Characterization of the oxygenase activity in a mutant of Chlamydomonas reinhardi exhibiting altered ribulosebisphosphate carboxylase

A previously described Mendelian mutant of Chlamydomonas reinhardi, ac i72, exhibiting altered ribulosebisphosphate carboxylase activity and unable to grow on minimal medium is examined for changes in ribulosebisphosphate oxygenase activity. The ribulosebisphosphate oxygenase activity of the enzyme purified from both wild type and ac i72 is compared over a pH range from 7.0 to 9.5. Both enzymes exhibit maximum activity at pH 9.0. However, the ac i72 enzyme is twice as active as the wild type enzyme at a physiological pH of 7.0. The studies in vivo of the products of CO2 fixation of ac i72 and wild type cells in the presence of high and low O2 concentration shows that due to a lower level of carboxylation, the ac i72 cells fix CO2 at half the rate of wild type cells. In ac i72, 24% of the photosynthetically fixed 14C is channelled into the water-soluble fraction as opposed to 6% in wild type. Thin-layer chromatography of the water-soluble fraction showed extensive accumulation of components of the glycolate pathway in ac i72 as compared to wild type. This indicates that the oxygenase activity of the enzyme prevails in ac i72 in vivo. Since a high concentration of glycolate is toxic to cells of C. reinhardi, the high oxygenase activity of ac i72 provides an explanation for the inability of ac i72 to grow phototrophically even though its rate of CO2 fixation is half that of wild type. This toxicity to glycolate is overcome by growth under amber illumination or low O2 concentration.

A mutant strain of Chlamydomonas reinhardi exhibiting altered ribulosebisphosphate carboxylase

A mutant, ac i72, of Chlamydomonas reinhardi possessing an altered ribulosebisphosphate carboxylase and unable to grow on minimal medium has been isolated and characterized. Comparison of ribulosebisphosphate carboxylase purified from both wild type and ac i72 strains is given. The enzyme from ac i72 shows alterations in several characteristics: (a) the specific activity is reduced to 35% that of wild type, (b) the V for both substrates is reduced 3-6 fold, (c) the Mg2+ requirement for maximal activity is 3 times greater, (d) the inhibitory effect of Cl- is greater, and (e) the isoelectric point is changed (6.0 for wild type and 5.8 for ac i72). However, the ribulosebisphosphate carboxylase from ac i72 is identical to that from wild type with respect to pH requirement, temperature sensitivity, subunit structure, and sedimentation characteristic. Other photosynthetic properties of wild type and ac i72 cells were also compared. CO2 fixation in ac i72 in vivo is reduced proportionally to the reduction in activity of the enzyme, but the level of O2 evolution is the same as in wild-type cells. Photosynthetic electron transport, 70-S ribosome content, and chlorophyll content are unaltered in ac i72. The chloroplast ultrastructure of ac i72 cells is distinctly different from that of wild-type cells. The inheritance of the mutation is Mendelian.

Nucleotide-metabolizing enzymes in Chlamydomonas flagella

Nucleotides have at least two functions in eukaryotic cilia and flagella. ATP, originating in the cells, is utilized for motility by energy-transducing protein(s) called dynein, and the binding of guanine nucleotides to tubulin, and probably certain transformations of the bound nucleotides, are prerequisites for the assembly of microtubules. Besides dynein, which can be solubulized from Chlamydomonas flagella as a heterogeneous, Mg2+ or Ca2+-activated ATPase, we have purified and characterized five other flagellar enzymes involved in nucleotide transformations. A homogeneous, low molecular weight, Ca2+-specific adenosine triphosphatase was isolated, which was inhibited by Mg2+ and was not specific for ATP. This enzyme was not formed by treating purified dynein with proteases. It was absent from extracts of Tetrahymena cilia. Its function might be an auxiliary energy transducer, or in steering or tactic responses. Two species of adenylate kinase were isolated, one of which was much elevated in regenerating flagella; the latter was also present in cell bodies. A large part of flagellar nucleoside diphosphokinase activity could not be solubilized. Two soluble enzyme species were identified, one of which was also present in cell bodies. Since these enzymes are of interest because they might function in microtubule assembly, we studied the extent to which brain nucleoside diphosphokinase co-polymerizes with tubulin purified by repeated cycles of polymerization. Arginine kinase was not detected in Chlamydomonas flagellar extracts.

