Mitchell Thomas K, Chilton William Scott, Daub Margaret E
Department of Plant Pathology, North Carolina State University, Raleigh, North Carolina 27695-7616, USA.
Appl Environ Microbiol. 2002 Sep;68(9):4173-81. doi: 10.1128/AEM.68.9.4173-4181.2002.
Cercosporin is a non-host-specific polyketide toxin produced by many species of plant pathogens belonging to the genus Cercospora. This red-pigmented, light-activated toxin is an important pathogenicity determinant for Cercospora species. In this study, we screened 244 bacterial isolates representing 12 different genera for the ability to degrade cercosporin. Cercosporin degradation was determined by screening for the presence of cleared zones surrounding colonies on cercosporin-containing culture medium and was confirmed by assaying the kinetics of degradation in liquid medium. Bacteria belonging to four different genera exhibited the cercosporin-degrading phenotype. The isolates with the greatest cercosporin-degrading activity belonged to Xanthomonas campestris pv. zinniae and X. campestris pv. pruni. Isolates of these pathovars removed over 90% of the cercosporin from culture medium within 48 h. Bacterial degradation of red cercosporin was accompanied by a shift in the color of the growth medium to brown and then green. The disappearance of cercosporin was accompanied by the appearance of a transient green product, designated xanosporic acid. Xanosporic acid and its more stable lactone derivative, xanosporolactone, are nontoxic to cercosporin-sensitive fungi and to plant tissue and are labile in the presence of light. Detailed spectroscopic analysis (to be reported in a separate publication) of xanosporolactone revealed that cercosporin loses one methoxyl group and gains one oxygen atom in the bacterial conversion. The resulting chromophore (4,9-dihydroxy-3-oxaperlylen-10H-10-one) has never been reported before but is biosynthetically plausible via oxygen insertion by a cytochrome P-450 enzyme.
尾孢菌素是由许多尾孢菌属的植物病原体产生的一种非寄主特异性聚酮化合物毒素。这种红色色素、光激活毒素是尾孢菌属物种的重要致病决定因素。在本研究中，我们筛选了代表12个不同属的244株细菌分离株降解尾孢菌素的能力。通过筛选含尾孢菌素培养基上菌落周围是否存在透明圈来确定尾孢菌素的降解情况，并通过测定液体培养基中的降解动力学来进行确认。属于四个不同属的细菌表现出降解尾孢菌素的表型。降解尾孢菌素活性最强的分离株属于野油菜黄单胞菌丁香致病变种和野油菜黄单胞菌李致病变种。这些致病变种的分离株在48小时内从培养基中去除了超过90%的尾孢菌素。红色尾孢菌素的细菌降解伴随着生长培养基颜色先变为棕色然后变为绿色的变化。尾孢菌素的消失伴随着一种瞬时绿色产物（命名为尾孢菌酸）的出现。尾孢菌酸及其更稳定的内酯衍生物尾孢菌内酯对尾孢菌素敏感真菌和植物组织无毒，并且在光照下不稳定。对尾孢菌内酯的详细光谱分析（将在另一篇出版物中报道）表明，在细菌转化过程中，尾孢菌素失去一个甲氧基并获得一个氧原子。所得发色团（4,9 - 二羟基 - 3 - 氧代 - 10H - 10 - 过氧戊环酮）以前从未报道过，但通过细胞色素P - 450酶插入氧在生物合成上是合理的。
