Regulation of initial rate of induction of nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase during the cell cycle of synchronous Chlorella

Heterologous expression of cDNAs encoding Chlorella sorokiniana NADP-specific glutamate dehydrogenase wild-type and mutant subunits in Escherichia coli cells and comparison of kinetic and thermal stability properties of their homohexamers

Full-length cDNAs encoding the alpha- and beta-subunits and a truncated mutant subunit of the Chlorella sorokiniana NADP-GDH isozymes were constructed and expressed in Escherichia coli cells. The kinetic and thermal stability properties of the resultant homohexamers were examined. The electrophoretic mobility of the recombinant alpha- and beta-subunits was identical to that of the native subunits as determined by immunoblotting. The homohexamers were purified by anion-exchange and gel-filtration chromatography. The alpha- and beta-homohexamers that were synthesized in the bacterial cells were shown to have similar Michaelis constants for their substrates as previously shown after synthesis in C. sorokiniana cells (Bascomb and Schmidt, 1987). The alpha homohexamer synthesized in the bacterium was allosteric with respect to NADPH but to a lesser degree than when isolated from the alga. The mutant homohexamer was composed of subunits that were truncated by 40 amino acids at their N-termini. This mutant isozyme was kinetically similar to the larger, anabolic alpha-homohexamer, but it did not display the allosteric response to NADPH shown by the alpha-homohexamer. The three isozymes had significant thermal tolerance and were stable at 50 degrees C. The temperature optimum for catalytic activity for the alpha- and beta-homohexamers was 60 degrees C, and 65 degrees C for the delta40N homohexamer. This study demonstrated that most of the kinetic properties of the Chlorella sorokiniana NADP-GDH isozymes were retained after their synthesis in a heterologous system, and that the distinctive N-terminal domains of these isozymes have dramatic effects on their biochemical characteristics.

Different Rates of Synthesis and Degradation of Two Chloroplastic Ammonium-Inducible NADP-Specific Glutamate Dehydrogenase Isoenzymes during Induction and Deinduction in Chlorella sorokiniana Cells

The kinetics of accumulation (per milliliter of culture) of the alpha- and beta- subunits, associated with chloroplast-localized ammonium inducible nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase (NADP-GDH) isoenzymes, were measured during a 3 hour induction of synchronized daughter cells of Chlorella sorokiniana in 29 millimolar ammonium medium under photoautotrophic conditions. The beta-subunit holoenzyme(s) accumulated in a linear manner for 3 hours without an apparent induction lag. A 40 minute induction lag preceded the accumulation of the alpha-subunit holoenzyme(s). After 120 minutes, the alpha-subunit ceased accumulating and thereafter remained at a constant level (i.e. steady state between synthesis and degradation). From pulsechase experiments, using (35)SO(4) and immunochemical procedures, the rate of synthesis of the alpha-subunit was shown to be greater than the beta-subunit during the first 80 minutes of induction. The alpha- and beta-subunits had different rates of degradation during the induction period (t((1/2)) = 50 versus 150 minutes, respectively) and during the deinduction period (t((1/2)) = 5 versus 13.5 minutes) after removal of ammonium from the culture. During deinduction, total NADP-GDH activity decreased with a half-time of 9 minutes. Cycloheximide completely inhibited the synthesis and degradation of both subunits. A model for regulation of expression of the NADP-GDH gene was proposed.

