Structural characterization of a pathogenicity-related superoxide dismutase codified by a probably essential gene in Xanthomonas citri subsp. citri

Citrus canker is a plant disease caused by the bacteria Xanthomonas citri subsp. citri that affects all domestic varieties of citrus. Some annotated genes from the X. citri subsp. citri genome are assigned to an interesting class named "pathogenicity, virulence and adaptation". Amongst these is sodM, which encodes for the gene product XcSOD, one of four superoxide dismutase homologs predicted from the genome. SODs are widespread enzymes that play roles in the oxidative stress response, catalyzing the degradation of the deleterious superoxide radical. In Xanthomonas, SOD has been associated with pathogenesis as a counter measure against the plant defense response. In this work we initially present the 1.8 Å crystal structure of XcSOD, a manganese containing superoxide dismutase from Xanthomonas citri subsp. citri. The structure bears all the hallmarks of a dimeric member of the MnSOD family, including the conserved hydrogen-bonding network residues. Despite the apparent gene redundancy, several attempts to obtain a sodM deletion mutant were unsuccessful, suggesting the encoded protein to be essential for bacterial survival. This intriguing observation led us to extend our structural studies to the remaining three SOD homologs, for which comparative models were built. The models imply that X. citri subsp. citri produces an iron-containing SOD which is unlikely to be catalytically active along with two conventional Cu,ZnSODs. Although the latter are expected to possess catalytic activity, we propose they may not be able to replace XcSOD for reasons such as distinct subcellular compartmentalization or differential gene expression in pathogenicity-inducing conditions.柑橘溃疡病（Citrus canker）是一种由柑橘黄单胞菌柑橘亚种（Xanthomonas citri subsp. citri）引发的植物病害，可侵染所有栽培柑橘品种。该菌基因组中的部分注释基因被归类至一个名为"致病性、毒力与适应性"的功能类群，颇具研究价值。其中sodM基因编码产物XcSOD，是该基因组预测的4种超氧化物歧化酶（superoxide dismutase，SOD）同源蛋白之一。超氧化物歧化酶是一类广泛分布的酶类，参与氧化应激响应过程，可催化有害超氧自由基的降解。在黄单胞菌属中，SOD与病原菌致病过程密切相关，作为抵御植物防御反应的应对策略之一。本研究首先解析了柑橘黄单胞菌柑橘亚种来源的含锰超氧化物歧化酶XcSOD的1.8 Å晶体结构。该结构具备MnSOD家族二聚体成员的所有典型特征，包括保守的氢键网络残基。尽管该菌基因组存在明显的基因冗余现象，但多次尝试构建sodM基因缺失突变体均告失败，提示该编码蛋白对细菌存活至关重要。这一有趣的发现促使我们将结构研究拓展至其余3种SOD同源蛋白，并为其构建了比较模型。模型分析显示，柑橘黄单胞菌柑橘亚种还会产生一种含铁SOD（推测无催化活性）以及两种典型的铜锌超氧化物歧化酶（Cu,ZnSOD）。尽管后两类SOD理论上具备催化活性，但我们推测它们无法替代XcSOD的功能，原因可能包括亚细胞定位差异，或是在致病诱导条件下的基因表达模式不同。