布氏柠檬酸杆菌JPG1的SoxR对好氧/厌氧-甲萘醌胁迫生理响应的调控

Regulatory role of SoxR in Citrobacter braakii JPG1 in physiological response to aerobic/anaerobic-menadione stress

摘要:

SoxR是细菌应答氧化胁迫的转录调控因子之一，在细菌逆境非生物胁迫防御中发挥着至关重要的作用。目前对细菌应答氧化胁迫的认知主要来自包括大肠杆菌在内的少数模式菌，为了研究非模式细菌应答氧化胁迫的方式，本研究以肠杆菌科中常见的布氏柠檬酸杆菌(Citrobacter braakii)为模型，基于全基因组背景分析SoxR进化关系，构建soxR基因缺失菌株(ΔsoxR)，并以氧化还原循环化合物甲萘醌为诱导剂，通过细胞存活能力测定，探究转录因子SoxR在应答好氧/厌氧-甲萘醌胁迫中的作用。好氧条件下，低浓度甲萘醌(0.1 mmol/L)胁迫24 h后，野生型细胞数量比0 h增加了4.2倍，ΔsoxR仅增加了1.3倍；高浓度甲萘醌(0.3 mmol/L)胁迫的野生型和ΔsoxR细胞数量分别为0 h时的29%和0.2%。厌氧条件下，甲萘醌不影响野生型和ΔsoxR细胞的生长，与对照组(0 mmol/L)相比无显著差异(P>0.05)。结果表明，C. braakii JPG1的SoxR仅在好氧条件下响应氧化还原循环化合物甲萘醌的胁迫并增强了菌株的抗氧化能力，对厌氧-甲萘醌无响应。本研究为探究细菌在不同氧条件下转录调控因子SoxR对甲萘醌胁迫响应的调控作用提供了理论基础。

Abstract:

SoxR, one of bacterial transcriptional regulators, plays a crucial role in bacterial responses to oxidative stress induced by unfavorable environmental conditions. So far, the understanding of bacterial responses to oxidative stress mainly stems from a handful model bacteria such as Escherichia coli and the studies on non-model bacterial responses to oxidative stress are limited. In this study, Citrobacter braakii JPG1, a commonly occurring strain of enterobacteria, was used as a model for the first time to explore the role of SoxR in the responses to aerobic/anaerobic-menadione stress. First, we analyzed the phylogenetic relationship of SoxR based on the whole genome and constructed the soxR-deleted strain (ΔsoxR). Then, the cell counts of the wild type (WT) and ΔsoxR were compared under aerobic/anaerobic-menadione stress. The results showed that the cell count of WT exposed to the aerobic-low concentration menadione (0.1 mmol/L) stress for 24 h increased by 4.2 times compared with that at the time point of 0 h, while that of ΔsoxR only increased by 1.3 times. The vast majority of WT and ΔsoxR cells died after exposure to the aerobic-high concentration menadione (0.3 mmol/L) stress for 24 h, with the cell counts only 29% and 0.2% of those at the time point of 0 h, respectively. Interestingly, the cell counts of WT showed no significant difference between the anaerobic-menadione stress and the control (P>0.05), and the same was true for ΔsoxR. All these results indicated that SoxR of C. braakii JPG1 only has a regulatory effect on the redox cycling compound menadione under aerobic conditions and enhance the antioxidant capacity. Under anaerobic conditions, menadione failed to activate SoxR. The findings from this study provide new insights into understanding both the physiological responses to menadione stress and the regulatory role of SoxR under different oxygen conditions.