Acid phosphatase mutants in Chlamydomonas: isolation and characterization by biochemical, electrophoretic and genetic analysis

Identification and characterization of the COPII vesicle-forming GTPase Sar1 in Chlamydomonas

Eukaryotic cells are highly compartmentalized, requiring elaborate transport mechanisms to facilitate the movement of proteins between membrane-bound compartments. Most proteins synthesized in the endoplasmic reticulum (ER) are transported to the Golgi apparatus through COPII-mediated vesicular trafficking. Sar1, a small GTPase that facilitates the formation of COPII vesicles, plays a critical role in the early steps of this protein secretory pathway. Sar1 was characterized in yeast, animals and plants, but no Sar1 homolog has been identified and functionally analyzed in algae. Here we identified a putative Sar1 homolog (CrSar1) in the model green alga Chlamydomonas reinhardtii through amino acid sequence similarity. We employed site-directed mutagenesis to generate a dominant-negative mutant of CrSar1 (CrSar1DN). Using protein secretion assays, we demonstrate the inhibitory effect of CrSar1DN on protein secretion. However, different from previously studied organisms, ectopic expression of CrSar1DN did not result in collapse of the ER-Golgi interface in Chlamydomonas. Nonetheless, our data suggest a largely conserved role of CrSar1 in the ER-to-Golgi protein secretory pathway in green algae.

Determination of heavy metal toxicity by using a micro-droplet hydrodynamic voltammetry for microalgal bioassay based on alkaline phosphatase

We developed an electrochemical microalgal bioassay for the determination of heavy metal toxicity in water on the basis of the alkaline phosphatase (ALP) enzyme inhibition of Chlamydomonas reinhardtii. Five heavy metals were chosen as toxicants: Hg, Cd, Pb, Zn, and Cu. The induced ALP activity of C. reinhardtii was inhibited using the phosphate starvation method, and the results were evaluated by measuring the electrochemical oxidation of p-aminophenol (PAP) following the enzymatic conversion of p-aminophenyl phosphate (PAPP) as a substrate. The rapid determination of enzymatic activity was achieved using hydrodynamic voltammetry in a 50 μL micro-droplet with a rotating disk electrode (RDE). Enzymatic activity over a PAPP substrate is affected by heavy metal ions, and this phenomenon decreases the chronoamperometric current signal. The concentrations of Hg, Cd, Pb, Zn, and Cu in which the ALP activity was half that of the control (EC₅₀) were found to be 0.017, 0.021, 0.27, 1.30, and 1.36 μM, respectively. The RDE system was demonstrated to be capable of detecting enzymatic activity by using a small amount of regent, a reaction time of only 60 s, and a detection limit of 5.4 × 10^-7 U.

Biosynthesis of carbonic anhydrase in Chlamydomonas reinhardtii during adaptation to low CO(2)

The unicellular green alga Chlamydomonas reinhardtii synthesizes carbonic anhydrase in response to low levels of CO(2) (i.e., air levels of CO(2)). This enzyme, localized predominantly in the periplasmic space of the alga (or associated with the cell wall), is an important component of the machinery required for the active accumulation of inorganic carbon by C. reinhardtii and the saturation of ribulose-1,5-bisphosphate carboxylase at low extracellular carbon concentrations. We have begun to examine the synthesis and compartmentalization of carbonic anhydrase in C. reinhardtii. The monomeric species associated with carbonic anhydrase activity is synthesized as a precursor on 80S cytoplasmic ribosomes. This precursor can be detected immunologically in the profiles of translation products when a reticulocyte lysate, cell-free system is primed with poly(A)-RNA from either air-grown C. reinhardtii or cells shifted from growth on 5% CO(2) to air for 12 hr. It is not synthesized when the in vitro system is primed with poly(A)-RNA from CO(2)-grown algae. Since translatable RNA for the polypeptide responsible for carbonic anhydrase activity was only present in cells that experienced low levels of CO(2), the adaptation process either involves the regulation of transcription of the carbonic anhydrase gene (and perhaps other genes involved in adaptation) or the post-transcriptional processing of the messenger RNA. Furthermore, the appearance of the mature polypeptide associated with carbonic anhydrase activity in the periplasmic space of C. reinhardtii is inhibited by tunicamycin, an antibiotic that prevents core glycosylation of polypeptides on the endoplasmic reticulum. Together, these results suggest that the biosynthesis of this extracellular algal enzyme involves the translation of mRNA for the carbonic anhydrase monomer on ribosomes bound to the endoplasmic reticulum, the cleavage of a signal sequence during transport of the nascent polypeptide into the lumen of the endoplasmic reticulum, and subsequent glycosylation events prior to export across the plasmalemma.

