Together with PhnG, PhnH and PhnL is required for the transfer of the ribose triphosphate moiety from ATP to methyl phosphonate. PhnI alone has nucleosidase activity, catalyzing the hydrolysis of ATP or GTP forming alpha-D-ribose 5-triphosphate and adenine or guanine, respectively. ATP + methylphosphonate = adenine + alpha-D-ribose 1-methylphosphonate 5-triphosphate ATP + H2O = adenine + D-ribose 5-triphosphate kcat is 1.4 sec(-1) for ATP hydrolysis in the presence of PhnI alone, and 20 sec(-1) for RPnTP synthesis from ATP in the presence of PhnI, PhnG, PhnH and PhnL. Forms a complex with PhnG, PhnH, PhnJ and PhnK with the suggested composition PhnG(4)H(2)I(2)J(2)K. The sequence shown is that of strain K12. Belongs to the PhnI family.