Purification and Partial Kinetic and Physical Characterization of Two Chloroplast-Localized NADP-Specific Glutamate Dehydrogenase Isoenzymes and Their Preferential Accumulation in Chlorella sorokiniana Cells Cultured at Low or High Ammonium Levels

Two ammonium-inducible, chloroplast-localized NADP-specific glutamate dehydrogenase isoenzymes were purified to homogeneity from Chlorella sorokiniana. These isoenzymes were homopolymers of either alpha- or beta-subunits with molecular weights of 55,500 or 53,000, respectively. The alpha-isoenzyme was preferentially induced at low ammonium concentrations (2 millimolar or lower), whereas only the beta-isoenzyme accumulated after cells were fully induced (120 minutes) at high ammonium concentrations (29 millimolar). Purification of isoenzymes was achieved by (NH(4))(2)SO(4) fractionation, gel-filtration, anion-exchange fast protein liquid chromatography, and affinity chromatography. The alpha- and beta-isoenzymes were separated by their differential binding to Type 4 nicotinamide adenine dinucleotide phosphate-Sepharose. Both isoenzymes bound to an antibody affinity column to which purified antibody (prepared against beta-isoenzyme) was covalently attached. Peptide mapping of the subunits showed them to have a high degree of sequence homology. Both subunits were synthesized in vitro from precursor protein(s) with a molecular weight of 58,500. Although the subunits have similar chemical, physical, and antigenic properties, their holoenzymes have strikingly different ammonium K(m) values. The ammonium K(m) of the beta-isoenzyme remained constant at approximately 75 millimolar, whereas this K(m) of the alpha-isoenzyme ranged from 0.02 to 3.5 millimolar, depending upon nicotinamide adenine dinucleotide phosphate concentration.

Effect of Different Carbon Sources on the Ammonium Induction of Different Forms of NADP-Specific Glutamate Dehydrogenase in Chlorella sorokiniana Cells Cultured in the Light and Dark

The ammonium induction of the chloroplast-localized NADP-specific glutamate dehydrogenase (NADP-GDH) was shown not to be a light-dependent process per se in Chlorella sorokiniana. In the dark without exogenous organic substrates, the cells synthesized low levels of fully active NADP-GDH, provided endogenous starch reserves had not been depleted. When cells were supplied with exogenous acetate, the rate of induction of NADP-GDH activity per milliliter of culture in the dark was equal to or slightly greater than the rate observed under photosynthetic conditions without an organic carbon source. Glucose supported only a low rate of induction of NADP-GDH activity in the dark. Both acetate and glucose inhibited induction of enzyme activity in the light. The NADP-GDH holoenzyme had at least 7 different electrophoretic forms. These forms differed in net charge and/or molecular weight. Their difference in molecular weight was due to the presence of 2 subunits with similar antigenic properties but different molecular weights (M(r) = 55,500 and 53,000; alpha-and beta-subunits, respectively). Depending upon the cultural conditions and length of the induction period, a wide variation was observed in the alpha:beta subunit ratio and in the numbers and sizes of the NADP-GDH holoenzymes.

Specific Polysome Immunoadsorption to Purify an Ammonium-Inducible Glutamate Dehydrogenase mRNA from Chlorella sorokiniana and Synthesis of Full Length Double-Stranded cDNA from the Purified mRNA

A specific polysome immunoadsorption procedure, employing soluble rabbit anti-NADP-GDH IgG and sheep anti-rabbit IgG covalently-linked to an insoluble cellulose matrix, was used to immunoselect polysomes translating mRNA for a chloroplastic ammonium-inducible NADP-GDH in fully induced cells of Chlorella sorokiniana. The immunoselected polysomes were dissociated, and the NADP-GDH mRNA was recovered by oligo (dT)cellulose chromatography. The translatable NADP-GDH mRNA was estimated to be 0.07 and 90% of the total polysomal poly(A)(+)RNA before and after immunoselection of the polysomes, respectively. The immunoadsorption procedure resulted in an 83% recovery and 1,291-fold purification of translatable NADP-GDH mRNA. In vitro translation of the immunoselected poly(A)(+)RNA yielded a single radioactive protein (on sodium dodecyl sufate polyacrylamide gels) with a molecular weight of 58,500, i.e. size of the putative precursor-protein of the NADP-GDH subunit in the holoenzyme in fully induced cells. The purified NADP-GDH mRNA was used for synthesis of a high proportion of nearly full-length single-stranded cDNA and double-stranded cDNA molecules.