The LPB1 gene is important for acclimation of Chlamydomonas reinhardtii to phosphorus and sulfur deprivation

Organisms exhibit a diverse set of responses when exposed to low-phosphate conditions. Some of these responses are specific for phosphorus limitation, including responses that enable cells to efficiently scavenge phosphate from internal and external stores via the production of high-affinity phosphate transporters and the synthesis of intracellular and extracellular phosphatases. Other responses are general and occur under a number of different environmental stresses, helping coordinate cellular metabolism and cell division with the growth potential of the cell. In this article, we describe the isolation and characterization of a mutant of Chlamydomonas reinhardtii, low-phosphate bleaching (lpb1), which dies more rapidly than wild-type cells during phosphorus limitation. The responses of this mutant to nitrogen limitation appear normal, although the strain is also somewhat more sensitive than wild-type cells to sulfur deprivation. Interestingly, depriving the cells of both nutrients simultaneously allows for sustained survival that is similar to that observed with wild-type cells. Furthermore, upon phosphorus deprivation, the lpb1 mutant, like wild-type cells, exhibits increased levels of mRNA encoding the PHOX alkaline phosphatase, the PTB2 phosphate transporter, and the regulatory element PSR1. The mutant strain is also able to synthesize the extracellular alkaline phosphatase activity upon phosphorus deprivation and the arylsulfatase upon sulfur deprivation, suggesting that the specific responses to phosphorus and sulfur deprivation are normal. The LPB1 gene was tagged by insertion of the ARG7 gene, which facilitated its isolation and characterization. This gene encodes a protein with strong similarity to expressed proteins in Arabidopsis (Arabidopsis thaliana) and predicted proteins in Oryza sativa and Parachlamydia. A domain in the protein contains some similarity to the superfamily of nucleotide-diphospho-sugar transferases, and it is likely to be localized to the chloroplast or mitochondrion based on programs that predict subcellular localization. While the precise catalytic role and physiological function of the putative protein is not known, it may function in some aspect of polysaccharide metabolism and/or influence phosphorus metabolism (either structural or regulatory) in a way that is critical for allowing the cells to acclimate to nutrient limitation conditions.

MACRONUTRIENT UTILIZATION BY PHOTOSYNTHETIC EUKARYOTES AND THE FABRIC OF INTERACTIONS

Organisms acclimate to a continually fluctuating nutrient environment. Acclimation involves responses specific for the limiting nutrient as well as responses that are more general and occur when an organism experiences different stress conditions. Specific responses enable organisms to efficiently scavenge the limiting nutrient and may involve the induction of high-affinity transport systems and the synthesis of hydrolytic enzymes that facilitate the release of the nutrient from extracellular organic molecules or from internal reserves. General responses include changes in cell division rates and global alterations in metabolic activities. In photosynthetic organisms there must be precise regulation of photosynthetic activity since when severe nutrient limitation prevents continued cell growth, excitation of photosynthetic pigments could result in the formation of reactive oxygen species, which can severely damage structural and functional features of the cell. This review focuses on ways that photosynthetic eukaryotes assimilate the macronutrients nitrogen, sulfur, and phosphorus, and the mechanisms that govern assimilatory activities. Also discussed are molecular responses to macronutrient limitation and the elicitation of those responses through integration of environmental and cellular cues.

