The expression of the virulence-associated effector protein gene avrA is dependent on a Salmonella enterica-specific regulatory function.

Although the avrA gene is prevalent among 80% of the Salmonella enterica serovars, only a small number of them usually express the respective virulence-associated effector protein AvrA. However, under culture conditions below pH 6.0 many of the AvrA non-producer strains (e.g. S. Agona, S. Bovismorbificans, S. Virchow) begin to produce AvrA, while some others remain silent. Four phenotypical classes of S. enterica were identified under defined standard culture conditions: class 1 comprises strains with a constitutive synthesis of AvrA; class 2 comprises strains with an acid induction of AvrA; class 3 comprises strains with silent avrA genes; and the fourth class comprises strains which do not contain the avrA gene (class 0 strains). The expression of avrA was found to be controlled by a Salmonella-specific mechanism because cloned avrA genes from classes 1, 2, and 3 strains remain silent in Escherichia coli strains, while easily expressed in S. enterica strains. The expression of AvrA in classes 1, 2, and 3 strains does not coincide with the nucleotide sequences of the respective promoter or structural regions of the avrA genes, but depends directly on this Salmonella-specific regulatory system which appears to be differently modulated in the distinct Salmonella serovars.