Effects of α-amanitin on coordination of two mRNAs of ribulose-bisphosphate carboxylase in greening pea leaves

Switch from heterotrophy to autotrophy of apple cotyledons depends on NO signal

NO accelerates transition of germinated embryos from heterotrophy to autotrophy by stimulation of chloroplasts maturation. NO-mediated autotrophy of apple seedlings correlates to increased content of RuBisCO small subunit and improvement of parameters of chlorophyll a fluorescence. Nitric oxide (NO) acts as signaling molecule involved in regulation of various physiological processes in plants, although its involvement in cotyledons greening is poorly recognized. To identify the importance of NO signal for plant growth and development we investigated the effects of short-term application of NO at various developmental stages of seedlings of apple (Malus domestica Borkh.) on cotyledons' chlorophyll a to b ratio, chlorophyll a fluorescence, photosynthetic activity, carbohydrates and RuBisCO both subunits content. NO-dependent biochemical alterations were linked to cytological observation of developing plastids in cotyledons of apple plants. Abnormal plantlets developing from dormant apple embryos are characterized by anatomical malformations of cotyledons. Short-term pre-treatment with NO of isolated embryos or seedlings with developmental anomalies resulted in formation of plants with cotyledons of equal size and chlorophyll content; these responses were blocked by NO scavenger. NO independently of time point of application accelerated embryos transition from heterotrophy to autotrophy by stimulation of photosynthetic activity, improvement of parameters of chlorophyll a fluorescence (F v/F m, F v/F 0) and increased content of RuBisCO small subunit. Further analysis showed that NO application modified glucose and hydrogen peroxide concentration in cotyledons. Beneficial effect of NO on development of seedlings without any abnormalities was manifested at ultrastructural level by decline in amount of proplastids and induction of formation and maturation of chloroplasts. Our data suggest that progress of autotrophy of young seedlings is governed by NO acting as stimulator of chloroplast-to-nucleus signaling.

Multiple coordinate controls contribute to a balanced expression of ribulose-1,5-bisphosphate carboxylase/oxygenase subunits in rye leaves

In the leaves of rye (Secale cereale L.), control mechanisms acting at multiple molecular levels contribute to a coordinate expression of the subunit polypeptides of ribulose-1,5-bisphosphate carboxylase. The relevance and hierarchy of the different control steps were evaluated by comparing the time courses of changes in levels of translatable mRNA, rates of in vivo amino acid incorporation, and the turnover of subunit polypeptides after selective interference with translation at either cytoplasmic 80S ribosomes, or at the 70S ribosomes of the chloroplast, by compartment-specific inhibitors, or by the use of 70S-ribosome-deficient leaves. The latter were generated by growing the plants at a non-permissive elevated temperature of 32 degrees C. The rates of synthesis of the two ribulose-1,5-bisphosphate carboxylase subunits were most rapidly adapted to each other by translational controls. Within 0.5-2.5 h after selective inhibition of the synthesis of either subunit, that of the other subunit made in the unaffected compartment also declined by more than 90% without any marked change in its mRNA. After prolonged inhibition (24 h) of either cytoplasmic or chloroplast protein synthesis, the levels of mRNAs for both subunits were greatly diminished. In rye, the mRNA levels for both subunits changed under all experimental conditions tested in a closely parallel manner and appeared to be always maintained in a balanced, fairly constant ratio by strong coordinate controls. Even 70S-ribosome-deficient leaves contained mRNAs for both the small and the large subunits, although only in small amounts. The mRNAs for both subunits were also markedly further decreased in 70S-ribosome-deficient leaves after application of an inhibitor of cytoplasmic translation. MDMP [2-(4-methyl-2,6-dinitroanilino)-N-methylpropionamide], suggesting that the suppression of the large subunit mRNA in the plastids was not mediated through feedback effects of accumulating unassembled large subunits. Coordinate controls at both the mRNA and the translational level require a bidirectional exchange of regulatory signals between chloroplast and cytoplasm. However, these controls were not absolutely restrictive and allowed low rates of uncoupled synthesis of either large or small subunits. Large subunits made in the presence of MDMP were stable over 24 h. However, unassembled small subunits synthesized in 70S-ribosome-deficient leaves were degraded with a half-time of 10.5 h, in contrast to their behavior after integration into the holoprotein in normal leaves, where no turnover was detected. The proteolytic removal of surplus free small subunits is regarded as a final post-translational fine-tuning step to establish a balanced subunit stoichiometry in leaves.