Acclimation of Chlamydomonas reinhardtii to its nutrient environment

To cope with low nutrient availability in nature, organisms have evolved inducible systems that enable them to scavenge and efficiently utilize the limiting nutrient. Furthermore, organisms must have the capacity to adjust their rate of metabolism and make specific alterations in metabolic pathways that favor survival when the potential for cell growth and division is reduced. In this article I will focus on the acclimation of Chlamydomonas reinhardtii, a unicellular, eukaryotic green alga to conditions of nitrogen, sulfur and phosphorus deprivation. This organism has a distinguished history as a model for classical genetic analyses, but it has recently been developed for exploitation using an array of molecular and genomic tools. The application of these tools to the analyses of nutrient limitation responses (and other biological processes) is revealing mechanisms that enable Chlamydomonas to survive harsh environmental conditions and establishing relationships between the responses of this morphologically simple, photosynthetic eukaryote and those of both nonphotosynthetic organisms and vascular plants.

Biochemical characterization of the extracellular phosphatases produced by phosphorus-deprived Chlamydomonas reinhardtii

We have examined the extracellular phosphatases produced by the terrestrial green alga Chlamydomonas reinhardtii in response to phosphorus deprivation. Phosphorus-deprived cells increase extra-cellular alkaline phosphatase activity 300-fold relative to unstarved cells. The alkaline phosphatases are released into the medium by cell-wall-deficient strains and by wild-type cells after treatment with autolysin, indicating that they are localized to the periplasm. Anion-exchange chromatography and analysis by nondenaturing polyacrylamide gel electrophoresis revealed that there are two major inducible alkaline phosphatases. A calcium-dependent enzyme composed of 190-kD glycoprotein subunits accounts for 85 to 95% of the Alkaline phosphatase activity. This phosphatase has optimal activity at pH 9.5 and a Km of 120 to 262 microns for all physiological substrates tested, with the exception of phytic acid, which it cleaved with a 50-fold lower efficiency. An enzyme with optimal activity at pH 9 and no requirement for divalent cations accounts for 2 to 10% of the alkaline phosphatase activity. This phosphatase was only able to efficiently hydrolyze arylphosphates. The information reported here, in conjunction with the results of previous studies, defines the complement of extracellular phosphatases produced by phosphorus-deprived Chlamydomonas cells.

Characterization of Sulfate Transport in Chlamydomonas reinhardtii during Sulfur-Limited and Sulfur-Sufficient Growth

We have characterized sulfate transport in the unicellular green alga Chlamydomonas reinhardtii during growth under sulfur-sufficient and sulfur-deficient conditions. Both the Vmax and the substrate concentration at which sulfate transport is half of the maximum velocity of the sulfate transport (K1/2) for uptake were altered in starved cells: the Vmax increased approximately 10-fold, and the K1/2 decreased approximately 7-fold. This suggests that sulfur-deprived C. reinhardtii cells synthesize a new, high-affinity sulfate transport system. This system accumulated rapidly; it was detected in cells within 1 h of sulfur deprivation and reached a maximum by 6 h. A second response to sulfur-limited growth, the production of arylsulfatase, was apparent only after 3 h of growth in sulfur-free medium. The enhancement of sulfate transport upon sulfur starvation was prevented by cycloheximide, but not by chloramphenicol, demonstrating that protein synthesis on 80S ribosomes was required for the development of the new, high-affinity system. The transport of sulfate into the cells occurred in both the light and the dark. Inhibition of ATP formation by the antibiotics carbonylcyanide m-chlorophenylhydrazone and gramicidin-S and inhibition of either F- or P-type ATPases by N,N-dicyclohexylcarbodiimide and vanadate completely abolished sulfate uptake. Furthermore, nigericin, a carboxylate ionophore that exchanges H+ for K+, inhibited transport in both the light and the dark. Finally, uptake in the dark was strongly inhibited by valinomycin. These results suggest that sulfate transport in C. reinhardtii is an energy-dependent process and that it may be driven by a proton gradient generated by a plasma membrane ATPase.

