C3 unit addition reaction accompanied by formation of C-C bond in phytotoxin biosynthesis of phytopathogen.

Sorokinianin是由一株（OB-25-l） sorokiniana菌株产生的一种植物毒素，该菌株是一种引起小麦和其他禾草的斑疹病或足腐病和根腐病的病原体。这种毒素被认为是由线虫孢醇型倍半萜和C_3部分组成的。为了弄清生物合成的来源，我们进行了三个实验：[1,2-^<13>C_2]醋酸钠饲养实验；用C_3单元前体候选物和菌株OB-25-1的主要代谢物-前孢菌素进行替代培养实验；[2,3-^<13>C_2]琥珀酸饲喂试验。这些实验结果表明，由前螺孢醇通过添加TCA循环成员，脱羧和内酯化反应合成了sorokinianin。为确定直接C_3单元前驱体，利用富马酸酶和H_2_<18>O，以富马酸为原料制备了L-[羟基-^<18>O]苹果酸，并进行了L-[羟基-^<18>O]苹果酸加料实验。实验结果表明，直接C_3单元前驱体不是苹果酸/草乙酸，而是富马酸/琥珀酸。

Sorokinianin is a phytotoxin produced by a strain (OB-25-l) of Bipolaris sorokiniana, a pathogen causing spot blotch or foot and root rot diseases of wheat and other grasses. This toxin is assumed to be composed of a helminthosporol-type sesquiterpene and a C_3 portion. To clarify the biosynthetic origin we conducted three experiments : a [1,2-^<13>C_2] sodium acetate feeding experiment ; a replacement culture experiment with the candidates for the C_3 unit precursor, together with prehelminthosporol, the major metabolite of the strain OB-25-1 ; and a [2,3-^<13>C_2] succinic acid feeding experiment. The results of these experiments suggested that sorokinianin was synthesized from prehelminthosporol by reactions that include the addition of a member of the TCA cycle, decarboxylation and lactonization. To determine the direct C_3 unit precursor we prepared L-[hydroxy-^<18>O] malic acid from fumaric acid by fumarase and H_2_<18>O and conducted a L-[hydroxy-^<18>O] malic acid feeding experiment. The results of the experiment indicated that the direct C_3 unit precursor was not malic acid/oxalacetic acid but fumaric acid/succinic acid.