Evidence for Chloroplastic Localization of an Ammonium-Inducible Glutamate Dehydrogenase and Synthesis of Its Subunit from a Cytosolic Precursor-Protein in Chlorella sorokiniana

Chlorella sorokiniana cells, cultured for 12 hours in 30 millimolar ammonium medium, contained an ammonium inducible nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase (NADP-GDH) isoenzyme with subunits having a molecular weight of 53,000. In vitro translation of total cellular poly(A)(+) RNA, isolated from fully induced cells, resulted in synthesis of an NADP-GDH antigen with a molecular weight of 58,500. The 58,500 dalton antigen was processed in vitro, with a 100,000g supernatant prepared from broken fully induced Chlorella cells, to a protein with a molecular weight of 53,000. These data support the inference that the NADP-GDH subunit (M(r) = 53,000) is initially synthesized as a larger precursor protein (M(r) = 58,500). By use of a cytochemical staining procedure, dependent upon NADP-GDH catalytic activity, the holoenzyme was shown to be chloroplast-localized. An immunoelectron microscopy procedure, employing anti-NADP-GDH immunoglobulin G and Protein A-gold complex, showed that NADP-GDH antigen was absent from the nucleus but present in both the chloroplast and cytosol. Since synthesis of the enzyme can be inhibited by cycloheximide, the detection of NADP-GDH antigen in the cytosol was probably due to binding of the NADP-GDH antibody to nascent polypeptide chains of the precursor-protein being synthesized on cytosolic 80S ribosomes.

Comparison of Patterns of Accumulation of Ribulose Bisphosphate Carboxylase Antigen and Catalytic Activity and Measurement of Antigen Half-Life during the Cell Cycle of Chlorella sorokiniana

By use of specific immunochemical procedures, ribulose-1,5-bisphosphate carboxylase (RuBPCase), antigen and catalytic activity were shown to have coincident step-patterns of accumulation during the cell cycle of Chlorella sorokiniana. Pulse-chase studies, employing radioactive sulfate, were performed during the period of rapid accumulation of enzyme activity and during the period of constant enzyme activity in the cell cycle. No degradation of RuBPCase antigen could be detected during either of these cell cycle periods. Thus, the step-pattern of accumulation of RuBPCase activity resulted from periodic synthesis of an enzyme that was stable under steady-state cell cycle conditions. Although inhibition of protein synthesis by cycloheximide, at different times in the cell cycle in the light, resulted in rapid decay of RuBPCase activity, this loss in activity occurred without detectable loss in enzyme antigen. When synchronous cells were placed into the dark, to slow the rate of protein synthesis in the absence of cycloheximide, the levels of enzyme antigen and activity decreased by 30 and 50%, respectively, during the 10-hour dark period. Thus, in C. sorokiniana changes in RuBPCase activity do not necessarily reflect parallel changes in enzyme antigen, particularly when cell growth is perturbed by changes from steady-state cultural conditions.

Light requirement for induction and continuous accumulation of an ammonium-inducible NADP-specific glutamate dehydrogenase in chlorella

The ammonium-inducible NADP-specific glutamate dehydrogenase of Chlorella sorokiniana was shown to require light for both its induction by ammonia in uninduced cells, and its continuous accumulation in fully induced cells. Addition of ammonia to uninduced cells in the light resulted in a 35-minute induction lag followed by linear and coincident increases in enzyme activity and antigen. Enzyme activity was not induced in the dark; however, transfer of these cells to the light resulted in an immediate increase in enzyme activity and antigen. The absence of an induction lag suggested that mRNA sequences and/or an enzyme precursor with different antigenic properties than the active holoenzyme accumulated in cells in the dark in ammonium medium. When fully induced cells were transferred to the dark, the activity of the enzyme quickly ceased to accumulate. In contrast to the NADP-specific isozyme, the cells also contain a constitutive NAD-specific isozyme which was shown to accumulate in cells in the dark in either ammonium or nitrate medium.

