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

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.Phosphorus (P) is an essential nonmetal nutrient for all living cells. Despite its relative abundance in the ecosystem, P is sometimes a limiting factor for organisms in terrestrial, oceanic, and freshwater environments (1, 11). This is because while all organisms can utilize soluble inorganic phosphate (P i ), the P-containing organic compounds generally found in nature are complex insoluble forms that are not readily available to cells (1, 11). Alkaline phosphatases (APases) release free P i from organic compounds. To date, three prokaryotic APase gene families-PhoA, PhoX, and PhoD-have been documented (7). In addition to different levels of homology among these APases, dissimilar metal requirements for their activities have been reported (7). A recent metagenomics analysis revealed that PhoX, a recently characterized phosphatase, is more abundant in marine bacteria than the previously considered classical PhoA (12). However, it has also been shown that PhoD is more abundant in marine bacteria than PhoA or PhoX (7), suggesting an important role for PhoD in marine bacteria. PhoD encompasses a family of phosphatase/phosphodiesterases (APase/APDase). Except for Bacillus subtilis PhoD (PhoD Bs ) (3), little is known on other PhoD proteins. PhoD Bs was shown to be secreted into extracellular medium by the Tat pathway, which recognizes targeted proteins by their N-terminal twin-arginine signal peptides containing the Tat consensus (SRRXFLK) motif (3, 9).Cyanobacteria inhabit a broad range of ecosystems and play a vital role in the global cycling of nutrients, including P.Hitherto, extensive studies on the regulation of gene expression during phosphate stress have been carried out (14). In contrast, functional properties of cyanobacterial APases have been reported only on classical PhoA (8), atypical APase (11), PhoV (21), and PhoX (5), and there is no report on the cyanobacterial PhoD. Moreover, little is known on the role of APase under high-salinity conditions, and this encouraged us to examine the role of cyanobacterial PhoD under high-salinity conditions.Aphanot...

phoD Alkaline Phosphatase Gene Diversity in Soil

“…PhoD and PhoX are exported by the twin-arginine translocation pathway (5, 6), while PhoA is secreted via the Sec protein translocation pathway (9). There is high sequence variability in the PhoA, PhoD, and PhoX proteins, not only between the families but also within each family (5,9). PhoD is widespread in both terrestrial and aquatic ecosystems (8,10).…”

“…It is a homodimeric enzyme that hydrolyzes phosphomonoesters and is activated by Mg 2ϩ and Zn 2ϩ (7). PhoD and PhoX are monomeric enzymes that hydrolyze both phosphomonoesters and phosphodiesters and are activated by Ca 2ϩ (5,6). Enzymes of all three families are predominantly periplasmic, membrane bound, or extracellular (8).…”

Physiological, biochemical and molecular responses of the halophilic cyanobacteriumAphanothece halophyticato Pi-deficiency

“…Increased APase activity under Pi-deficiency has been reported in cyanobacteria (Healey, 1973;Kageyama et al, 2011) and is an effective indicator of Pi deficiency in environments (Turner et al, 2001). The rapid increase in and high activity of cell-bound APase compared with cellfree APase indicates that Pi deprivation initially upregulates the synthesis of APase, which is secreted via twin-arginine translocation (Tat) pathways (Kageyama et al, 2011).…”

Improved Alkane Production in Nitrogen-Fixing and Halotolerant Cyanobacteria via Abiotic Stresses and Genetic Manipulation of Alkane Synthetic Genes

“…A. halophytica was grown in BG11 plus Turk Island salt solution containing 0.5 M NaCl, and S. elongatus PCC 7942 and Anabaena sp. PCC7120 were grown in BG11 alone [7]. For nitrogen-deficient treatment, the cells at exponential growth phase (OD 730 = *0.6) were washed with BG11 media without sodium nitrate (BG11 0 ), and then cultured in BG11 0 .…”