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Synthesis of PS II

The regulation of the synthesis of the polypeptides of PS II appears to be particularly complex, with evidence for regulation at transcriptional, translational and post-translational levels. The synthesis of the individual polypeptides does not appear to be tightly coordinated, with certain polypeptides accumulating in the absence of other PS II polypeptides under a variety of experimental conditions. The synthesis of a functional PS II complex is strongly dependent on light, both for its effect on the transcription of PS II genes and for its absolute requirement for Chi synthesis. [Pg.327]

Transcripts of cab genes are translated on cytoplasmic polysomes to produce soluble precursor forms of the polypeptides approximately 3-4 kDa larger than the mature form [76]. The precursors bind to the outer envelope membrane of the chloroplasts [77] before being imported into the chloroplasts by an energy-de-pendent process [78]. The mechanism by which the soluble precursors cross the two envelope membranes is not known, but translocation may occur in regions where the two envelope membranes are appressed. After uptake, the transit pep- [Pg.327]

The synthesis of the polypeptides of the core complex takes place exclusively in the chloroplasts, as indicated by synthesis in isolated chloroplasts or by translation of chloroplast RNA in vitro. Regulation of the synthesis appears to operate at transcriptional, translational and post-translational levels, with light playing a major role. [Pg.328]

Cyt b-559, detected spectroscopically, also accumulates in dark-grown plants [93] and its regulation therefore appears to be different from that of the other core polypeptides. The genes, psbE and psbF, for the 9 and 4 kDa polypeptides of Cyt b-559 are co-transcribed [56] and, due to the close spacing of the genes, may also be translationally coupled. This may ensure the synthesis of equal amounts of the two polypeptides, but as yet the stoichiometry of these two polypeptides in PS II preparations has not been established. [Pg.329]

The polypeptides of the water-splitting complex accumulate in dark-grown plants [89,94] and there is no absolute requirement for light for the expression of the nuclear genes. The polypeptides are synthesized on cytoplasmic ribosomes as precursor forms, with an additional 4-7 kDa extension [65]. This additional sequence is necessary for uptake by chloroplasts and presumably for targetting to the lu-menal space of the thylakoid membrane. The primary sequences of the 33, 23 and 16 kDa polypeptides and their transit peptides should soon be forthcoming from analysis of cDNA clones. [Pg.329]


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Synthesis of PS

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