Molecular Genetics of Host-Specific Toxins in Plant Disease

从链格孢属和旋孢属病原菌中克隆了许多与产生宿主特异性毒素有关的基因。在美日联合研究项目中，对其中的一些蛋白质的结构和功能进行了阐述。苹果叶斑病AM毒素环肽合成酶基因（4 MT）全长3 kb，无内含子，有4个氨基酸激活区，对组成肽毒素的氨基酸具有特异性。对日本梨黑斑病病原菌AK-毒素生物合成相关基因簇AKT 1（脂肪酸合成）、AKT 3（脂肪酸修饰）、AKTR（调节子）、AKTS 1（类似于苏氨酸脱氢酶，其真正作用尚不清楚，但特异于日本梨致病型）和AKT 2（与数据库中已知蛋白质无相似性）进行了结构和功能分析。草莓致病型具有AKT 1、AKT 3和AKTR同源基因，草莓致病型具有AKTR同源基因。基因的共享分布与毒素的分子相似性是一致的。并对Cochijobolus中TOX 1和TOX 2的结构和功能进行了深入的研究。从寄主粗柠檬的线粒体DNA中克隆了一个对ACR毒素敏感的基因，并通过蛋白质组分析从感病苹果叶片中分离到一个对AM毒素敏感的特异蛋白。大多数寄主特异性毒素生物合成的真菌基因会水平转移。

Many fungal genes responsible for the producion of host-specific toxins were cloned from Alternaria and Cochliobolus pathogens. Some of them were elucidated thier structures and function in the course of US/Japan Joint Research Project. A cyclic peptide synthetase gene (4MT) for AM-toxin of leaf blotch of apple pathogen was found to bel3kb in length with no introns and have four domains of amino acid activation specific for the amino acids constituting the peptide toxin. The gene cluster involved in AK-toxin biosynthesis in the black spot pathogen of Japanese pear were made structural and functional analyses AKT1 (fatty acid synthesis), AKT3(modification of fatty acid), AKTR(regulator), AKTS1 (similar to threonine dehydratase, its true role is unclear, but specific to the Japanese pear pathotype) and AKT2 (no similarity to known proteins in the databases). The strawberry pathotye had AKTl AKT3 and AKTR homologues, and the tangerine pathotype possessed AKTR homologue. The shared distribution of the genes were compatible with the molecular similarity of toxins. Also, structure and function of TOX1 and TOX2 in Cochijobolus were elucidated in depth. A gene for the sensitivity to ACR-toxin was cloned from mtDNA of host rough lemon, and a protein specific to the AM-toxin sensitivity was isolated from susceptible apple leaves by proteome analysis. Most fungal genes for host specific toxin biosynthesis would be horizontally transferred.