Calcium-regulated phosphorylation of proteins in the membrane-matrix compartment of the Chlamydomonas flagellum

Crosslinking of surface-exposed domains on certain Chlamydomonas flagellar membrane glycoproteins induces their movement within the plane of the flagellar membrane. Previous work has shown that these membrane glycoprotein movements are dependent on a critical concentration of free calcium in the medium and are inhibited reversibly by calcium channel blockers and the protein kinase inhibitors H-7, H-8, and staurosporine. These observations suggest that the flagellum may use a signaling pathway that involves calcium-activated protein phosphorylation to initiate flagellar membrane glycoprotein movements. In order to pursue this hypothesis, we examined the calcium dependence of phosphorylation of flagellar membrane-matrix proteins using an in vitro system containing [gamma-32P]ATP or [35S]ATP gamma S. Using only endogenous enzymes and endogenous substrates found in the membrane-matrix fraction obtained by extraction of flagella with 0.05% Nonidet P-40, we observed both calcium-independent protein phosphorylation and calcium-dependent protein phosphorylation in addition to an active protein dephosphorylation activity. Addition of micromolar free calcium increased the amount of protein phosphorylation severalfold. Calcium-activated protein kinase activity was inhibited by H-7, H-8, and staurosporine, the same protein kinase inhibitors that inhibit the calcium-dependent glycoprotein redistribution in vivo. A small group of polypeptides in the 26-58 kDa range exhibited a dramatic increase in phosphorylation in the presence of 20 microM free calcium. We suggest that Chlamydomonas utilizes the intraflagellar free calcium concentration to regulate the phosphorylation of specific flagellar proteins in the membrane-matrix fraction, one or more of which may be involved in regulating the machinery responsible for flagellar membrane glycoprotein redistribution.

New polypeptides and in-vitro-translatable mRNAs are produced by phosphate-starved cells of the unicellular algaChlamydomonas reinhardtii

Cells of the unicellular algaChlamydomonas reinhardtii Dang. deprived of inorganic phosphate (Pi) secrete into the culture medium large amounts of glycoproteins which are not produced by cells grown in the presence of Pi. One of the polypeptides (P6: Mr 73000 ± 2 000) resolved by sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE) is absent from a mutant lacking neutral-phosphatase activity, and probably corresponds to a subunit of this enzyme. The antibodies raised to P6, however, were able to cross-react on Western blots with most of the secreted proteins which indicates that they recognize oligosaccharide epitopes common to all of these de-novo-formed molecules. In order to verify whether the response to Pi deprivation takes place at the transcriptional level, the in-vitro translation products directed by poly(A)(+) RNA preparations obtained from cells grown with or without Pi were analyzed by two-dimensional SDS-PAGE. Three polypeptides specific to Pi-starved cells were detected but were considered unlikely to correspond to subunits of the neutral phosphatase. Whereas in-vivo-labelled proteins (notably P6) were precipitable by our antibodies, all attempts at precipitating in-vitro translation products have failed. This result is in agreement with the hypothesis that the antibodies recognize the oligosaccharide side chains but not the polypeptide backbone of the glycoproteins, a situation already described for monoclonal antibodies to the major structural glycoprotein of theChlamydomonas cell wall.

Structure and expression of the gene encoding the periplasmic arylsulfatase of Chlamydomonas reinhardtii

Chlamydomonas reinhardtii produces a periplasmic arylsulfatase in response to sulfur deprivation. We have isolated and sequenced arylsulfatase cDNAs from a lambda gt11 expression library. The amino acid sequence of the protein, as deduced from the nucleotide sequence, has features characteristic of secreted proteins, including a signal sequence and putative glycosylation sites. The gene has a broad codon usage with seven codons, all having A residues in the third position, not previously observed in C. reinhardtii genes. Arylsulfatase transcription is tightly regulated by sulfur availability. The approximately 2.7 kb arylsulfatase transcript is very susceptible to degradation, disappearing in less than an hour after sulfur starved cells are administered either sulfate or alpha-amanitin. The accumulation of the arylsulfatase transcript is also suppressed by the addition of cycloheximide. Transcription initiation from the arylsulfatase gene occurs approximately 100 bp upstream of the initiation codon, in a region that is 5' to a 43 bp imperfect inverted repeat. Preceding the transcription start site are sequences similar to those present in promoter regions of other genes from C. reinhardtii.

