An exopolyphosphatase of Escherichia coli. The enzyme and its ppx gene in a polyphosphate operon.

A gene, ppx, that encodes a novel exopolyphosphatase of 513 amino acids (58,133 Da) was found downstream of the gene for polyphosphate kinase, ppk. Transcription of the ppx gene depends on the ppk promoters, indicating a polyphosphate (polyP) operon of ppk and ppx. Exopolyphosphatase, purified to homogeneity from overproducing cells, is judged to be a dimer of 58-kDa subunits. Orthophosphate is released processively from the ends of polyP approximately 500 residues long, but chains of approximately 15 residues compete poorly with polyP as substrate; ATP is not a substrate. Mg2+ (1 mM) and a high concentration of K+ (175 mM) support optimal activity.