Genomic reconstruction of σ regulons in Clostridiales.

Background: The σ factor controls unique promoters and interacts with a specialized activator (enhancer binding proteins [EBP]) for transcription initiation. Although σ is present in many Clostridiales species that have great importance in human health and biotechnological applications, the cellular processes controlled by σ remain unknown.

Results: For systematic analysis of the regulatory functions of σ, we performed comparative genomic reconstruction of transcriptional regulons of σ in 57 species from the Clostridiales order. The EBP-binding DNA motifs and regulated genes were identified for 263 EBPs that constitute 39 distinct groups. The reconstructed σ regulons contain the genes involved in fermentation and amino acid catabolism. The predicted σ binding sites in the genomes of Clostridiales spp. were verified by in vitro binding assays. To our knowledge, this is the first report about direct regulation of the Stickland reactions and butyrate and alcohols synthesis by σ and the respective EBPs. Considerable variations were demonstrated in the sizes and gene contents of reconstructed σ regulons between different Clostridiales species. It is proposed that σ controls butyrate and alcohols synthesis in solvent-producing species, regulates autotrophic metabolism in acetogenic species, and affects the toxin production in pathogenic species.

Conclusions: This study reveals previously unrecognized functions of σ and provides novel insights into the regulation of fermentation and amino acid metabolism in Clostridiales species, which could have potential applications in guiding the treatment and efficient utilization of these species.