Purification and biosynthesis of a derepressible periplasmic arylsulfatase from Chlamydomonas reinhardtii

The unicellular green alga Chlamydomonas reinhardtii responds to sulfate deprivation by producing an arylsulfatase (Lien, T., and O. Schreiner. 1975. Biochim. Biophys. Acta. 384:168-179; Schreiner, O., 1975. Biochim. Biophys. Acta. 384:180-193) and by developing the capacity to transport sulfate more rapidly (our unpublished data). The arylsulfatase activity, detectable 3 h after the transfer of the cells to low sulfate medium (less than or equal to 10 microM sulfate), is a periplasmic protein released into the culture medium by cw15, a cell wall-less mutant of C. reinhardtii. We have purified the derepressible arylsulfatase to homogeneity and have raised monospecific antibodies to it. The protein monomer (67.6 kD) associates into a dimer, and the enzyme activity shows an alkaline pH optimum and a Km of 0.3 mM for p-nitrophenylsulfate. Studies focused on arylsulfatase biosynthesis demonstrate that it is glycosylated and synthesized as a higher molecular mass precursor. The mature protein contains complex N-linked oligosaccharides and the primary translation product has an apparent molecular mass approximately 5 kD larger than the deglycosylated monomer. Since translatable RNA encoding the arylsulfatase can only be detected in cells after sulfate starvation, it is likely that accumulation of the enzyme is regulated at the level of transcription, although posttranscriptional processes may also be involved.

Topography of cell wall lytic enzyme in Chlamydomonas reinhardtii: form and location of the stored enzyme in vegetative cell and gamete

Chlamydomonas lytic enzyme of the cell wall (gamete wall-autolysin) is responsible for shedding of cell walls during mating of opposite mating-type gametes. This paper reports some topographic aspects of lytic enzyme in cells. Both vegetative and gametic cells contain the same wall lytic enzyme. The purified enzyme is a glycoprotein with an apparent molecular mass of 67 kD by gel filtration and 62 kD by SDS PAGE, and is sensitive to metal ion chelators and SH-blocking agents. These properties are the same as those of the gamete wall-autolysin released into the medium by mating gametes. However, the storage form of the enzyme proves to be quite different between the two cell types. In vegetative cells, the lytic enzyme is found in an insoluble form in cell homogenates and activity is released into the soluble fraction only by sonicating the homogenates or freeze-thawing the cells, whereas gametes always yield lytic activity in the soluble fractions of cell homogenates. When vegetative cells are starved for nitrogen, the storage form of enzyme shifts from its vegetative state to gametic state in parallel with the acquisition of mating ability. Adding nitrogen to gametes converts it to the vegetative state concurrently with the loss of mating ability. We also show that protoplasts obtained by treatment of vegetative cells or gametes with exogenously added enzyme have little activity of enzyme in the cell homogenates, suggesting that lytic enzyme is stored outside the plasmalemma. When the de-walled gametes or gametes of the wall-deficient mutant, cw-15, of opposite mating types are mixed together, they mate normally but the release of lytic enzyme into the medium is practically negligible. When the de-walled vegetative cells are incubated, the lytic enzyme is again accumulated in the cells after the wall regeneration is almost complete.

