Bacterial Regulatory Proteins 2.0

Dear Colleagues,

Sigma factors are important regulatory subunits of RNA polymerase that confer promoter specificity to the RNA polymerase catalytic core enzyme. Upon association with core RNA polymerase, sigma factors allow the recognition of specific promoters, thus directing the expression of a specific set of genes (so-called regulon of the corresponding sigma factor). Most bacteria contain an essential primary sigma factor directing expression of “house-keeping” genes, and one or more alternative sigma factors responsive to various stimuli necessary to survive adverse stress conditions (including immune responses during host infection with pathogenic bacteria) and to differentiate bacterial cells. The complexity of cell life corresponds to a number of sigma factors. For example, the obligate intracellular pathogen Mycoplasma pneumoniae has only a single primary sigma factor, and the other obligate pathogen Chlamydia trachomatis has three sigma factors, but the saprophytic differentiating unicellular Bacillus subtilis contains 18 sigma factors and the hyphal differentiating Streptomyces coelicolor contains even 65 different sigma factors. Sigma factors dissociate from the RNA polymerase holoenzyme following the transcription initiation process. Therefore, a free core RNA polymerase pool is available to competitively bind various sigma factors, thereby reprogramming gene expression for the actual bacterial needs. Due to this competitive step of regulation, the most important step in almost all sigma factors is the regulation of their levels, which takes place at the transcriptional, translational, and post-translational level.

One common mechanism of the posttranslational regulation is the reversible interaction with their cognate negative regulators, anti-sigma factors, which sequester them from their interaction with the core RNA polymerase. After a variety of signals, this complex is released very rapidly, making the sigma factor free for their association with the core RNA polymerase to direct transcription of a set of cognate genes. This allows a very quick response to various adverse stress conditions. In addition to sigma factors, many other regulatory proteins contribute to the regulation of gene expression at the transcriptional level. Negative regulator (repressors) generally bind to promoters and block the access of the RNA polymerase holoenzyme to activate transcription. Positive regulators usually bind in the upstream regions of promoters, associate with the RNA polymerase holoenzyme, and allow activation of transcription of the cognate promoters.

This Special Issue will address the role of sigma factors of RNA polymerase and other specific regulatory proteins in various bacterial processes, including stress response, bacterial pathogenesis, and morphological cell differentiation. Research articles and reviews focusing on this topic are welcome.

Dr. Jan Kormanec
Guest Editor

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