The virulence regulator VirB from Shigella flexneri uses a CTP-dependent switch mechanism to activate gene expression
Sara Jakob, Wieland Steinchen, Juri Hanßmann, Julia Rosum, Katja Langenfeld, Manuel Osorio‐Valeriano, Niklas Steube, Pietro I. Giammarinaro, Georg Hochberg, Timo Glatter, Gert Bange, Andreas Diepold, Martin Thanbichler
Abstract
The transcriptional antisilencer VirB acts as a master regulator of virulence gene expression in the human pathogen Shigella flexneri. It binds DNA sequences (virS) upstream of VirB-dependent promoters and counteracts their silencing by the nucleoid-organizing protein H-NS. However, its precise mode of action remains unclear. Notably, VirB is not a classical transcription factor but related to ParB-type DNA-partitioning proteins, which have recently been recognized as DNA-sliding clamps using CTP binding and hydrolysis to control their DNA entry gate. Here, we show that VirB binds CTP, embraces DNA in a clamp-like fashion upon its CTP-dependent loading at virS sites and slides laterally on DNA after clamp closure. Mutations that prevent CTP-binding block VirB loading in vitro and abolish the formation of VirB nucleoprotein complexes as well as virulence gene expression in vivo. Thus, VirB represents a CTP-dependent molecular switch that uses a loading-and-sliding mechanism to control transcription during bacterial pathogenesis.