Circular oligomeric particles formed by Ros/MucR family members mediate DNA organization in α-proteobacteria.

The transcriptional regulator MucR from Brucella species controls the expression of many genes, including those involved in virulence, by binding AT-rich DNA regions. MucR and its homologs belong to the Ros/MucR family, whose members occur in α-proteobacteria. MucR is a recent addition to the family of histone-like nucleoid structuring (H-NS) proteins. Indeed, despite the lack of sequence homology, MucR bears many functional similarities with H-NS and H-NS-like proteins, structuring the bacterial genome and acting as global regulators of transcription. Here we present an integrated cryogenic electron microscopy (cryo-EM), nuclear magnetic resonance, modeling and biochemical study shedding light on the functional architecture of MucR from Brucella abortus and its homolog Ml5 from Mesorhizobium loti. We show that MucR and Ml5 fold in a circular quaternary assembly, which allows it to bridge and condense DNA by binding AT-rich sequences. Our results show that Ros/MucR family members are a novel type of H-NS-like proteins and, based on previous studies, provide a model connecting nucleoid structure and transcription regulation in α-proteobacteria.