Studies on the synthesis of 'chlorophyll-binding proteins',
which play a major role in the primary reactions of photosynthesis have also provided new information about chloroplast translation and its regulation. In many plant species, in the absence of light the mRNA of chlorophyll binding proteins, including those encoded by chloroplast genome, remain associated with thylakoid bound polysomes, but are not translated. Transfer of seedlings to light induces synthesis of proteins, although no increase in transcription has been observed. This suggests that post-transcriptional processes play a key role in light induced chloroplast gene expression.
It has also been shown that the polycistronic mRNA encoded by the psbB
operon in maize, need not be cleaved into tri-, di- or monocistronic forms for successful translation (psbB
operon = psbB
+ petB + petD
This suggests that plastid ribosomes can bind directly to internal initiation regions for initiation of translation as in prokaryotes. However, RNA processing does take place in chloroplasts and mitochondria, and the transcripts may differ in their translatability.
There are also nuclear genes, whose products are essential for translation of specific genes. Several of these nuclear encoded factors interact with the 5' untranslated region of chloroplast massages, but little is known about the identity and precise function of these factors. These products may be translational activators of the type described in Regulation of Gene Expression 1. Operon Circuits in Bacteria and other Prokaryotes
and Regulation of Gene Expression 3. A Variety of Mechanisms in Eukaryotes
for prokaryotic and eukaryotic genes respectively.
Protein synthesis in isolated chloroplasts of pea was studied by R.J. Ellis of U.K. The proteins which could be synthesized in isolated chloroplasts included (i) large subunit of Fraction I protein, (ii) five unidentified proteins of the internal lamellar system, and (iii) two or three unidentified polypeptides of the envelope. These are only few of a large number of proteins found in chloroplasts. It has also been demonstrated that while the large subunit of Fraction I protein is synthesized under the influence of chloroplast DNA the small subunit is synthesized in the cytoplasm under the influence of nuclear DNA. Small subunit is then transported to the chloroplast where it associates with large subunit to give rise to Fraction I protein (Fig. 34.1.4).
Proteins synthesized in mitochondria have also been identified by inhibiting protein synthesis in cytoplasm by cycloheximide. Although, some proteins are synthesized in mitochondria, not all proteins present in mitochondria are synthesized there. There are some mitochondrial proteins which are synthesized in cytoplasm and then transported. There are still others having dual origin, some polypeptides having cytoplasmic origin and others having mitochondrial origin. Some proteins, which are known to be synthesized in mitochondria include the following : (i) Cytochrome oxidase contains seven different kinds of polypeptides, of which three are synthesized on mitochondrial ribosomes, the remaining four being synthesized on cytoplasmic ribosomes. (ii) Adenosine triphosphatase (ATPase) is an important component of mitochondrial membrane, which plays an important role in coupling respiration to ATP formation. There are other polypeptides present in this complex and only two are definitely known to be synthesized on mitochondrial ribosomes. (iii) In mitochondrial ribosomes, although ribosomal proteins are known to be synthesized outside mitochondria and under the influence of nuclear DNA, but rRNA is transcribed from mtDNA, since both 21S rRNA and 15S rRNA (in yeast mitochondria, 21S rRNA and 15S rRNA occur instead 23S and 16S rRNAs;) hybridize with specific regions of mtDNA.
|Fig. 34.14. A model of co-operation between nuclear DNA and chloroplast DNA in pea as studied by R.J. Ellis and his colleagues (redrawn from Nature Vol. 254, 1975).