Chloroplast directed protein synthesis, product

In this paper, we have summarized our current understanding of the biochemical nature of the triazine binding site within the PS II complex. Studies using the proteolytic enzyme trypsin as a selective, surface-specific modifier of membrane polypeptides and the use of a photoaffinity triazine have been utilized separately to identify the triazine receptor protein as a 32-34 kilodalton (kDal) polypeptide of the PS II complex in peas (Pisum sativum L.). The nature of the covalent attachment of the photoaffinity probe has also enabled us to identify the triazine receptor protein as a product of chloroplast-directed protein synthesis this implies that the structural gene for the triazine receptor polypeptide is encoded on chloroplast DNA. 

The Triazine Receptor Protein is a Product of Chloroplast-Directed Protein Synthesis. Triazine resistance has been demonstrated from reciprocal crossing experiments to be inherited uniparentally through the female parent in campestris (16). 

Like Complex III of mitochondria, cytochrome b6f conveys electrons from a reduced quinone—a mobile, lipid-soluble carrier of two electrons (Q in mitochondria, PQb in chloroplasts)—to a water-soluble protein that carries one electron (cytochrome c in mitochondria, plastocyanin in chloroplasts). As in mitochondria, the function of this complex involves a Q cycle (Fig. 19-12) in which electrons pass, one at a time, from PQBH2 to cytochrome bs. This cycle results in the pumping of protons across the membrane in chloroplasts, the direction of proton movement is from the stromal compartment to the thylakoid lumen, up to four protons moving for each pair of electrons. The result is production of a proton gradient across the thylakoid membrane as electrons pass from PSII to PSI. Because the volume of the flattened thylakoid lumen is small, the influx of a small number of protons has a relatively large effect on lumenal pH. The measured difference in pH between the stroma (pH 8) and the thylakoid lumen (pH 5) represents a 1,000-fold difference in proton concentration—a powerful driving force for ATP synthesis. 

The truncated PHYLLO menFS module product showed homology to the N-terminal part of ICS, and hence it was important to establish whether it is functional. Hence, plants containing single and double knockouts of the genes ICSl and ICS2) were tested. It was found that only the double knockout plants showed the expected pha phenotype, demonstrating that ICSl and ICSl overlap in their function for the synthesis of isochorismate required for K. This experiment in addition established that the product of the menFS module is unable to complement K deficiency. The PHYLLO product was found to contain 1715 deduced amino acid residues with the four Men modules and a predicted transit peptide that directs the protein to the chloroplasts. 

S.2 Fatty acyl-CoA transferases. The enzyme systems involved with fatty acyl-CoA utilization in the cytosol appear to be membrane-bound. Consequently, detailed knowledge of their individual structure, specificity and genetic control is generally lacking due to the particular inability to obtain ready isolation and purification of the relevant proteins. Studies, however, support the concept of the operation of the eukaryotic pathway for the production of glycerolipids and polyunsaturated fatty acid (Browse et al., 1990 Stymne et al., 1990). While this pathway may contribute a significant quantity of fatty acid for use in membrane synthesis in the plastid (chloroplast) (Browse et al., 1990), its major importance would seem to lie with the production of unsaturated oils (Frentzen, 1986). On the other hand the occurrence of the prokaryotic pathway in the plastid permits more direct membrane lipid formation in both 16 3 and 18 3 plants (Browse et al., 1990 Somerville and Browse, 1991). Different sets of acyltransferase may be associated with the two pathways (Hills and Murphy, 1991). 

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