Phosphorylation of the prokaryotic histone-like protein H-NS modulates bacterial virulence in Salmonella Typhimurium.

H-NS is a prokaryotic histone-like protein that binds to bacterial chromosomal DNA with important regulatory roles in gene expression. Unlike histone proteins, hitherto post-translational modifications of H-NS are still largely uncharacterized, especially in bacterial pathogens. Salmonella Typhimurium is a primary enteric pathogen and its virulence is mainly dependent on specialized type III secretion systems (T3SSs), which were evolutionarily acquired via horizontal gene transfer. Previous studies have shown that H-NS plays a critical role in silencing foreign T3SS genes. Here, we found that H-NS is phosphorylated at multiple residues in S. Typhimurium, including S45, Y61, S78, S84, T86, and T106. Notably, we demonstrated that phosphorylation of H-NS S78 promotes its dissociation from DNA via a mechanism dependent on dimer formation, thereby leading to transcriptional activation of target genes. Functionally, phosphoryl-H-NS contributes to the expression of T3SS-associated proteins and hence increases bacterial virulence during infection. Therefore, our study reveals a novel mechanism by which covalent modifications of prokaryotic histone-like proteins regulate bacterial virulence of an important human pathogen.