An Alkaline Phosphatase/Phosphodiesterase, PhoD, Induced by Salt Stress and Secreted Out of the Cells of Aphanothece halophytica, a Halotolerant Cyanobacterium

Abstract

ABSTRACT Alkaline phosphatases (APases) are important enzymes in organophosphate utilization. Three prokaryotic APase gene families, PhoA, PhoX, and PhoD, are known; however, their functional characterization in cyanobacteria largely remains to be clarified. In this study, we cloned the phoD gene from a halotolerant cyanobacterium, Aphanothece halophytica ( phoD Ap ). The deduced protein, PhoD Ap , contains Tat consensus motifs and a peptidase cleavage site at the N terminus. The PhoD Ap enzyme was activated by Ca 2+ and exhibited APase and phosphodiesterase (APDase) activities. Subcellular localization experiments revealed the secretion and processing of PhoD Ap in a transformed cyanobacterium. Expression of the phoD Ap gene in A. halophytica cells was upregulated not only by phosphorus (P) starvation but also under salt stress conditions. Our results suggest that A. halophytica cells possess a PhoD that participates in the assimilation of P under salinity stress.