The Third Pillar of Bacterial Signal Transduction: Signaling Diversity and Evolution of Extracytoplasmic Function (ECF) Sigma Factors

细菌对环境条件的适应需要跨膜信号转导来协调足够的细胞反应。启动子决定了转录启动的特异性和效率，因此是协调差异基因表达的重要调控开关。胞质外功能σ因子是一组规模最大、种类最多的替代σ因子，它通过第二个通常是膜锚定的感应器蛋白--相应的抗σ因子--来响应环境刺激。尽管它们的数量和总体重要性很大，但到目前为止，只有非常有限的数量的这些系统进行了实验研究。在这里，我们建议对在综合比较基因组学和转录组学研究中发现的三组新的和不寻常的ECFσ因子成员进行分析：(I)参与地衣芽孢杆菌细胞膜应激反应的ECFσ因子。(Ii)ECFσ因子在功能上与反应调节因子相关，但缺乏抗σ因子，表明一种新的信号转导机制。(Iii)与氧化应激反应有关的ECFσ因子。这些蛋白质在功能上与羧基粘液内酯脱羧酶同系物相连，而不是标准的抗σ因子。我们的分析(正在进行的和拟议的)将被嵌入到比较基因组学数据库中，并用于建立生物信息学工具，以重建和预测微生物基因组中依赖ECFσ因子的调控网络。

Adaptation of a bacterium to environmental conditions necessitates transmembrane signaltransduction to orchestrate adequate cellular responses. The promoter determines the specificity and efficiency of transcriptional initiation, and is therefore an important regulatory switchboard to coordinate differential gene expression. Extracytoplasmic function (ECF) σ factors, the largest and most diverse group of alternative σ factors, respond to environmental stimuli through a second, normally membraneanchored sensor protein, the corresponding anti-σ factor. Despite their abundance and overall importance, only a very limited number of these systems have been experimentally studied to date. Here we propose the analysis of members of three novel and unusual groups of conserved ECF σ factors that were identified during comprehensive comparative genomics and transcriptomics studies, (i) ECF σ factors involved in the cell envelope stress response of Bacillus licheniformis. (ii) ECF σ factors that are functionally linked to response regulators, but lack an anti-σ factor, indicative for a novel mechanism of signal transduction. (iii) ECF σ factors implicated to be involved in oxidative stress response. These proteins are functionally linked to carboxymuconolactone decarboxylase homologs instead of a standard anti-σ factor. Our analyses (ongoing and proposed) will be embedded in a comparative genomics database and used to establish bioinformatics tools to reconstruct and predict ECF σ factor-dependent regulatory networks in microbial genomes.