Comparison of the exoS Gene and Protein Expression in Soil and Clinical Isolates of Pseudomonas aeruginosa

Exoenzyme S (ExoS) is translocated into eukaryotic cells by the type III secretory process and has been hypothesized to function in conjunction with other virulence factors in the pathogenesis of Pseudomonas aeruginosa. To gain further understanding of how ExoS might contribute to P. aeruginosa survival and virulence, ExoS expression and the structural gene sequence were determined in P. aeruginosa soil isolates and compared with ExoS of clinical isolates. Significantly higher levels of ExoS ADP-ribosyltransferase (ADPRT) activity were detected in culture supernatants of soil isolates compared to those of clinical isolates. The higher levels of ADPRT activity of soil isolates reflected both the increased production of ExoS and the production of ExoS having a higher specific activity. ExoS structural gene sequence comparisons found the gene to be highly conserved among soil and clinical isolates, with the greatest number of nonsynonymous substitutions occurring within the region of ExoS encoding GAP function. The lack of amino acid changes in the ADPRT region in association with a higher specific activity implies that other factors produced by P. aeruginosa or residues outside the ADPRT region are affecting ExoS ADPRT activity. The data are consistent with ExoS being integral to P. aeruginosa survival in the soil and suggest that, in the transition of P. aeruginosa from the soil to certain clinical settings, the loss of ExoS expression is favored.

外酶S（Exoenzyme S，ExoS）可通过Ⅲ型分泌过程转位进入真核细胞，学界此前曾推测其可与其他毒力因子协同参与铜绿假单胞菌（Pseudomonas aeruginosa）的致病过程。为进一步阐明ExoS如何促进铜绿假单胞菌的存活与致病能力，研究人员对土壤分离株中的ExoS表达情况及其结构基因序列进行了测定，并与临床分离株的ExoS展开对比分析。结果显示，相较于临床分离株，土壤分离株的培养上清液中检测到的ExoS ADP-核糖基转移酶（ADP-ribosyltransferase，ADPRT）活性显著更高。土壤分离株所呈现的更高ADPRT活性，既源于ExoS表达量的提升，也得益于ExoS本身比活性的增强。对ExoS结构基因序列的比对结果显示，该基因在土壤分离株与临床分离株中均高度保守，其中编码GAP功能的区域内发生的非同义替换数量最多。ADPRT区域未出现氨基酸改变，却伴随更高的比活性，这一现象暗示，铜绿假单胞菌产生的其他因子，或是ADPRT区域以外的氨基酸残基，共同影响了ExoS的ADPRT活性。本研究数据表明，ExoS是铜绿假单胞菌在土壤环境中存活的关键因子，同时也提示，当铜绿假单胞菌从土壤转移至特定临床环境时，其丧失ExoS表达的演化路径更受青睐。