Evidence for messenger ribonucleic acid of an ammonium-inducible glutamate dehydrogenase and synthesis, covalent modification, and degradation of enzyme subunits in uninduced Chlorella sorokiniana cells

The cells of Chlorella sorokiniana cultured in nitrate medium contain no detectable catalytic activity of an ammonium-inducible nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase (NADP-GDH). However, several lines of experimental evidence indicated that the NADP-GDH messenger ribonucleic acid was present at high levels and was being translated in uninduced cells. First, binding studies with 125I-labeled anti-NADP-GDH immunoglobulin G and total polysomes isolated from uninduced and induced cells showed that NADP-GDH subunits were being synthesized on polysomes from both types of cells. Second, when polyadenylic acid-containing ribonucleic acid was extracted from polysomes from uninduced and induced cells and placed into a messenger ribonucleic acid-dependent in vitro translation system, NADP-GDH subunits were synthesized from the ribonucleic acid from both sources. Third, when ammonia was added to uninduced cells, NADP-GDH antigen accumulated without an apparent induction lag. Fourth, by use of a specific immunoprecipitation procedure coupled to pulse-chase studies with [35S]sulfate, it was shown that the NADP-GDH subunits are rapidly synthesized, covalently modified, and then degraded in uninduced cells.

Regulation of accumulation of ammonium-inducible glutamate dehydrogenase catalytic activity and antigen during the cell cycle of fully induced, synchronous Chlorella sorokiniana cells

By use of a rocket immunoelectrophoresis-activity stain procedure, it was shown that catalytic activity of an ammonium-inducible nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase (NADP-GDH) was accompanied by a coincident increase in enzyme antigen during the cell cycle of preinduced synchronous Chlorella sorokiniana cells growing in the continuous presence of ammonia. Between the fourth and fifth hours of the G-1 phase of the cell cycle, a three- to fourfold increase in linear accumulation of enzyme antigen was observed. Pulse-chase studies with [35S]sulfate, coupled with a specific indirect immunoadsorption procedure for enzyme antigen, showed that NADP-GDH antigen undergoes continuous degradation (i.e., a half-life of 88 to 110 min) during its linear pattern of accumulation during the cell cycle. The apparent half-life of the enzyme increased by approximately 23% of the 4.5-h positive rate change in antigen accumulation during the cell cycle. This increase in half-life is insufficient in itself to account for the large change in rate of NADP-GDH antigen accumulation. The data from immunoelectrophoresis, pulse-chase, and initial 35S incorporation rate experiments taken together support the inference that changes in the rate of NADP-GDH synthesis are primarily responsible for the accumulation patterns of NADP-GDH activity during the C. sorokiniana cell cycle.

Turnover of ammonium-inducible glutamate dehydrogenase during induction and its rapid inactivation after removal of inducer from Chlorella sorokiniana cells

When ammonia was removed from Chlorella sorokiniana cells, which contain an ammonium-inducible nicotinamide adenine dinucleotide phosphate-specific glutamate dehydrogenase (NADP-GDH), the activity of this enzyme decayed with a half-life of approximately 8 min. By use of rocket immunoelectrophoresis, indirect immunoprecipitation, and indirect immunoadsorption (coupled with pulse-chase experiments with 35S-labeled sulfate), the rapid initial loss in activity was shown to be due to enzyme inactivation rather than degradation of NADP-GDH antigen. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of immunoprecipitates obtained with anti-NADP-GDH immunoglobulin G showed that enzyme inactivation is accompanied by the conversion of enzyme subunits (Mr = 59,000) to a protein with a molecular weight of 118,000. Because this protein was stable during boiling and in the presence of sodium dodecyl sulfate and high concentrations of mercaptoethanol or dithiothreitol, it was tentatively assumed to be a covalently linked dimer of enzyme subunits. Pulse-chase experiments showed that total NADP-GDH antigen was subject to rapid degradation (t 1/2 = 88 min) in induced cells, and the same degradation rate was maintained after removal of ammonia from induced cells.