The nitrate-reducing enzyme system of Chlamydomonas reinhardii

In Chlamydomonas reinhardii the reduction of nitrate to ammonia occurs in two independent enzymatic steps: 1. the two-electrons reduction of nitrate to nitrite catalyzed by NADH-nitrate reductase, and, 2. the six-electrons reduction of nitrite to ammonia catalyzed by ferredoxin-nitrite reductase. Both enzymes have been purified and characterized, and some of their properties have been studied.

Genetic control of chlorophyll biosynthesis in chlamydomonas: analysis of a mutant affecting synthesis of delta-aminolevulinic acid

A Mendelian mutation, r-1, in Chlamydomonas reinhardtii has been isolated which elevates protoporphyrin accumulation of the Mendelian protoporphyrin mutants brS-1 and brC-1 more than 20 fold. This increased protoporphyrin accumulation is shown to result from increased delta-aminolevulinic acid synthesis in the double mutants brS-1 r-1 and brC-1 r-1 over that of brS-1 and brC-1 alone. By itself, the r-1 mutation has no detectable protoporphyrin accumulation and has reduced levels of delta-aminolevulinic acid synthesizing activity, chlorophyll, protoheme, and cytochrome oxidase activity. The low levels of chlorophyll and protoheme in r-1 can be increased by feeding delta-aminolevulinic acid. We hypothesize that r-1 may be a mutation of the gene coding for the delta-aminolevulinic acid synthesizing enzyme which reduces the sensitivity of this enzyme to feedback inhibition by protoporphyrin or heme as well as reducing the overall activity of the enzyme. Evidence is also presented for a single delta-aminolevulinic acid synthesizing enzyme serving both chlorophyll and heme biosynthesis.

The induction of allophanate lyase during the vegetative cell cycle in light-synchronized cultures of Chlamydomonas reinhardi

Allophanate lyase can be induced by urea or acetamide 20-40-fold within 4 h in NH4 + -deprived cultures of Chlamydomonas reinhardi. In light-synchronized cultures, allophanate lyase induction appeared to be limited to the light phase of the cell cycle, provided that culture samples were induced under ongoing illumination conditions (i.e. light induction of light phase cells and dark induction of dark phase cells). However, when culture samples were induced under constant light conditions this cell cycle pattern was abolished. Light was found to be required for allophanate lyase induction and this was shown to be due, in part, to the light requirement for inducer uptake. The relationship between allophanate lyase induction and gametogenesis is discussed.

Isolation and study of mutants lacking a derepressible phosphatase in Chlamydomonas reinhardi

In the green alga Chlamydomonas reinhardi, removal of inorganic phosphate from the culture medium results in the increase of phosphatase activity (derepression) in the wild-type (WT) strain as well as in a double mutant (P2Pa)) lacking the two main constitutive acid phosphatases. Following treatment of WT and P2Pa with N-methyl-N-nitro-N-nitrosoguanidine (MNNG), mutants were recovered which display very low phosphatase activities when grown in the absence of phosphate; as shown by electrophoresis, they lack one non-migrating phosphatase (PD mutants). This enzyme is active over a wide range of pH with an optimum at pH 7.5. The comparison of elctropherograms form WT and mutants grown on media with or without phosphate allowed us to provide a tentative definition of the pool of derepressible phosphatases in Chlamydomonas: in addition tothe neutral phosphatase lacking in PD mutants, Chlamydomonas produces two electrophoretic forms of alkaline phosphatase showing an optimal activity at pH 9.5.

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.

Purification of a derepressible arylsulfatase from Chlamydomonas reinhardti. Properties of the enzyme in intact cells and in purified state

Arylsulfatase (aryl-sulfate sulfohdydrolase, EC 3.1.6.1) has been purified from SO4-2-minus-starved cells of Chlamydomonas reinhardti. The enzyme was isolated from acetone-powder extract by (NH4)2SO4 precipitation, Sephadex G-200 filtration and ion-exchange chromatography. Only one fraction of aryl-sulfatase was found. The final preparation was homogenous by the criteria of sedimentation, diffusion and polyacrylamide gel electrophoresis. The purified enzyme had a molecular weight of about 150 000, estimated by ultracentrifugation and gel filtration, and an isoelectric point of 9.0. The properties of the enzyme as investigated in intact cells and in the purified state were found to be very similar except for the temperature optimum. Imidazole strongly increased the enzyme by increasing the V, but reduced the affinity for the substrate. The enzyme activity was competitively inhibited by borate with a greater affinity for borate than for the substrate. The Chlamydomonas enzyme is a Type I arylsulfatase since it was inhibited by CN-minus, but not SO4-2-minus and phosphate.

