Stromal lamella

The localization of PSI and PSII between the granal and stromal lamellae can change and is indirectly controlled by light intensity, optimizing the distribution of excitons between PSI and PSII for efficient energy capture. 

Because Photosystem II tends to occur in the grana and Photosystem I in the stromal lamellae, the intervening components of the electron transport chain need to diffuse in the lamellar membranes to link the two photosystems. We can examine such diffusion using the time-distance relationship derived in Chapter 1 (Eq. 1.6 x je = 4Djtife). In particular, the diffusion coefficient for plastocyanin in a membrane can be about 3 x 10 12 m2 s-1 and about the same in the lumen of the thylakoids, unless diffusion of plastocyanin is physically restricted in the lumen by the appres-sion of the membranes (Haehnel, 1984). For such a D , in 3 x 10-4 s (the time for electron transfer from the Cyt b(f complex to P ), plastocyanin could diffuse about [(4)(3 x 10-12 m2 s-1) (3 x 10-4 s)]1/2 or 60 nm, indicating that this complex in the lamellae probably occurs in relatively close proximity to its electron acceptor, Photosystem I. Plastoquinone is smaller and hence would diffuse more readily than plastocyanin, and a longer time (2 x 10-3 s) is apparently necessary to move electrons from Photosystem II to the Cyt b(f complex hence, these two components can be separated by greater distances than are the Cyt b f complex and Photosystem I. 

For cyclic electron flow, an electron from the reduced form of ferredoxin moves back to the electron transfer chain between Photosystems I and II via the Cyt bCyclic electron flow does not involve Photosystem II, so it can be caused by far-red light absorbed only by Photosystem I — a fact that is often exploited in experimental studies. In particular, when far-red light absorbed by Photosystem I is used, cyclic electron flow can occur but noncyclic does not, so no NADPH is formed and no O2 is evolved (cyclic electron flow can lead to the formation of ATP, as is indicated in Chapter 6, Section 6.3D). When light absorbed by Photosystem II is added to cells exposed to far-red illumination, both CO2 fixation and O2 evolution can proceed, and photosynthetic enhancement is achieved. Treatment of chloroplasts or plant cells with the 02-evolution inhibitor DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethyl urea], which displaces QB from its binding site for electron transfer, also leads to only cyclic electron flow DCMU therefore has many applications in the laboratory and is also an effective herbicide because it markedly inhibits photosynthesis. Cyclic electron flow may be more common in stromal lamellae because they have predominantly Photosystem I activity. 

Precursor Processing. Hie precursor protein is synthesized on stromal lamellae and is processed there to the 32kDa mature form (21). Processing of the precursor is a posttranslational event (12) and takes place at the carboxy terminus (22). Following processing, the mature protein translocates to spatially-distinct chloroplast membranes, the grana, where functional photosystem II reaction centers are mainly located (21, 23). 

Moreover, even in the precursor state, the stromal-lamellae associated protein shows similar trypsin-sensitive domains (P. Goloubinoff, unpublished). Hius, these data raise the possibility that membrane integration and orientation are independent of processing and translocation of the 32kDa protein. 

PSI are most abundant in the unstacked stromal lamellae. In contrast, PSII are located primarily in the stacked regions of thy-lakoid membrane. Cytochrome b6f is found in both areas of thylakoid membrane. The ATP synthase is found only in thylakoid membrane that is directly in contact with the stroma. 

The biogenesis of thylakoid proteins is a complex issue since these proteins are synthesised by two distinct genetic systems. Of particular complexity and interest is the biogenesis of cytoplasmically-synthesised thylakoid lumen proteins, since these proteins must cross three membranes to reach their sites of function. Previous work on one such protein, plastocyanin, indicated that the import of this protein can be divided into two phases. Plastocyanin is initially synthesised as a precursor, which is post-translationally imported across the double-membrane envelope and processed to an intermediate form by a stromal processing peptidase, SPP. The intermediate is then transferred across the thylakoid membrane and processed by a second thylakoidal processing peptidase, TPP (1-3). TPP is an integral thylakoid membrane protein with the active site on the lumenal face of the membrane the enzyme is located exclusively in non-appressed, "stromal" lamellae (4.5). 

UPID MOLECULAR SPECIES COMPOSITI( I OF 6RANAL AND STROMAL LAMELLAE... 

Granal and stromal lamellae fractloned from whole thylakolds of Splrodela were > 98% homogeneous based on electron microscopic examination (data not presented). Figure 1 reveals the distinct polypeptide compositions relative to the whole thylakolds. The granal lamellae are enriched In photosystem 11 (PS 11) polypeptides (rc 11, LHCP, PS 11 extrinsic 33KDa) while stromal lamellae are enriched In photosystem 1 (PS 1) and ATPase (a,B) proteins. 

Stromal Lamellae of Splrodela ollgorrhlza. Values are given as nmol/mg ail. 

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