VirB, a transcriptional activator of virulence in Shigella flexneri, uses CTP as a cofactor

• Published in: Communications biology Vol. 6; no. 1; pp. 1204 - 10
• Main Authors: Antar, Hammam, Gruber, Stephan
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 25-11-2023; Nature Publishing Group; Nature Portfolio
• Subjects: 14/35; 42; 42/44; 631/326/88; 631/45/147; 82/16; 82/29; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Biology; Biomedical and Life Sciences; Chromosomes; Diarrhea; DNA - metabolism; DNA-Binding Proteins - genetics; DNA-Binding Proteins - metabolism; Gene silencing; Gram-negative bacteria; Life Sciences; Microenvironments; Nucleotide sequence; Promoter Regions, Genetic; Shigella flexneri; Shigella flexneri - genetics; Shigellosis; Supercoiling; Transcription activation; Transcription Factors - genetics; Transcription Factors - metabolism; Virulence; Virulence - genetics; Virulence Factors - genetics
• ISSN: 2399-3642; 2399-3642
• DOI: 10.1038/s42003-023-05590-8

VirB is a transcriptional activator of virulence in the gram-negative bacterium
Shigella flexneri
encoded by the large invasion plasmid, pINV. It counteracts the transcriptional silencing by the nucleoid structuring protein, H-NS. Mutations in
virB
lead to loss of virulence. Studies suggested that VirB binds to specific DNA sequences, remodels the H-NS nucleoprotein complexes, and changes DNA supercoiling. VirB belongs to the superfamily of ParB proteins which are involved in plasmid and chromosome partitioning often as part of a ParABS system. Like ParB, VirB forms discrete foci in
Shigella flexneri
cells harbouring pINV. Our results reveal that purified preparations of VirB specifically bind the ribonucleotide CTP and slowly but detectably hydrolyse it with mild stimulation by the
virS
targeting sequences found on pINV. We show that formation of VirB foci in cells requires a
virS
site and CTP binding residues in VirB. Curiously, DNA stimulation of clamp closure appears efficient even without a
virS
sequence in vitro. Specificity for entrapment of
virS
DNA is however evident at elevated salt concentrations. These findings suggest that VirB acts as a CTP-dependent DNA clamp and indicate that the cellular microenvironment contributes to the accumulation of VirB specifically at
virS
sites.
This study sheds light on the mechanism of transcription activation by which VirB, a virulence transcription activator in
Shigella flexneri
, the causative agent of the diarrheal disease shigellosis, uses the ribonucleotide CTP as a cofactor to load at specific DNA sites.