Genetic effects of N-methyl-N'-nitro-N-nitrosoguanidine and its homologs

Since the discovery of the mutagenic activity of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in 1960, this compound has become one of the most widely used chemical mutagens. The present paper gives a survey on the chemistry, metabolism, and mode of interaction of MNNG with DNA and proteins, and of the genotoxic effects of this agent on microorganisms, plants, and animals, including human cells cultured in vitro. Data on the carcinogenicity and teratogenicity of MNNG as well as on the genotoxic effects of homologs of MNNG are also presented.

Acid phosphatase isoenzymes of Chlamydomonas reinhardii

Acid phosphatase isoenzymes of Chlamydomonas reinhardii were investigated by isoelectric focusing in polyacrylamide gel systems. In this paper we describe in detail an original method for isoelectric focusing of acid phosphatases extracted from wild-type and acid phosphatase-lacking mutant algae, obtained from Laboratoire de Génetique of University of Liège. Three isoenzymes can be separated from the buffer-soluble components of these cells. An additional isoenzyme type can be visualized using the nonionic detergent NP40 as solubilizer. We conclude that these four isoenzymes are related to the structural gene of the soluble constitutive acid phosphatase, which was shown by their appearance in P2 and their total absence in mutant Pa. The pI values of soluble constitutive acid phosphatase isoenzymes range between pH 5.2 and 6.2. As a result of treatment with NP40 the extracts from both wild-type and mutant lines contain two additional active phosphatase forms which can be characterized by their high heat resistance and low pI values. These enzymes are fully active using either alpha-naphthyl phosphate or different acetate esters as substrates.

A mutation altering some properties of the neutral phosphatase in Chlamydomonas reinhardi: possible post-translational modification of phosphatase structure

A mutant (PDs-) of Chlamydomonas reinhardi has been isolated which produces an altered neutral phosphatase. The wild-type (PDs+) and mutant (PDs-) phosphatases markedly differed in their thermosensitivities and electrophoretic mobilities. The heterozygous PDs-/PDs+ diploids produced only the wild-type electrophoretic form of the phosphatase. Mixing extracts of PDs- with extracts of various other strains in vitro resulted in the rapid transformation of the PDs- enzymic form into an enzymic variety, the properties (heat sensitivity, electrophoretic mobility) of which were similar to those of the wild-type neutral phosphatase. The results are discussed in relation to the idea that the PDs mutation is located not in the structural gene but rather in a modifying gene acting at the post-translational level.

Regulation of the neutral phosphatase in Chlamydomonas reinhardi: an immunogenetic study of wild-type and mutant strains

In Chlamydomonas reinhardi, the activity of the neutral phosphatase considerably increases when the cells are grown in the absence of inorganic phosphate (Pi). A comparative immunological study of cells grown on media containing Pi or not indicated that the neutral phosphatase was synthesized de novo. Ten mutants lacking the neutral phosphatase and distributed among three genetic loci (PD2, PD3, PD24) were investigated for their ability to produce cross-reacting material (CRM) antigenically related to the wild enzyme. All mutants were shown to form much less CRM than the wild-type strain. It is proposed that the three genes are involved in the regulation of neutral phosphatase synthesis.

Dissociation-recombination of sunflower seed acid phosphatase

Formal genetic studies of sunflower (Helianthus annuus) seed acid phosphatase (ACP, E.C. 3.1.3.2) had suggested that the functional enzyme consists of two polypeptide subunits. The dimeric quaternary structure was demonstrated by dissociation-recombination procedures. Dissociation of electrophoretically distinct homodimers was effected upon freezing of extracts in a pH 8-9 buffer containing 1 M NaCl and 0.1 M 2-mercaptoethanol. Reassociation, as indicated by the formation of the hybrid isozyme, occurred during 12 hr dialysis against a pH 7.0 buffer.

