Protein synthesis, in chloroplast

Barraclough BR, Ellis RJ. Protein synthesis in chloroplasts. IX. Assembly of newly synthesized large subunits of ribulose bis-phosphate carboxylase in isolated intact chloroplasts. Biochim. Biophys. Acta 1980 608 19-31. 

Transcription and protein synthesis in the chloroplast can be inhibited by antibiotics because chloroplast ribosomes are similar in structure to those of bacteria. Chloroplast ribosomes are 70S in size, comprising a large 50S subunit (containing... 

DonnerB, Helmboldt-CaesarU, ResingL. Ciradianrhythmoftotal protein synthesis in the cytoplasm and chloroplasts of Gonyaulaxpolyedra. Chronobiolnt 1985 2 1-10. 

The enzyme responsible for ATP synthesis in chloroplasts is a large complex with two functional components, CF0 and CFi (C denoting its location in chloroplasts). CF0 is a transmembrane proton pore composed of several integral membrane proteins and is homologous to mitochondrial F0. CFi is a peripheral... 

The individual steps involved in the conversion of glutamate into ALA are shown in Scheme 6 [1,2,13]. First the glutamate is converted by an ATP-depen-dent ligase into a RNA ester 18, which appears to be the same as the gluta-myl- RNA used for protein synthesis in the plant chloroplast. This gluta-rnyl- RNA is then reduced by an NADPH-dependent reductase to glutamate 1-semialdehyde 19. As expected of an a-amino aldehyde, 19 is not particularly stable under neutral or basic conditions but can be isolated under acidic conditions, under which it cyclises to the corresponding lactol [2]. 

The answer is c. (Murray, pp 452—467. Scriver, pp 3—45. Sack, pp 1—40. Wilson, pp 101-120.) Prokaryotic ribosomes have a sedimentation coefficient of 70S and are composed of SOS and 30S subunits. Eukaryotic cytoplasmic ribosomes, either free or bound to the endoplasmic reticulum, are larger—60S and 40S subunits that associate to an SOS ribosome. Nuclear ribosomes are attached to the endoplasmic reticulum of the nuclear membrane. Ribosomes in chloroplasts and mitochondria of eukaryotic cells are more similar to prokaryotic ribosomes than to eukaryotic cytosolic ribosomes. Like bacterial ribosomes, chloroplast and mitochondrial ribosomes use a formylated tRNA. In addition, they are sensitive to many of the inhibitors of protein synthesis in bacteria. 

Since protein synthesis in eukaryotes occurs both in the cytoplasm and in certain cellular organelles such as mitochondria (and chloroplasts in the case of plants), the mechanisms of cytoplasmic protein synthesis are described first, followed by a discussion of the similarities and differences in protein synthesis in the organelles, particularly the mitochondria. 

This protein is product of chloroplast psbA gene, so called photogene . It is the main product of chloroplast protein synthesis in light and its mRNA is the most abundant in the chloroplasts. 

Effect of temperature on protein Synthesis by isolated Chloroplasts Intact chloroplasts of V. Sinensis mediated protein synthesis in a light-dependent reaction without the requirement for ATP. No protein synthesis was seen in the dark. Fig.l shows the fluorographic profile of the proteins synthesized at various temperatures. The general profile of the polypeptides synthesized was not altered very much by the change in incubation temperature. However, at 35 C and above, there was a distinct increase in the synthesis and/or accumulation of four polypeptides in the molecular sizes of 85,70, 60 and 23 kDa. All four polypeptides could be observed, at least as faint bands, even at 25 C. The synthesis of HSPs at 40 C in the isolated chloroplasts were completely inihibited by chloramphenicol but not by cycloheximide. The transcriptional inihibitors like actinomycin-D and rifamycin were also inihibitory to the HSP synthesis (Fig 2). 

In vitro Protein Synthesis in the Chloroplasts isolated from the... 

FIGURE 10 Electron transport and ATP synthesis in chloroplasts. The jagged arrows represent light striking the two photosystems (PS I and PS II) in the thylakoid membrane. Other members of the electron transport chain shown are a quinone (Q), the cytochrome complex (heO plastocyanin (PC), and an iron-sulfur protein (FeS). The chloroplast ATP synthase is shown making ATP at the expense of the electrochemical proton gradient generated by electron transport. 

Chloroplasts are also capable of protein synthesis. In Chapter 8, elegant evidence is presented showing that one of the proteins made in chloroplasts is the large subunit of ribulose-1,5-diphosphate carboxylase. The... 

Fig. 37. Schematic representation of the processes of chlorophyll and protein synthesis in the cytoplasm and chloroplast of C. reinhardi leading to the formation of functional chloroplast membranes.

Membrane proteins (I) (peptides a, b, c in Hoober s nomenclature L-protein, or peptides Va, Vb are synthesized by cytoplasmic 80 S ribosomes (II) and coded for by nuclear DNA (III). The transcription process can be inhibited by actinomycin D, but not by rifampicin. The translation process can occur in the dark and is inhibited by cycloheximide (II). The proteins are transferred to the chloroplast and become incorporated into chloroplast membrane (IV), which as such, are photosynthetically inactive. Activation proteins " synthesized in the chloroplast (V), coded for by chlorplast—as well as nuclear DNA—and translated on 70 S ribosomes inhibited by chloramphenicol are added to the inactive membrane which becomes photosynthetically functional (VI). The activation proteins include components which are essential for the activity of the photosystem II and I reaction centers and can be made and inserted into the growing membrane in absence of concomitant chlorophyll synthesis or protein synthesis in the cytoplasm, provided that the inactive membranes containing the cytoplasmic proteins are already formed. ... 

Information is accumulating concerning the mechanism of initiation of protein synthesis. In bacterial, mitochondrial and chloroplast systems the initial amino acid in a polypeptide is always formyl-methionine—the formyl residue being removed from the completed... 

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