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

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.

Alternative splicing of a precursor-mRNA encoded by the Chlorella sorokiniana NADP-specific glutamate dehydrogenase gene yields mRNAs for precursor proteins of isozyme subunits with different ammonium affinities

Chlorella sorokiniana has seven ammonium-inducible, chloroplastic NADP-specific glutamate dehydrogenase (NADP-GDH) isozymes composed of varying ratios of alpha- and beta-subunits. Southern blot and allele-specific PCR analyses indicate that the C. sorokiniana genome possesses a single 7178 bp nuclear NADP-GDH gene. cDNA cloning and sequencing, 5'-RACE-PCR analysis, and RNase protection analysis identified two NADP-GDH mRNAs that are identical with the exception of a 42 nt sequence located within the 5'-coding region of the longer mRNA. The 42 nt sequence, termed an auxon because it serves as an exon or intron, appears to undergo alternative splicing from the precursor mRNA by a process that is regulated by both nutritional and environmental signals. Depending upon whether the auxon is included or excluded in a mature mRNA, the gene can be considered to consist of 22 or 23 exons, respectively. The 2074 and 2116 nt mRNAs encode precursor proteins of 56,350 and 57,850 Da, respectively. The N-termini of the purified mature alpha- and beta-subunits were sequenced, identifying full-length subunits of 53,501 and 52,342 Da, respectively. The sequences of the subunits are identical except for an 11 amino acid extension at the N-terminus of the alpha-subunit. The alpha-subunit has an additional alpha-helical domain at its N-terminus compared with the beta-subunit. By correlating the abundances of the two mRNAs with the levels (and relative turnover rates) of the alpha- and beta-subunit antigens during induction in Chlorella, the larger mRNA is proposed to encode the larger subunit.

Induction, isolation, and some properties of the NADPH-dependent glutamate dehydrogenase from the nonheterocystous cyanobacterium Phormidium laminosum

The level of the NADPH-dependent glutamate dehydrogenase activity (EC 1.4.1.4) from nitrate-grown cells of the thermophilic non-N2-fixing cyanobacterium Phormidium laminosum OH-1-p.Cl1 could be significantly enhanced by the presence of ammonium or nitrite, as well as by L-methionine-DL-sulfoximine and other sources of organic nitrogen (L-Glu, L-Gln, and methylamine). The enzyme was purified more than 4,400-fold by ultracentrifugation, ion-exchange chromatography, and affinity chromatography, and at 30 degrees C it showed a specific activity of 32.9 mumol of NADPH oxidized per min per mg of protein. The purified enzyme showed no aminotransferase activity and catalyzed the amination of 2-oxoglutarate preferentially to the reverse catabolic reaction. The enzyme was very specific for its substrates 2-oxoglutarate (Km = 1.25 mM) and NADPH (Km = 64 microM), for which hyperbolic kinetics were obtained. However, negative cooperativity (Hill coefficient h = 0.89) and [S]0.5 of 18.2 mM were observed for ammonium. The mechanism of the aminating reaction was of a random type with independent sites. The purified enzyme showed its maximal activity at 60 degrees C (Ea = 5.1 kcal/mol [21.3 kJ/mol]) and optimal pH values of 8.0 and 7.5 when assayed in Tris hydrochloride and potassium phosphate buffers, respectively. The native molecular mass of the enzyme was about 280 kilodaltons. The possible physiological role of the enzyme in ammonia assimilation is discussed.

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.