Trapp S C, Hohn T M, McCormick S, Jarvis B B
Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742, USA.
Mol Gen Genet. 1998 Feb;257(4):421-32. doi: 10.1007/s004380050666.
Macrocyclic trichothecenes are toxic sesquiterpenoids that are produced by certain fungi and plants. The unique structural features of macrocyclic trichothecenes result in increased toxicity relative to other trichothecene structural types. Here we report the sequences and relative locations of the MRTRI5, MRTRI6, and MRTRI4 genes in the biosynthetic pathway for macrocyclic trichothecenes in Myrothecium roridum. The deduced sequences of the products of MRTRI5 and MRTRI4 display overall identities of 75 and 63%, respectively, with the corresponding proteins in Fusarium sporotrichioides. Based on sequence comparisons, MRTRI5 encodes the enzyme trichodiene synthase, which has been shown to catalyze the first step in the trichothecene pathways of Fusarium and Trichothecium species. MRTRI6 encodes a transcription factor (392 amino acids) required for pathway gene expression, and the predicted MRTRI4 product (533 amino acids) is a cytochrome P450 monooxygenase responsible for the initial oxygenation step in the pathway. The sizes of the predicted products of MRTRI5 and MRTRI4 show good agreement with their apparent counterparts in the Fusarium pathway; however, the protein specified by MRTRI6 is almost twice the size of its putative homolog in F. sporotrichioides. Only the C-terminal 124 residues of MRTRI6, containing the proposed Cys2His2 zinc finger motifs, show significant similarity (65% identity) to the TRI6 sequence in F. sporotrichioides. MRTRI4 can successfully complement a TRI4-mutant in F. sporotrichioides, although the resulting trichothecene profile differed from that observed in wild-type strains. Complemented mutants accumulated low levels of T-2 toxin, in addition to sambucinol, deoxysambucinol, and the pathway intermediates trichothecene and isotrichodiol. Mapping data indicate that the genes of the macrocyclic trichothecene pathway in M. roridum are clustered, but that their organization and orientation differ markedly from those of the trichothecene gene cluster found in F. sporotrichioides. These results show that the biosynthetic pathways for macrocyclic trichothecenes are closely related to other trichothecene pathways and that the evolution of gene clusters for the biosynthesis of natural products in fungi can involve significant rearrangements.
大环单端孢霉烯族毒素是由某些真菌和植物产生的有毒倍半萜类化合物。与其他单端孢霉烯族毒素结构类型相比，大环单端孢霉烯族毒素独特的结构特征导致其毒性增强。在此，我们报告了在漆斑菌中参与大环单端孢霉烯族毒素生物合成途径的MRTRI5、MRTRI6和MRTRI4基因的序列及相对位置。MRTRI5和MRTRI4基因产物的推导序列与拟分枝孢镰刀菌中的相应蛋白质总体一致性分别为75%和63%。基于序列比较，MRTRI5编码 trichodiene合酶，该酶已被证明催化镰刀菌属和木霉属物种中单端孢霉烯族毒素生物合成途径的第一步。MRTRI6编码途径基因表达所需的转录因子（392个氨基酸），预测的MRTRI4产物（533个氨基酸）是一种细胞色素P450单加氧酶，负责该途径中的初始氧化步骤。MRTRI5和MRTRI4预测产物的大小与其在镰刀菌途径中明显对应的产物大小相符；然而，MRTRI6指定的蛋白质几乎是其在拟分枝孢镰刀菌中假定同源物大小的两倍。只有MRTRI6的C末端124个残基，包含推测的Cys2His2锌指基序，与拟分枝孢镰刀菌中的TRI6序列显示出显著相似性（65%一致性）。MRTRI4可以成功互补拟分枝孢镰刀菌中的TRI4突变体，尽管产生的单端孢霉烯族毒素谱与野生型菌株中观察到的不同。互补突变体除了积累低水平的T - 2毒素外，还积累了接骨木醇、脱氧接骨木醇以及途径中间体单端孢霉烯族毒素和异单端孢二醇。定位数据表明，漆斑菌中大环单端孢霉烯族毒素途径的基因是成簇的，但它们的组织和方向与拟分枝孢镰刀菌中发现的单端孢霉烯族毒素基因簇明显不同。这些结果表明，大环单端孢霉烯族毒素的生物合成途径与其他单端孢霉烯族毒素途径密切相关，并且真菌中天然产物生物合成基因簇的进化可能涉及重大重排。
