The σ^I sigma factor from Bacillus subtilis is a σ factor associated with RNA polymerase (RNAP) that was previously implicated in adaptation of the cell to elevated temperature. Here, we provide a comprehensive characterization of this transcriptional regulator. By transcriptome sequencing (RNA-seq) of wild-type (wt) and σ^I-null strains at 37°C and 52°C, we identified ∼130 genes affected by the absence of σ^I Further analysis revealed that the majority of these genes were affected indirectly by σ^I The σ^I regulon, i.e., the genes directly regulated by σ^I, consists of 16 genes, of which eight (the dhb and yku operons) are involved in iron metabolism. The involvement of σ^I in iron metabolism was confirmed phenotypically. Next, we set up an in vitro transcription system and defined and experimentally validated the promoter sequence logo that, in addition to -35 and -10 regions, also contains extended -35 and -10 motifs. Thus, σ^I-dependent promoters are relatively information rich in comparison with most other promoters. In summary, this study supplies information about the least-explored σ factor from the industrially important model organism B. subtilisIMPORTANCE In bacteria, σ factors are essential for transcription initiation. Knowledge about their regulons (i.e., genes transcribed from promoters dependent on these σ factors) is the key for understanding how bacteria cope with the changing environment and could be instrumental for biotechnologically motivated rewiring of gene expression. Here, we characterize the σ^I regulon from the industrially important model Gram-positive bacterium Bacillus subtilis We reveal that σ^I affects expression of ∼130 genes, of which 16 are directly regulated by σ^I, including genes encoding proteins involved in iron homeostasis. Detailed analysis of promoter elements then identifies unique sequences important for σ^I-dependent transcription. This study thus provides a comprehensive view on this underexplored component of the B. subtilis transcription machinery.

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