紫色非硫细菌的砷代谢机制

Arsenic metabolism in purple nonsulfur bacteria

【目的】系统阐述紫色非硫细菌(PNSB)砷代谢机制和砷代谢基因簇的进化关系。【方法】通过生物信息学方法分析了PNSB砷代谢基因簇的分布、组成、排布方式。采用UV-Vis和HPLC-ICP-MS方法,研究了3个PNSB种类对砷的抗性、砷形态及价态的转化、砷在细胞中的积累和分布以及磷酸盐对As细胞毒性的影响。【结果】砷基因簇分析表明:已公布全基因组序列的17个PNSB菌株基因组中均含有以ars operon为核心的砷代谢基因簇,由1-4个操纵子组成,主要含有与细胞质砷还原和砷甲基化代谢相关的基因,但基因的组成和排列方式因种和菌株而异,尤其是arsM和两类进化来源不同的arsC。实验结果表明:光照厌氧条件下,3个PNSB种类对As(Ⅴ)和As(Ⅲ)均具有抗性,As(Ⅴ)和As(Ⅲ)均能进入细胞;在胞内As(Ⅴ)能够还原为As(Ⅲ)并被排出胞外,但不能将As(Ⅲ)氧化为As(Ⅴ),也未检测到甲基砷化物;磷酸盐浓度升高,能够抑制As(Ⅴ)进入细胞,降低As(Ⅴ)对细胞的毒性,而不能抑制As(Ⅲ)进入细胞。【结论】PNSB砷代谢机制主体为细胞质As(Ⅴ)还原,也还有砷甲基化途径。通过对砷代谢基因簇结构多样性特点和进化方式分析,提出了与Rosen不同的ars operon进化途径。这对深入开展PNSB砷代谢和基因之间的相互作用研究奠定基础。

[Objective]To elucidate the arsenic metabolic pathway of purple nonsulfur bacteria（PNSB）.[Methods]We investigated the distribution within their genomes,organization,composition,arrangement,core genes and coding proteins of arsenic gene clusters found in complete genome from 17 strains of PNSB by comparing the genomes analysis,and studied the arsenic metabolism in 3 members of PNSB under anaerobic conditions by UV-Vis and HPLC-ICP-MS.[Results]Arsenate reduction and arsenite methylation pathways mediated by ars operon are the dominating arsenic metabolic processes.The arsenic gene clusters differ vastly in composition and arrangement.Some members of PNSB evolved two independently families of arsenate reduction genes（arsC）.The cells of Rhodopseudomonas palustris CQV97,Rhodobacter azotoformans 134K20 and Rhodobacter capsulatus XJ-1 could reduce As（Ⅴ） to As（Ⅲ）,whereas As（Ⅲ） could not be transformed back to As（Ⅴ）.Higher concentration phosphate competitively inhibited arsenate toxicity to cells.[Conclusion]Our investigations shed light on the evolution and functional implications in arsenic gene clusters of PNSB,and support the notion that arsenate reduction and arsenite methylation appears to be the dominant process in PNSB.