Phosphorus is an essential nutrient for all living organisms. Under conditions of inorganic phosphate starvation, genes from the Pho regulon are induced, allowing microorganisms to use phosphonates as a source of phosphorus. The phnO gene was previously annotated as a transcriptional regulator of unknown function due to sequence homology with members of the GCN5-related N-acyltransferase family (GNAT). PhnO can now be functionally annotated as an aminoalkylphosphonic acid N-acetyltransferase which is able to acetylate a range of aminoalkylphosphonic acids. Studies revealed that PhnO proceeds via an ordered, sequential kinetic mechanism with AcCoA binding first followed by aminoalkylphosphonate. Attack by the amine on the thioester of AcCoA generates the tetrahedral intermediate that collapses to generate the products. The enzyme also requires a divalent metal ion for activity, which is the first example of this requirement for a GNAT family member.