Aromatic patch in whiB-like transcription factors facilitates primary sigma factor interaction in mycobacterium tuberculosis.

WhiB-like (Wbl) family proteins are a unique family of iron-sulfur ([4Fe-4S]) cluster-bound transcription factors found exclusively in Actinobacteria and actinobacteriophages, including the notoriously persistent pathogen Mycobacterium tuberculosis (Mtb). Despite their critical roles in cell development, stress response and antibiotic resistance, the mechanisms of gene regulation by the Wbl family proteins are not fully understood due to the lack of a canonical DNA-binding motif in most Wbl proteins. Here, we present structural and biochemical evidence demonstrating that all Mtb Wbl proteins bind to the same site in the conserved region 4 of the primary sigma 70 factor facilitated by a previously unrecognized structural motif, the aromatic patch, in the Wbl family. Our phylogenetic findings provide compelling evidence for a complex evolutionary relationship of Wbls between actinobacteria and the associated phages. Together, this work fills a critical gap in our understanding of the function, mechanism and evolutionary origin of Wbls.