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

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.

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.

Purification and Partial Characterization of Ribulose Bisphosphate Carboxylase Holoenzyme and Its Subunits from Chlorella sorokiniana and Use of Its Antigen Affinity-Purified Antibodies in Specific Immunoprecipitation and Immunoadsorption Procedures

Chlorella sorokiniana ribulose-1,5-bisphosphate carboxylase (RuBPCase) was purified to homogeneity with yields of 35 to 40%. Molecular weights of the holoenzyme and its large subunit (LS) and small subunit (SS) were estimated to be 562,000, 55,000, and 15,800, respectively. Amino acid compositions of LS from C. sorokiniana and spinach were similar, whereas the compositions of their SS were very different. Antisera prepared against holoenzyme, LS, and SS were purified by antigen-affinity column chromatography. Purified anti-holoenzyme immunoglobulin G (IgG) and anti-LS IgG cross-reacted with holoenzyme and LS but not with SS. Anti-SS IgG reacted neither with holoenzyme nor with LS. Because purified anti-holoenzyme IgG or the anti-LS IgG inhibited RuBPCase activity, antibody preparations were titered by the amount of (35)S-labeled RuBPCase immunoprecipitated. Approximately 40% of the total RuBPCase activity in cell homogenates was tightly particulate-bound and was solubilized with 0.5% Nonidet P-40 without inhibition of enzyme activity. Direct-immunoprecipitation and indirect-immunoadsorption procedures, with affinity-purified anti-holoenzyme IgG, gave specific and quantitative recovery of (35)S-labeled RuBPCase from cell extracts containing Nonidet P-40. Affinity-purified anti-LS IgG and anti-SS IgG were used to immunoprecipitate either the LS or SS antigens synthesized in vitro in a mRNA-dependent in vitro translation assay system. Rocket immunoelectrophoresis was used to quantify as little as 50 nanograms of RuBPCase antigen in cell extracts.