Differential regulation of the plasmid-encoded genes in the Shigella flexneri virulence regulon

Expression of the Shigella flexneri virulence gene regulon is controlled by multiple environmental signals acting through a regulatory cascade. The primary regulator is VirF, which is a positive regulator of the secondary regulatory gene virB and the structural gene icsA. The product of the virB gene in turn activates transcription of the genes coding for the invasion proteins, and for the type III secretion system which promotes export of the invasion proteins to the bacterial cell surface. The genes making up the regulon were studied in their native locations on the 230-kb virulence plasmid. Transcriptional control was detected at each level of the regulatory cascade. A gearing effect was detected upon thermal induction of transcription in the regulon, with the virF gene being induced by about two fold, virB by 10-fold and the structural genes by 100-fold. In addition, each gene studied displayed individual characteristics in its response to stimuli such as growth medium osmolarity, pH, variations in DNA superhelicity and the presence or absence of H-NS. The primary regulatory gene virF, displayed loose regulation under standard laboratory growth conditions. Regulation was tighter at the secondary regulator virB, while control of structural gene expression was tighter still. It is proposed that this regulatory pattern ensures that energetically wasteful expression of the structural genes under inappropriate conditions is avoided while allowing the regulatory genes to be expressed sufficiently under non-permissive conditions to ensure a rapid response to inducing conditions when these arise. Once induced, fine tuning of the response can be achieved through the different sensitivities of the individual regulon members to external stimuli.