Identification and Purification of a Derepressible Alkaline Phosphatase from Anacystis nidulans

We have examined the increase in alkaline phosphatase activity in the cyanobacterium Anacystis nidulans R2 upon phosphate deprivation. Much of the activity is released into the medium when A. nidulans is osmotically shocked, indicating that the enzyme is located either in the periplasmic space or is loosely bound to the cell wall. The polypeptide associated with phosphatase activity has been identified as a single species of Mr 160,000. Several lines of evidence demonstrate that this polypeptide is responsible for alkaline phosphatase activity: (a) It is absent when cells are grown in the presence of phosphate and specifically accumulates during phosphate deprivation. (b) It is the major periplasmic polypeptide extracted by osmotic shock. (c) It represents over 90% of the protein in a fraction of periplasmic polypeptides enriched for phosphatase activity. (d) Antibodies raised against the purified species of Mr 160,000 inhibit phosphatase activity by approximately 70%. Phosphate, an essential nutrient for all organisms, is present in nucleic acids, phospholipids, and a number of biochemically important solutes such as ATP. It is required by cells in large amounts, frequently limits the growth of both terrestrial and aquatic organisms, and is actively transported into most organisms as the phosphate anion. During periods of phosphate limitation many microorganisms employ emergency systems which aid in its acquisition. Anacystis nidulans (Synechococcus 6301), a unicellular cyanobacterium, exhibits marked physiological and biochemical changes when deprived of phosphate. Following deprivation, growth stops and the cells become bleached; the light harvesting phycobilisome complex is degraded, Chl levels decline, and cyanophycin granules accumulate (21). In addition, both acid soluble and insoluble phosphorus pools begin to decline and phosphate uptake is significantly enhanced (8). One of the most well documented changes that occurs in this microorganism during phosphate deprivation is the appearance of an alkaline phosphatase (12). The production of alkaline phosphatase in response to phosphate limitation has been extensively examined in Escherichia coli (23, 26) and several different prokaryotic (11) and eukaryotic (28) organisms. The enzyme is a nonspecific phosphomonoesterase generally associated with zinc. In several organisms much of the alkaline phosphatase activity is localized to the periplasmic space. Mechanisms involved in regulated expression of alkaline phosphatase activity have been examined in E. coli (22, 24), 1 Carnegie Institution of Washington Publication, No. 1005. M. A. B. was supported by the National Science Foundation and Centre National de la Recherche Scientifique postdoctoral fellowship. 2Permanent address: Laboratoire de Physiologie Cellulaire Vdgdtale/ DRF, UA CNRS 576, CENG 85X, F-38041 Grenoble Cedex, France. Bacillus licheniformis (11), and the yeast Saccharomyces cerevisiae (28). In E. coli the alkaline phosphatase gene is part of the complex pho regulon which is controlled by the single repressor substance inorganic phosphate (22, 24). The genes of the pho regulon are scattered on the E. coli chromosome and the products of these genes are both structural elements required for elevated phosphate acquisition and regulatory elements involved in transcriptional control. While both genetic and biochemical approaches have increased our understanding of several events which accompany phosphate limitation and the functions of certain polypeptides encoded by member genes of the pho regulon, many aspects of the regulation are still unclear. In the eukaryotic organism S. cerevisiae (1, 28) the genes involved in acclimation to low phosphate conditions are also dispersed on the genome and encode numerous structural and regulatory elements. As in E. coli, elevated synthesis of alkaline phosphatase in this organism during phosphate deprivation has been used to analyze the molecular control of gene regulation during acclimation. In this paper, we confirm the appearance of an alkaline phosphatase activity in the cyanobacterium A. nidulans during phosphate deprivation and demonstrate that much of the activity is localized to the periplasmic space. Furthermore, the polypeptide responsible for the phosphatase activity was purified from periplasmic proteins extracted from phosphate-starved cells. Antibodies prepared against the denatured monomer were used to confirm the identification of the enzyme. MATERIALS AND METHODS Culture Conditions. Anacystis nidulans, strain R2, was grown to mid-logarithmic phase (2-5 x 108 cells/ml) at 30°C in BG-11 medium (20) bubbled with 3% CO2 in air. Growth was in continuous light at an intensity of 100 ,umol m-2 s. To achieve phosphate deprivation, cells were harvested by centrifugation at 3500g for 5 min, washed in BG-11 medium in which KH2PO4 was replaced by KCl (phosphate-free medium) and then resuspended in phosphate-free medium at a 3 to 4 times lower cell density. Measurement of Phosphatase Activity. Either whole cell suspensions (unlysed) or preparations of periplasmic proteins (see below) were made to 0.2 M Tris-HC1 (pH 8.5), 2 mM MgCI2, and 3.6 mM p-nitrophenylP (Sigma) (final volume 1.5 ml), unless otherwise specified, and incubated at 37°C for 5 to 15 min. The reaction was stopped with 150 gl of 4 N NaOH, the mixture centrifuged 4 min in an Eppendorf microfuge (7000g), and the absorbance of the supernatant measured at 400 nm and compared to a standard absorbance curve forp-nitrophenol (Sigma). Under these conditions the reaction was linear with respect to both time and enzyme concentration. Extraction of Total Soluble Proteins. Cells from approximately 50 ml of culture were harvested by centrifugation at 3500g for 5 min and resuspended in 6 ml of ice-cold 5 mm Tris-HCl (pH 7.5), 1 mm benzamidin-HCI, 1 mm phenylmethylsulfonylfluor1179 www.plantphysiol. org on July 22, 2017 Published by Downloaded from Copyright © 1988 American Society of Plant Biologists. All rights reserved.