Identification and Characterization of the Cognate Anti-Sigma Factor and Specific Promoter Elements of a T. tengcongensis ECF Sigma Factor

Extracytoplasmic function (ECF) σ factors, the largest group of alternative σ factors, play important roles in response to environmental stresses. Tt-RpoE1 is annotated as an ECF σ factor in Thermoanaerobacter tengcongensis. In this study, we revealed that the Tt-tolB gene located downstream of the Tt-rpoE1 gene encoded the cognate anti-σ factor, which could inhibit the transcription activity of Tt-RpoE1 by direct interaction with Tt-RpoE1 via its N-terminal domain. By in vitro transcription assay, the auto-regulation ability of Tt-RpoE1 was determined, and band shift assay showed that Tt-RpoE1 preferred to bind a fork-junction promoter DNA. With truncation or base-specific scanning mutations, the contribution of the nucleotides in -35 and -10 regions to interaction between Tt-RpoE1 and promoter DNA was explored. The promoter recognition pattern of Tt-RpoE1 was determined as 5' tGTTACN(16)CGTC 3', which was further confirmed by in vitro transcription assays. This result showed that the Tt-RpoE1-recognized promoter possessed a distinct -10 motif (-13CGTC-10) as the recognition determinant, which is distinguished from the -10 element recognized by σ(70). Site-directed mutagenesis in Region 2.4 of Tt-RpoE1 indicated that the "D" residue of DXXR motif was responsible for recognizing the -12G nucleotide. Our results suggested that distinct -10 motif may be an efficient and general strategy used by ECF σ factors in adaptive response regulation of the related genes.