The capacity for arylsulfatase synthesis in synchronous and synchronized cultures of Chlamydomonas reinhardti

The green algae Chlamydomonas reinhardti synthesizes arylsulfatase (arylsulfate sulfohydrolase EC 3.1.6.1) by derepression when the concentration of SO4-2-minus in the growth medium is less than about 5-10-minus 5 M. The following observations indicate that the arylsulfatase enzyme is stable while its mRNA was unstable: (1) The increase in enzyme activity stopped and remained constant after addition of cycloheximide to derepressed cells. (2) After readdition of SO4-2-minus the increase in enzyme activity continued at a lower rate whereafter it remained constant. (3) No decay of radioactivity was observed after readdition of SO4 2-minus in labelled enzyme protein isolated from pulse-labelled --S cells. The maximum rate of arylsulfatase synthesis. Measurements of this capacity in cells taken at different developmental stages from a selection synchronous and from a light-dark synchronized culture showed that: (1) Arylsulfatase was derepressible at all stages of the life cycle. (2) The same periodic capacity patterns were found, both with the synchronized and the synchronous cells. Furthermore, the rate of accummulation of RNA and protein changed in the same periodic manner during the life cycle as did the enzyme capacity.

Metabolic control of urea catabolism in Chlamydomonas reinhardi and Chlorella pyrenoidosa

In the unicellular green alga Chlamydomonas reinhardi (strain y-1), synthesis of the enzymes required for urea hydrolysis is under substrate induction control by urea and under end product repression control by ammonia. Hydrolysis of urea if effected by the sequential action of the discrete enzymes urea carboxylase and allophanate lyase, collectively called urea amidolyase. The carboxylase converts urea to allophanate in a reaction requiring biotin, adenosine 5'-triphosphate, and Mg2+. The lyase hydrolzyes allophanate to ammonium ions and bicarbonate. Neither activity is present in more than trace amounts when cultures are grown with ammonia or urea plus ammonia, or when they are starved for nitrogen for 8 h. Urea in the absence of ammonia induces both activities 10 to 100 times the basal levels. Addition of ammonia to an induced culture causes complete cessation of carboxylase accumulation and an 80% depression of lyase accumulation. Ammonia does not reduce urea uptake by repressed cells, so it does not prevent induction by the mechanism of inducer exclusion. The unicellular green alga Chlorella pyrenoidosa (strain 3 Emerson) also has discrete carboxylase and lyase enzymes, but only the carboxylase exhibits metabolic control.

Synthesis and turnover of ribulose biphosphate carboxylase and of its subunits during the cell cycle of Chlamydomonas reinhardtii

The chloroplast enzyme ribulose-1,5-bisphosphate (Ru-1,5-P2) carboxylase (EC 4.1 1.39) is made up ot two nonidentical subunits, one synthesized in the chloroplast and the other outside. Both of these subunits of the assembled enzyme are synthesized in a stepwise manner during the synchronous cell cycle of the green alga Chlamydomonas reinhardtii. The activity of this enzyme increases in the light and this increase is due to de novo protein synthesis as shown by the measurement of the amount of protein and by the pulse incorporation of radioactive arginine in the 18S enzyme peak in linear sucrose density gradients. During the dark phase of the cell cycle, there is little change in the enzymatic activity as well as in the amount of this enzyme. Pulse-labeling studies using radioactive arginine indicated that there is a slow but detectable rate of synthesis of the carboxylase and of its subunits in the dark. Ru-1,5-P2 carboxylase, prelabeled with radioactive arginine throughout the entire light period, shows a similarly slow rate of degradation in the following dark period. This slow turnover of the enzyme in the dark accounts for the steady levels of carboxylase protein and of enzymatic activity during this period. A wide variety of inhibitors of protein synthesis by 70S and 80S ribosomes abolished the incorporation of [3H]arginine into total Ru-1,5-P2 carboxylase during short-term incubation. These results suggest a tight-coordinated control of the biosynthesis of the small and large subunits of the enzyme. This stringent control is further substantiated by the finding that both subunits are synthesized in sychrony with each other, that the ratio of radioactivity of the small to the large subunit remains constant throughout the entire light-dark cycle, and that the rates of synthesis and of degradation of both subunits are similar to that of the assembled enzyme.

