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Pinto, Daniela; Liu, Qiang; Mascher, Thorsten (2019): ECF sigma factors with regulatory extensions: the one-component systems of the sigma universe. In: Molecular Microbiology, Vol. 112, No. 2: pp. 399-409
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The sigma subunit of the bacterial RNA polymerase determines promoter specificity. The extracytoplasmic function sigma factors (ECFs) represent the most abundant and diverse group of alternative sigma factors and are present in the vast majority of bacterial genomes. Typically, ECFs are regulated by anti-sigma factors that sequester their cognate ECFs, thereby preventing their interaction with the RNA polymerase. Beyond these ECF paradigms, a number of distinct modes of regulation have been proposed and experimentally investigated. Regulatory extensions represent one such alternative mechanism of ECF regulation that can be found in 18 phylogenetically distinct ECF groups. Here, the sigma factors contain additional domains that are fused to the ECF core domains and are involved in stimulus perception and modulation of sigma factor activity. We will summarize the current state of knowledge on regulating ECF activity by C-terminal extensions. We will also discuss newly identified ECF groups containing either N- or C-terminal extensions and propose possible mechanisms by which these extensions have been generated and affect ECF sigma factor activity. Based on their modular architecture and the resulting physical connection between stimulus perception and transcriptional output, these ECFs are analogous to one-component systems, the primary mechanism of bacterial signal transduction.