Extracellular phosphatases of Chlamydomonas reinhardi and their regulation

Chlamydomonas reinhardi, cultured under normal growth conditions, secreted significant amounts of protein and carbohydrates but not lipids or nucleic acids. A fivefold increase in light intensity led to a tenfold increase in secreted protein and carbohydrate. Among the proteins secreted was acid phosphatase with a pH optimum at 4.8 like the enzyme in the cells. Phosphorus depleted algae grown on minimal orthophosphate contained and secreted both acid and alkaline phosphatase. The pH optimum of the intracellular alkaline phosphatase was 9.2. When phosphorus-depleted cells were grown with increasing orthophosphate, intra- and extracellular alkaline phosphatase was almost completely repressed and intra- and extracellular acid phosphatase was partially repressed. Extracellular acid and alkaline phosphatase increased with the age of the culture. Electrophoresis indicated only one acid and one alkaline phosphatase in phosphorus-satisfied and phosphorus-depleted cells. Chlamydomonas cells suspended in an inorganic salt solution secreted only acid phosphatase; the absence of any extr-cellular cytoplasmic marker enzyme indicated that there was little, if any, autolysis to account for the extracellular acid enzyme. Phosphorus-depleted cells were able to grow on organic phosphates as the sole source of orthophosphate. Ribose-5-phosphate was the best for cell multiplication, and its utility was shown to be due to the cell's ability to use the ribose as well as the orthophosphatase for cell multiplication.

Genes involved in the regulation of the neutral phosphatase in Chlamydomonas reinhardi

In Chlamydomonas reinhardi, mutations in either of two unlinked genes (PD2 and PD3) abolish the activity of the derepressible neutral phosphatase. The question arose whether these genes (or one of them) specify the structure of the enzyme or whether they have a regulatory function. Three mutants producing an active phosphatase at 25 degrees C but not at 35 degrees C were isolated and investigated. One of these mutants (PDts11) was allelic with PD2, another one (PDts12) was linked to PD3 and the third one (PDts13) was linked to PD2. PDts11 and PDts13 affected the formation of the neutral phosphatase only whereas PDts12 interfered with the formation of both neutral and alkaline phosphatases at 35 degrees C. The neutral phosphatase produced by the three mutants at low temperature was not more thermosensitive in vitro than the wild enzyme. Moreover, quite similar Km values were found in WT, PDts11 and PDts12 using naphthyl phosphate as a substrate. On the other hand, revertants of PD-2 and PD-3 were isolated: their neutral phosphatases could not be distinguished from the wild enzyme on the basis of their thermosensitivities and Km values for naphthyl phosphate. These results are consistent with the idea that PD2 and PD3 are regulatory genes. Other possible regulatory genes were revealed through PDts12 and PDts13 mutations.

Release of enzymes by normal and wall-free cells of Chlamydomonas

The phosphatase produced by the wild-type strain of Chlamydomonas reinhardi in media deprived of inorganic phosphate are found partly inside and partly outside the cells. The same enzymes are almost completely released by a mutant strain defective in cell wall formation. It is proposed that the failure of cell wall mutants to survive in certain conditions is related to their inability to retain certain essential compounds that are normally associated to the cell wall.

Phosphatase of Chlamydomonas reinhardi: biochemical and cytochemical approach with specific mutants

The unicellular alga Chlamydomonas reinhardi produces two constitutive acid phosphatases and three depressible phosphatases (a neutral and two alkaline ones) that can utilize napthyl phosphate as a substrate. Specific mutants depressible phosphatase were used to investigate biochemical properties and the cytochemical localization of these enzymes. The two constitutive phosphatases show similar pH optima (about 5.0) and Km values (2 x 10(-3) to 3.3 x 10(-3) M) but differ in their heat sensitivity and affinity for glycerophosphate.

Changes in phosphatase activity associated with cell wall defects in Chlamydomonas reinhardi

Experiments were performed to isolate mutants lacking alkaline phosphatase in Chlamydomonas reinhardi. Mutants with null enzyme activity were obtained. A cytological study of these mutants however revealed cell wall defects, suggesting that the loss of phosphatase activity in these strains is not due to the inactivation of the corresponding phosphatase structural gene but rather to the leakage of this enzyme as a consequence of the cell wall abnormality. Incidentally, this finding provides the basis of a convenient method for selecting easily cell wall mutants of Chlamydomonas.