Cyclic di-GMP rescues H-NS–mediated silencing of bacterial type VI secretion systems

Abstract

Abstract The type VI secretion system (T6SS) is a widespread versatile machine that is encoded by many gram-negative bacteria and plays crucial roles in interbacterial competition and bacteria-host interactions. Histone-like nucleoid structuring protein (H-NS) is a global regulator that represses the expression of T6SS genes in various pathogens and environmental isolates. Bacteria appear to have evolved regulatory mechanisms to relieve the transcriptional repression mediated by H-NS when the T6SS activity is required, but the underlying molecular mechanism awaits elucidation. Here we show that elevated intracellular cyclic di-GMP (c-di-GMP) levels alleviate the H-NS–mediated repression of the T6SS in Salmonella enterica serovar Typhimurium.Mechanistically, c-di-GMP directly binds to the H-NS protein to abrogate its binding to the T6SS promoters, thus derepressing expression of the T6SS genes. Furthermore, we provide evidence that bile salts-induced increase in intracellular c-di-GMP levels leads to activation of the S. TyphimuriumT6SS within the host gut, facilitating its killing of commensal bacteria and successful colonization. The observations that c-di-GMP derepresses gene expression via targeting the H-NS or H-NS-like proteins in Vibrio parahaemolyticus, Pseudomonas aeruginosaand Pseudomonas putida suggest a general mechanism through which the H-NS family of proteins act as environment-sensing regulators in Gram-negative bacteria.