The gene for albicidin detoxification from Pantoea dispersa encodes an esterase and attenuates pathogenicity of Xanthomonas albilineans to sugarcane

Albicidin phytotoxins are pathogenicity factors in a devastating disease of sugarcane known as leaf scald, caused by Xanthomonas albilineans. A gene (albD) from Pantoea dispersa has been cloned and sequenced and been shown to code for a peptide of 235 amino acids that detoxifies albicidin. The gene shows no significant homology at the DNA or protein level to any known sequence, but the gene product contains a GxSxG motif that is conserved in serine hydrolases. The AlbD protein, purified to homogeneity by means of a glutathione S-transferase gene fusion system, showed strong esterase activity on p-nitrophenyl butyrate and released hydrophilic products during detoxification of albicidins. AlbD hydrolysis of p-nitrophenyl butyrate and detoxification of albicidins required no complex cofactors. Both processes were strongly inhibited by phenylmethylsulfonyl fluoride, a serine enzyme inhibitor. These data strongly suggest that AlbD is an albicidin hydrolase. The enzyme detoxifies albicidins efficiently over a pH range from 5.8 to 8.0, with a broad temperature optimum from 15 to 35°C. Expression of albD in transformed X. albilineans strains abolished the capacity to release albicidin toxins and to incite disease symptoms in sugarcane. The gene is a promising candidate for transfer into sugarcane to confer a form of disease resistance.

albicidin植物毒素（Albicidin phytotoxins）是引发甘蔗毁灭性病害——叶灼病的致病因子，该病害由线条黄单胞菌（Xanthomonas albilineans）引起。研究人员已对分散泛菌（Pantoea dispersa）中的albD基因进行了克隆与测序，结果显示该基因编码一条由235个氨基酸组成的肽链，可对albicidin进行解毒。该基因在DNA或蛋白质水平上与所有已知序列均无显著同源性，但其编码产物含有丝氨酸水解酶（serine hydrolases）中保守的GxSxG基序。通过谷胱甘肽S-转移酶（glutathione S-transferase）基因融合系统纯化至均一的AlbD蛋白，对对硝基苯基丁酸酯（p-nitrophenyl butyrate）表现出较强的酯酶活性，且在albicidin解毒过程中会释放亲水性产物。AlbD对对硝基苯基丁酸酯的水解反应以及对albicidin的解毒过程均无需复杂辅因子；两类反应均可被丝氨酸酶抑制剂苯甲基磺酰氟（phenylmethylsulfonyl fluoride）强烈抑制。上述数据有力表明，AlbD是一种albicidin水解酶。该酶可在pH 5.8至8.0的范围内高效解毒albicidin，最适温度范围较广，为15至35℃。在转化后的线条黄单胞菌（X. albilineans）菌株中表达albD基因，可使其丧失释放albicidin毒素以及在甘蔗中引发病害症状的能力。该基因是将其转入甘蔗以赋予其病害抗性的极具潜力的候选基因。