苯甲酸衍生物，包括放线菌素生物合成假定途径中的中间体，能够支持放线菌素合成酶I催化的ATP-焦磷酸交换反应。有趣的是，三种合成吩恶嗪也在交换反应中作为底物发挥作用。事实证明，从为纯化放线菌素合成酶I而制备的相同提取物中，通过类似的程序，可以纯化参与放线菌素生物合成的第二种酶，即3-羟基邻氨基苯甲酸4-甲基转移酶。先前已证明淡青链霉菌含有吩恶嗪合酶的沉默基因，但显然不含有完整合成放线菌素的沉默途径，因为在含有激活吩恶嗪合酶表达的克隆序列的菌株中，既检测不到放线菌素合成酶I，也检测不到甲基转移酶。
