FIG. 6.

NPP1, NPP2, and PHO5 are the major contributors for NPPase activity. (A) Extracellular phosphate hydrolysis by the triple-null strain was monitored through measuring phosphate import over 24 h. Extracellular [γ-³²P]ATP was utilized as a substrate. At the end of the time course, a 50% defect in phosphate uptake was measured in the triple-null strain. (B) Extracellular [α-³²P]ATP was utilized to test phosphate hydrolysis at the proximal position. The absence of all three genes caused a nearly complete loss of phosphate hydrolysis at the α-position of ATP. (C) NPPase activity was measured in the triple-null strain using the coupled enzyme assay with ATP as a substrate. The triple-null strain demonstrated nominal NPPase activity throughout the entire time course.