The cyanobacterium Synechocystis sp. strain PCC 6803 possesses two genes, named ppa and ppx, which, respectively, encode proteins involved in the hydrolysis of inorganic phosphate polymers, namely, inorganic pyrophosphatase (PPA, EC 3.6.1.1), an essential enzyme that hydrolyzes pyrophosphate, and exopolyphosphatase (PPX, EC 3.6. 1.11), a processive enzyme that releases the terminal orthophosphate group from linear polyphosphates. Northern blots showed that both single-copy genes are induced by long-term inorganic phosphate (P-i) starvation, transcript levels being markedly increased (ca. 10- and 20-fold, respectively) relative to P-i-sufficient cells. Concurrent increases of both PPA and PPX specific activities and protein levels by P-i deprivation were also observed. On the other hand, a knockout mutant was obtained by insertional mutagenesis of ppx, but it could not be achieved with ppa, thus indicating that PPA function is essential for cell viability. Moreover, whereas the ppx mutant exhibited under P-i-sufficient conditions lower growth rates than the wild-type and was certainly devoid of PPX activity, it showed a severe reduction of the PPA levels. These results are the first evidence on the involvement of both PPA and PPX in a possible intracellular P-i-recycling enzymatic process activated under P-i-starvation. (C) 2003 Elsevier Science (USA). All rights reserved.