This invention relates to a novel polyphosphate: AMP phosphotransferase (PAP), a gene coding this PAP, and their use. The PAP has the following properties:(A) action: catalyzing of the following two reactions: NMP+PolyP.sub.(n).fwdarw.NDP+PolyP.sub.(n-1) dNMP+POlyP.sub.(n).fwdarw.dNDP+PolyP.sub.(n-1) (wherein NMP represents nucleoside monophosphate, NDP represents nucleoside diphosphate, dNMP represents deoxynucleoside monophosphate, dNDP represents deoxynucleoside diphosphate, n represents degree of polymerization of the polyphosphate which is an integer of up to 100);(B) substrate specificity: specific to AMP, GMP, IMP, dAMP, and dGMP, also acting with CMP, UMP, dCMP, and TMP;(C) molecular weight: about 55 to 56 Kd (kilodalton); and(D) specific activity: at least 70 units per 1 mg of enzyme protein.