Sigma A‐bound iron–sulfur protein WhiB1 in <i>Mycobacterium tuberculosis</i>

Abstract WhiB1 belongs to the WhiB‐like family that is widely distributed in the phylum of Actinobacteria. It is a monomeric transcription factor that contains a [4Fe–4S] cluster in the holo‐form. In mycobacteria, the holo‐WhiB1 interacts with the domain 4 of the σ 70 ‐family primary sigma factor (σ A 4 ) in the RNA polymerase holoenzyme and plays an essential role in active cell growth. Domain 4 of the σ 70 ‐family primary sigma factors recognizes the −35 hexamer of promoter DNA and is commonly utilized by transcription factors for gene activation. However, the crystal structure of the WhiB1:σ A 4 complex reveals unexpectedly that WhiB1 binds to σ A 4 using a hydrophobic interface that differs markedly from previously characterized σ A 4 ‐bound bacterial transcription activators. Moreover, holo‐WhiB1 represses transcription by interaction with σ A 4 in vitro and WhiB1's essential role in supporting mycobacterial growth requires its interaction with σ A in vivo . Together, these results demonstrate that holo‐WhiB1 regulates gene expression by a noncanonical mechanism relative to well‐characterized σ A 4 ‐dependent transcription activators.