Ribosome carboxylase

Insulin stimulates the phosphorylation of serine side chains of many proteins, including ATP citratelyase, acetyl-CoA carboxylase, and ribosomal subunit S6. 

Among the proteins found in the chloroplast stroma are the enzymes of the Calvin cycle, which functions in fixing carbon dioxide into carbohydrates during photosynthesis (Chapter 8). The large (L) subunit of ribulose 1,5-bisphosphate carboxylase (rubisco) is encoded by chloroplast DNA and synthesized on chloroplast ribosomes in the stromal space. The small (S) subunit of rubisco and all the other Calvin cycle enzymes are encoded by nuclear genes and transported to chloroplasts after their synthesis in the cytosol. The precursor forms of these stromal proteins contain an N-terminal stromal-import sequence (see Table 16-1). 

The chloroplast genome is similar to that of mitochondria, reflecting its similar bacterial origin. However, the circular chromosome is larger than its mitochondrial counterpart, encoding 30 membrane proteins that are involved in photosynthesis. It also encodes the four components of the bacterial ribosomal system (23S, 16S, 5S, and 4.5S), 20 ribosomal proteins, and 30 tRNAs. Notable among the proteins is one of the subunits of ribulose bisphosphate carboxylase (Rubisco). This enzyme catalyzes the carboxylation of ribulose 1,5-bisphosphate from COj and is responsible for carbon fixation in plants. It is the most abimdant protein on earth. 

The situation depicted in Fig. 21 Ilia is suggested by observations on the location of genes for chloroplast ribosome components and for ribu-lose-diphosphate carboxylase. Here, the nuclear-coded cytoplasmically-synthesized polypeptide combines with an organelle-coded polypeptide and the complex of these components is limited to the organelle due to membrane impermeability. 

One should also emphasize that during the greening of Euglena, as well as of the etiolated higher plants in which proplastids are present, not only photosynthetic membranes are formed, but also most of the soluble enzymes of the chloroplast—such as Ru-1, 5-diP carboxylase, G-3-P dehydrogenase, cytochrome 552, etc., as well as chloroplast ribosomes. 

Translation of mRNA of nuclear origin by chloroplastic ribosomes in C. reinhardi was also demonstrated for soluble chloroplast proteins, such as Ru-1,5 diP-carboxylase, by Armstrong et It was also reported that... 

The block in the synthesis of the cytoplasmic proteins of the chloroplast membrane causes a cessation of the synthesis of most of the other membrane components, including the synthesis of some chloroplast-made membrane proteins. This phenomenon is not yet understood. The plastid present in the dark-grown cells contains ribosomes which are functional, since the plastid continues to produce in the dark proteins such as Ru-1,5-diP carboxylase, cytochrome/, etc. Thus, cessation of synthesis of the chlo-... 

Finally, the fact that RNA, soluble proteins (Ru-l,5-diP carboxylase subunit), and insoluble (membrane and ribosomal proteins) are synthesized in the cytoplasm and become localized in the chloroplast, implies the existence of specific mechanisms for intracellular transport or transfer processes for macromolecules among the different cellular compartments. 

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