Assembly of Rubisco from native subunits

Large subunits of ribulosebisphosphate carboxylase/oxygenase (Rubisco) (3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39) from prokaryotic sources can assemble into intact enzyme either in vitro or in Escherichia coli cells. Large subunits of higher plant Rubisco do not assemble into Rubisco in E. coli cells, nor is it possible to reconstitute higher plant Rubisco from its dissociated subunits in vitro. This behavior represents an obstacle to any practical attempts at engineering the higher plant enzyme, and it suggests that the in vivo assembly mechanism of higher plant Rubisco must be more complex than is commonly expected for oligomeric proteins of organelles. In pea chloroplasts, a binding protein interacts with newly synthesized large subunits, in quantities expected for an intermediate in the assembly process, as judged by Western blotting. Radiotracer-labeled large subunits which interact with this binding protein can be shown to assemble into Rubisco in reactions which lead to changes in the aggregation state of the binding protein. Antibody to this binding protein specifically inhibits the assembly of these subunits into Rubisco. Rubisco synthesis appears to be subject to many types of control: gene dosage, transcription rate, selective translation of message, post-translational degradation and threshold concentration effects have been observed in various organisms' synthesis of Rubisco. The biochemical mechanisms underlying most of these effects have not been elucidated. The post-translational assembly mechanism in particular appears to require further study.

Nuclear-organelle interactions: nuclear antisense gene inhibits ribulose bisphosphate carboxylase enzyme levels in transformed tobacco plants

The biosynthesis of ribulose bisphosphate carboxylase (RUBISCO) provides a model system for studying the coordination of nuclear and organelle gene expression, since this abundantly transcribed and expressed chloroplast enzyme is composed of small (SS) and large subunits (LS) encoded by a nuclear multigene family and a single chloroplast gene, respectively. We have tested the possibility that SS mRNA or protein levels affect LS mRNA amounts or LS protein production and accumulation. We find that expression of antisense DNA sequences for the SS in transgenic tobacco plants drastically reduces the accumulation of SS mRNA and SS protein. These changes are accompanied by corresponding reductions of LS protein but not LS mRNA amounts; accumulation of the LS protein appears to be regulated by translational and posttranslational factors. We also find that the transgenic plants display striking variations in growth that are correlated with antisense gene dosage.

Effects of specific inhibitors on the coordination of the concentrations of ribulose-bisphosphate-carboxylase subunits and their corresponding mRNAs in the alga Chlorogonium

Investigations were carried out on the effects of inhibitors of transcription and translation on the concentrations of the subunits of the plastid enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBPCase) and their corresponding mRNAs in the unicellular green alga Chlorogonium elongatum Dangeard. The light-induced increase of nuclear-coded small-subunit mRNA was strongly inhibited by α-amanitin while the increase of plastid-coded large-subunit mRNA was only weakly affected: Consequently, the mRNAs of the two subunits were present in very different proportions. Nevertheless, the light-induced increase of both subunits was strongly reduced by α-amanitin to the same degree, and hence the ratio of their concentrations was not affected compared with the untreated control cells. The effect of cycloheximide on the subunit mRNAs was similar to but weaker than that of α-amanitin. Again the increases in the subunit levels were strongly inhibited to the same degree. By contrast, rifampicin and chloramphenicol inhibited the light-induced increase of large-subunit mRNA more strongly than that of small-subunit mRNA, but the differences were less distinct than those caused by α-amanitin and cycloheximide. Again, the increase in both subunits was inhibited almost to the same extent. These results - especially those of the α-amanitin experiments - clearly show that the fine coordination of the RuBPCase subunits occurs posttranscriptionally at the level of translation and-or degradation. This conclusion was confirmed by pulse-chase experiments. Inhibition of the synthesis of the large subunits by chloramphenicol resulted - as also found by other authors-in a degradation of excess small subunits in the plastid. On the other hand, inhibition of the concentration of small subunits caused a proportionate reduction in the synthesis of large subunits, but no rapid degradation of large subunits could be detected. Therefore, the fine coordination of both subunits of RuBPCase is achieved by the degradation of an excess of small subunits, while the level of large subunits is adapted to the small subunit concentration, probably by adjustment of translation of the large-subunit mRNA. Furthermore, our experiments with α-amanitin and cycloheximide allow us to conclude that in the blue-light induction of large-subunit mRNA in the plastid the nucleocytoplasmic compartment is not directly involved.

Regulation of gene expression of ribulose bisphosphate carboxylase in greening pea leaves

Ribulose bisphosphate carboxylase/oxygenase (RuBisCO) is composed of two subunits, the chloroplastcoded large subunit (LS) and the nuclear-coded small subunit (SS). The effects of different light doses on the levels of the large subunit gene (rbcL), the rbcL and the rbcS mRNAs, and the rate of synthesis of RuBisCO were followed during greening. The rbcL gene dosage changed in response to light whereas the level of the rbcL mRNA changed independently of the gene dosage. This suggests that the expression of the rbcL gene is transcriptionally regulated and that the change in gene dosage only partially contributes to the increase in the mRNA. It appeared that the RuBisCO synthesis rate was proportional to the rbcS mRNA level rather than rbcL mRNA level. These results, taken together with the earlier observations of many researchers, suggest that RuBisCO synthesis in greening pea leaves is controlled primarily at the level of transcription of both genes and is fine-tuned at the post-transcriptional level in chloroplasts, so that the amount of LS is almost stoichiometric to that of SS.