text: The Pho regulon is controlled by a two-component regulatory system composed of a histidine kinase sensor protein (PhoR) within the inner membrane and a transcriptional response regulator (PhoB/PhoR) on the cytoplasmic side of the membrane. These proteins bind to upstream promoters in the pho regulon in order to induce a general change in gene transcription. This occurs when the cell senses low concentrations of phosphate within its internal environment causing the response regulator to be phosphorylated inducing an overall decrease in gene transcription. This mechanism is ubiquitous within gram-positive, gram-negative, cyanobacteria, yeasts, and archaea.