Chloroplasts ribosomes

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... 

SN. Newman, TE. Boyton, NW. Gillham, BL. Randolph-Anderson, AM. Johnson, EH. Harris (1990) Transformation of chloroplast ribosomal RNA genes in Chlamydomonas molecular and genetic characterization of integration events. Genetics, 126 875-898... 

Chang has made observations on the polysomes of pinto bean leaves exposed to ozone (at 0.35 ppm for 20-50 min). He found that the chloroplast polysomes were more susceptible to oxidation than was the cytoplasmic ribosomes. The sulfhydryl content of the chloroplast ribosomes was also much more susceptible to oxidation than was that of the cytoplasmic ribosomes. Finally, it was found that the effects of ozone on ribosome composition could be reproduced by p-mer-curicbenzoate. Chang s results imply that either ozone itself or a product of ozone oxidation passes from the cytoplasmic membrane to the interior of the chloroplast before having its effect. These results connect with a number of papers on the oxidation of sulfhydryl compounds by ozone. Tomlinson and Rich have reported decreases in leaf sulfhydryl groups after ozone exposure (at 1 ppm for 30-60 min). 

Prokaryotic ribosomes have a similar structure, but are somewhat smaller than those of eukaryotes (sedimentation coef cient 70 S for the complete ribosome, 30 S and 50 S for the subunits). Mitochondrial and chloroplast ribosomes are comparable to prokaryotic ones. 

The ribosomes of eukaryotic cells (other than mitochondrial and chloroplast ribosomes) are larger and more complex than bacterial ribosomes (Fig. 27-9d), with a diameter of about 23 nm and a sedimentation coefficient of about 80S. They also have two subunits, which vary in size among species but on average are 60S... 

Chloroplast ribosomes more sensitive than cytoplasmic 77... 

Five of the eight subunits of photosystem II reaction center are synthesized on chloroplast ribosomes. The three Tris-soluble subunits are synthesized on cytoplasmic ribosomes as larger precursors and are transported into the chloroplast by a vectorial processing mechanism [29, 119-121,133]. The genes coding for all the... 

Two or three of the smaller polypeptides of the PS I complex may be encoded in chloroplast DNA, on the basis of their reported synthesis on chloroplast ribosomes, but the genes have not yet been located. The remaining polypeptides of the complex, including the LHC I polypeptides, are presumed to be encoded in nuclear DNA, but none of the genes for these components has been isolated. 

The sites of synthesis of the smaller polypeptides of the core complex are not clearly established. Several methods, including the use of specific inhibitors of protein synthesis and synthesis in isolated chloroplasts, have suggested that two of the polypeptides are synthesized on chloroplast ribosomes. Experiments examining the effect of chloramphenicol and cycloheximide on the labelling of PS I polypeptides have suggested that in Spirodela two polypeptides of 12 and 8 kDa are synthesized on chloroplast ribosomes [121], whereas in pea a polypeptide of about 6 kDa was labelled in the presence of cycloheximide but not chloramphenicol [136]. In both plants the 66 kDa polypeptides were synthesized in the presence of cycloheximide but not chloramphenicol, indicating their synthesis on chloroplast ribosomes [121,136]. Incorporation of labelled amino acids into PS I polypeptides in isolated chloroplasts has indicated that one or two polypeptides, in addition to the 66 kDa polypeptides [137], are synthesized on chloroplast ribosomes. In pea chloroplasts polypeptides of 15 kDa [136] and 17 and 11 kDa [138] have been reported to be labelled, and in wheat a polypeptide of 15 kDa has been reported to be labelled in isolated etiochloroplasts [139]. The different electrophoresis systems used preclude any direct comparison of the sizes of these polypeptides, but the conclusion must be that two, or more, of the smaller polypeptides of the PS I complex are synthesized on chloroplast ribosomes. 

The chloroplast genes for the CFq subunits IV, III and I and for the a subunit of CFj are co-transcribed to give a primary transcript which is then processed to remove the intron in atp and to generate a number of smaller transcripts [148]. Nothing is known of the mechanisms of these processing events. The chloroplast genes for the /3 and e subunits of CFx are also co-transcribed [151,152]. The transcripts are translated on chloroplast ribosomes, as indicated by the sensitivity of... 

There are at least four known actions of pyridazines on plants involving a one or more of the following effects (a) inhibition of the Hill reaction (photolysis of water during the photosynthesis), (b) inhibition of pigment formation (chlorophylls, carotenoids), (c) change in the linolenic/linoleic acid ratio, and (d) influence on the chloroplast ribosomes. In addition, pyridazines may be used in plant protection as biocides. 

Fig. 5. Secondary structure of the E. coli 16S rRNA region involved in streptomycin sensitivity. The encircled C at position 912 is mutated to U in streptomycin-resistant chloroplast ribosomes and in archaeal ribosomes, with the exception of those of Desulfuwcoccus mobilis.

In eukaryotic cells, a ribosome remains free in the cytoplasm unless it is directed to the endoplasmic reticulum (ER), the extensive membrane system that comprises about half the total membrane of a cell. The region that binds ribosomes is called the rough ER because of its studded appearance, in contra.sl with the smooth ER, which is devoid of ribosomes (Figure 30.28). Free ribosomes synthesize proteins that remain within the cell, either within the cytoplasm or directed to organelles bounded by a double membrane, such as the nucleus, mitochondria and chloroplasts. Ribosomes bound to the ER usually synthesize proteins destined to leave the cell or to at least contact the cell exterior from a position in the cell membrane. These proteins fall into three major classes secretory proteins (proteins exported by the cell), lysosomal proteins, and proteins spanning the plasma membrane. Virtually all integral membrane proteins of the cell, except those located in the membranes of mitochondria and chloroplasts, are formed by ribosomes bound to the ER. 

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). 

Continuous preparative ultracentrifugation was used in the mid-1970s for isolating mitochondria, chloroplasts, ribosomes, or RNA species from cell lysates... 

Nerve axon Thylakoid Visual receptor Mitochondria Chloroplast Ribosomes Conduction of nerve impulse Conversion of light into energy Conversion of light into chemical energy Oxidative phosphorylation Photosynthetic phosphorylation Protein synthesis... 

G1 Galling, G. Incorporation of uridine and of orotate into chloroplast ribosome RNA of Chlorella after treatment with antibiotics. Arch. Mikrobiol., 81, 245-259 (1972) (Germ.)... 

Mitochondrial ribosomes are preferentially associated with the inner mitochondrial membrane and are more diverse than cytoplasmic or chloroplast ribosomes. Particular features include post-transcriptional oUgoadenylation of the 3 ends of both the smaller and larger rRNA components. Except for mitochondrial ribosomes of higher plants, the essential sequence motifs 5S rRNA and 5.8S rRNA are present in the large rRNA component and are not foimd as individual species. 

Feierabend, J. (1982). Inhibition of chloroplast ribosome formation by heat in higher plants. IN "Methods in Chloroplast Molecular Biology" (Edelman, M., Hallick, R.B. and Chua, N.H., eds.), pp. 671-680, Elsevier Biochemical, Amsterdam... 

Membrane Development—Genes for Chloroplast Ribosomal Proteins—The Evolution of Eukaryotism ... 

---