Structure of the transcription open complex of distinct σ^I factors

Bacterial σ^I factors of the σ⁷⁰-family are widespread in Bacilli and Clostridia and are involved in the heat shock response, iron metabolism, virulence, and carbohydrate sensing. A multiplicity of σ^I paralogues in some cellulolytic bacteria have been shown to be responsible for the regulation of the cellulosome, a multienzyme complex that mediates efficient cellulose degradation. Here, we report two structures at 3.0 Å and 3.3 Å of two transcription open complexes formed by two σ^I factors, SigI1 and SigI6, respectively, from the thermophilic, cellulolytic bacterium, Clostridium thermocellum. These structures reveal a unique, hitherto-unknown recognition mode of bacterial transcriptional promoters, both with respect to domain organization and binding to promoter DNA. The key characteristics that determine the specificities of the σ^I paralogues were further revealed by comparison of the two structures. Consequently, the σ^I factors represent a distinct set of the σ⁷⁰-family σ factors, thus highlighting the diversity of bacterial transcription.