Polypeptides of PS

The Rieske Fe-S protein is synthesized on cytoplasmic ribosomes to give a 27 kDa precursor form [109]. This precursor is presumably required for targetting to the chloroplasts. There is no information on the regulation of expression of the gene for the Rieske Fe-S protein, nor on the assembly of the protein into the Cyt b-f complex. 

The polypeptides of the reaction centre core complex are most easily resolved by electrophoresis in the presence of SDS. This allows the separation of two polypeptides of approximately 66 kDa together with six or more polypeptides of lower relative molecular mass. In pea, these have been reported to be 21,17,16.5,11.5, 11 and 10.5 kDa [119], but the electrophoretic mobility of some of these polypeptides is highly dependent on the electrophoresis conditions, making comparisons between preparations from different laboratories difficult. A preparation containing only two smaller polypeptides of 18 and 15 kDa has been described from barley chloroplast membranes [122]. 

The LHC I complex consists of Chi a and b bound to three or four polypeptides of 22-25 kDa in pea chloroplasts [118]. Variation in the number and size of the LHC I polypeptides in different plants and in different laboratories may be due to differences in electrophoresis procedures. The LHC I polypeptides migrate in SDS-gel electrophoresis in a position similar to that of the LHC II polypeptides, and the reported antigenic similarity between LHC I and LHC II polypeptides [129] may be due to cross-contamination of the LHC complexes. 

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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. 

The regulation of the appearance of the 66 kDa polypeptides may take place at a post-transcriptional step. In maize, transcripts of the two genes, psaA and psaB, are present in dark-grown plants [90], and therefore, if maize does not accumulate 66 kDa polypeptides in the dark, there must be some mechanism to prevent translation of the mRNA or to degrade newly synthesized polypeptides. There is no information on the transcripts for other polypeptides of PS I in dark-grown plants. 

FIGURE 1. Precursor polypeptides of PS I subunits from C. reinhardtii. The protein sequences have been deduced from cDNA sequences. The processing sites between transit peptides and mature proteins ( ) were determined by comparing the open reading frames of cDNAs with the N-terminal sequences of the mature proteins. Hydrophobic domains are underlined. 

Several polypeptide components of PS II and OEC have been isolated from thy-lakoids and PS II preparations capable of O2 evolution, after the initial isolation by Kuwabara and Murata [31] of a 33-34 kDa polypeptide (see, for a review. Ref. 10). On the basis of several criteria, such as the extraction by different reagents and the accessibility to antibodies in thylakoids or in inside-out vesicles prepared from thylakoids, a tentative and certainly incomplete picture has been proposed... 

It has been demonstrated that phosphorylation of LHC causes the detachment of a fraction of it from PS II and its lateral migration in the membrane to become incorporated into PS I [134-136]. It has indeed been shown that the fluorescence quenching caused by LHC phosphorylation is qualitatively different from spillover, because only LHC is quenched, not PS II [136], and Fq as well as are quenched [136,137]. The phosphorylation of LHC and/or of other thylakoid polypeptides may have more complex effects, and their interactions are far from being understood. It has been reported that protein phosphorylation enhances PS I-de-pendent cyclic photophosphorylation even under light saturation conditions [133], which could not be explained merely on the basis of PS I antenna enlargement. 

All species contain two different chlorophylls, BChl a and an additional light-harvesting pigment, either BChl c, d ox e [40], As in Chloroflexus these light-harvesting pigments are housed in chlorosomes located adjacent to the cytoplasmic membrane [45,46]. In Chlorobium BChl c is associated with a 7.5-kDa polypeptide. (J.M. Olson and P. Roepstorff, unpublished). A small amount of energytransferring BChl a is also found in the chlorosomes [47,48], but most of the BChl a is found in a water-soluble protein associated with the cytoplasmic membrane [45]. The RC (see Fig. 4 and Ref. 36) is similar to that of PS I in cyanobacteria and chloroplasts. The primary electron donor P-840 = +0.25 V) is a BChl... 

This review is intended to summarize the PS II/OEC unit in terms of its polypeptide and electron transfer cofactor composition, its electron transfer pathways and its mode of operation in producing oxygen. The photochemical aspects of its operation wil be dealt with only cursorily as these are treated in detail in Chapter 4 of this volume. There has been considerable review activity recently on specific aspects of PS II/OEC function, including articles on polypeptide composition [7-9], manganese function [10-12], electron transfer and Oj-evolving properties [13-18] and the chloride requirement [19]. 

The fourth area of activity in which Mn valence and organization are addressed involves quantitation of the amount of Mn released from the PS II/OEC following perturbation. This method led to the early estimates of manganese stoichiometry [138,139] and has been refined and used to study the effects of a number of PS II inhibitory treatments [112]. With the realization of the role of the peripheral polypeptides in maintaining PS II/OEC integrity, these studies have continued and a clearer picture of the factors which control Mn binding is emerging. 

A number of PS II inhibitor treatments, such as Tris-washing and NH2OH extraction, had been well established as releasing Mn from the OEC [109]. Aker-lund, Andersson and co-workers showed that these treatments also released the peripheral polypeptides [71]. Mathis and co-workers showed that the ns phases in P-680 reduction that occur in untreated preparations are replaced by fxs components in inhibited samples [180]. The predominant decay phase is pH dependent, =2 ijs at pH 8 and =45 /is at pH 5, and is attributed to P-680 reduction by a secondary donor, usually designated D in optical work, which is identical to the EPR-detectable Z species [180,194,195]. The bulk pH and salt concentration dependencies of this phase implicate local membrane pH in influencing its time course... 

PS II [134], Primary structure information is available for linker polypeptides of Synechococcus 6301 [77] and M. laminosus [105]. The complete amino acid sequences of the and the have been established and large N-terminal... 

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