转录因子SoxR的结构、调控机制及生理功能

转录因子SoxR是典型的汞抗性操纵子调节因子家族的成员,广泛存在于变形菌门、放线菌门、酸杆菌门等微生物类群中。SoxR感受胞内氧化还原电势,通过铁硫簇失去一个电子,而激活目标基因的表达。SoxR在大肠杆菌中能应答过氧化物,负责抗氧化胁迫的全局性调控;在铜绿假单胞菌中受群体感应终端信号分子绿脓菌素的激活,参与群体对环境变化的协同应答过程。本文综述了SoxR的结构、作用机制和生理功能。近年来关于SoxR的研究虽然已取得许多令人瞩目的成果,但SoxR激活的分子机制仍有待确立和验证,同时也亟待在更多微生物类群中开展相关研究。对这些问题的深入研究,不仅可以更全面地认识SoxR,而且可以对微生物体的整个代谢调控网络有更加深入地了解,并有可能获得里程碑式的重大发现。     

Differential activities of the SoxR protein of Escherichia coli: SoxS is not required for gene activation under iron deprivation

When Escherichia coli cells are under superoxide stress, proteins SoxR and SoxS, acting sequentially, control the expression of a set of repair and defense genes. One of these genes, fumC, encoding fumarase C, was reported to be also activated by iron deprivation in a soxRS-dependent manner. However, the same condition failed to induce the expression of a soxS'::lacZ fusion. The expression of acnA (aconitase A) is also activated by SoxR alone when under iron deprivation, but not of sodA (Mn-superoxide-dismutase). SoxR completely inhibited the migration of a DNA fragment containing the promoter region of fumC, in gel-shift experiments. SoxR might bind to a different region than SoxS within the fumC promoter, or an unknown intermediate other than SoxS might be acting. It is possible that the regulatory role of SoxR is more complex than previously considered.     

A reducing system of the superoxide sensor SoxR in Escherichia coli

The soxRS regulon functions in protecting Escherichia coli cells against superoxide and nitric oxide. When SoxR is activated by oxidation of its [2Fe-2S] cluster, it increases the synthesis of SoxS, which then activates its target gene expression. How the oxidized SoxR returns to and is maintained in its reduced state has been under question. To identity genes that constitute the SoxR-reducing system, we screened an E.coli mutant library carrying a chromosomal soxSp::lacZ fusion, for constitutive mutants. Mutations mapped to two loci: the rsxABCDGE operon (named for reducer of SoxR) that is highly homologous to the rnfABCDGE operon in Rhodobacter capsulatus involved in transferring electrons to nitrogenase, and the rseC gene in the rpoE-rseABC operon. In-frame deletion of each open reading frame in the rsxABCDGE operon produced a similar constitutive phenotype. The double mutation of rsx and rseC suggested that rsxABCDGE and rseC gene products act together in the same pathway in reducing SoxR. Electron paramagnetic resonance analysis of SoxR and measurement of re-reduction kinetics support the proposal that rsx and rseC gene products constitute a reducing system for SoxR.