Sharp threshold gene activation in two-component system is due to
Two-component signal-transduction systems consisting of a histidine kinase and a response regulator are widespread in bacteria and plants but essentially absent from animals. The histidine kinase autophosphorylates on a conserved histidine and transfers the phosphoryl group to an aspartate residue on the response regulator, which then executes the cellular response. These systems enable rapid adaptation to environmental changes.
Ref: NCERT Biology Class 11–12 Alberts et al Molecular Biology of the Cell Lodish et al, Molecular Cell Biology Cooper & Hausman, The Cell Abbas et al., Cellular and Molecular Immunology (for immunology sections)