PhoA

PhoA, short for alkaline phosphatase A, is a periplasmic enzyme encoded by the phoA gene in Escherichia coli and many other Gram-negative bacteria. It catalyzes the hydrolysis of phosphate esters, releasing inorganic phosphate and an alcohol from a broad range of substrates. In E. coli, the enzyme is exported to the periplasm via a Sec-dependent signal peptide; once in the periplasm, the mature enzyme forms a homodimer and requires divalent metal cofactors for activity, including two zinc ions and two magnesium ions.

Biochemical properties include an alkaline pH optimum and broad substrate specificity. The activity of PhoA can be measured colorimetrically using substrates such as p-nitrophenyl phosphate, which yields a yellow product under alkaline conditions. The periplasmic localization makes PhoA suitable for certain diagnostic and research applications.

Genetic regulation occurs as part of the phosphate regulon. PhoA expression is controlled by the PhoR-PhoB two-component system, which senses extracellular inorganic phosphate levels. Under phosphate-replete conditions, PhoR represses PhoB and phoA transcription; during phosphate starvation, PhoB becomes activated and induces phoA along with other phosphate-scavenging genes.

Applications and significance include its use as a reporter gene in molecular biology, particularly because PhoA activity reflects periplasmic localization and can be distinguished from cytoplasmic reporters like LacZ. PhoA fusions are employed in topological mapping to infer the localization and orientation of proteins within the cell envelope.

Related enzymes include alkaline phosphatases in other organisms, which share functional similarities but may differ in localization and regulatory control.