Chorismate mutase of Chlamydomonas reinhardi. Partial purification and some properties

Chorismate mutase (chorismate pyruvatemutase, EC 5.4.99.5) was extracted from Chlamydomonas reinhardi by sonication. Fractionation of crude sonic extracts with (NH4)2SO4 and by DEAE-cellulose and Sephadex gel chromatography indicated a single peak of chorismate mutase activity with molecular weight 61 000. The Michaelis constant for 20-fold purified enzyme was 0.46 mM. Prephenate dehydrogenase (EC 1.3.1.9) and prephenate dehydratase (EC 4.2.1.40) activities were not detected in our crude or partially purified preparations of chorismate mutase. Tyrosine (1.25 mM) inhibited chorismate mutase activity by approx. 85% in crude and partially purified preparations. Phenylalanine (1.25 mM) inhibited 20%. Tryptophan (1.25 mM) by itself had no detectable effect on chorismate mutase activity but it completely reversed inhibition by tyrosine and phenylalanine. No repression of chorismate mutase was observed when the minimal growth medium was supplemented with aromatic end products.

Cell-cell interactions: enhancement of glycosyl transferase ectoenzyme systems during Chlamydomonas gametic contact

Glycosyl transferase ectoenzyme systems that transfer galactose, glucose, N-acetylglucosamine, N-acetylneuraminic acid, mannose, and fucose have been detected on vegetative cells and gametes of Chlamydomonas moewusii. Gametes have higher levels of activity of the transferase ectoenzyme systems than morphologically identical vegetative cells, as determined by transfer of monosaccharide onto endogenous cell surface acceptors. When (plus) and (minus) gametes are mixed, there is a significant increase in the activity of transferase ectoenzyme systems. No enhancement in activity of transferase ectoenzyme systems occurs when (plus) and (minus) vegetative cells are mixed. Flagellar membrane vesicles obtained from (plus) and (minus) gametes show high activity of transferase ectoenzyme systems per mg of protein and also demonstrate enhanced activity upon mixing. Therefore, glycosyl transferases and acceptors seem to be located on the flagellar membrane and appear to have a function particularly related to gametic cells. The mechanism of cellular adhesion or recognition proposed by Roseman (1970, Chem. Phys. Lipids 5, 270-297), involving glycosyl transferases and acceptors, is strongly suggested by our data for the mating reaction in Chlamydomonas.

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

In order to isolate acid phosphatase mutants in the green alga Chlamydomonas reinhardi, a staining method for detecting the enzyme activity in colonies has been developed. The occurrence of more than one acid phosphatase brought about some difficulty in the selection of mutants. We have, however, found an original method of selection based on the differential heat sensitivity of the enzymes. After treatment of the wild-type strain with N-methyl-N'-nitro-N-nitrosoguanidine, two types of mutants were recovered, then analyzed by biochemical and electrophoretic methods. In the first class of mutants (P(1), P(2), P(3),...) a heat-stable acid phosphatase bound to cellular debris of the crude extract was missing. The mutant P(a), representing the second class of mutations, was lacking a soluble heat-sensitive enzyme. These mutations were genetically different and exhibited mendelian inheritance.

Can a non-Mendelian mutation affect both chloroplast and mithchondrial ribosomes?

Chloroplast ribosomes isolated from a spectinomycin-resistant mutant (spr-1-27-3) of Chlamydomonas reinhardtii that displays non-Mendelian inheritance fail to bind labeled antibiotic, in contrast to ribosomes from wild-type cells. In vitro resistance of this mutant appears to result from the absence of a specific protein in the small subunit of the chloroplast ribosome. However, chloroplast protein synthesis in the mutant and wild type shows identical sensitivity to spectinomycin in short-term in vivo experiments where ribulosediphosphate carboxylase serves as the marker. Long-term experiments demonstrate that the mutant can grow in the presence of spectinomycin only when acetate is supplied as a carbon source. Mitochondrial structure and function of the mutant are not affected by the antibiotic, whereas chloroplast structure and function are. Apparently, the mitochondrion, rather than the chloroplast, of this mutant is resistant to spectinomycin in vivo. We hypothesize that the gene product of the spr locus is a protein common to both chloroplast and mitochondrial ribosomes. The mutant gene product, in vivo, confers resistance on mitochondrial, but not chloroplast, ribosomes. We suppose that the mutant spr protein loosely attaches to chloroplast ribosomes in vivo so that the antibiotic is bound and blocks protein synthesis, but it dissociates during isolation, resulting in loss of the binding site.