ERRATA 755 limited number of mutually specific cohesive sites implied thereby suggests a specialized biological function, one that remains to be identified. Summary.-The DNA of phage lambda undergoes reversible transitions from linear to characteristically folded molecules, and from linear monomers to open polymers. Some conditions favoring one state or another have been defined. It may be surmised that each molecule carries two specifically interacting cohesive sites.
The currently prevalent concept of the generation of photosynthetic reducing power in oxygen-evolving cells envisions a linear (noncyclic) electron flow from water to ferredoxin (and thence to NADP+) that requires the collaboration of photosystems I and 11 (PSI and PSII) joined by plastoquinone and other electron carriers (the Z scheme). The essence of the Z scheme is that only PSI can reduce ferredoxin-i.e., that, after being energized to an intermediate reducing potential by PSII, electrons from water are transported via plastoquinone to PSI which energizes the electrons to their ultimate reducing potential adequate for the reduction of ferredoxin. Basic to the Z scheme is the function of plastoquinone as the obligatory link in electron transport from PSIl to PSI. However, we have found that, when plastoquinone function was inhibited, ferredoxin was photoreduced by water without the collaboration of PSI. We now report evidence for an important function of plastoquinone in the translocation of protons liberated inside the thylakoid membrane by photooxidation of water. When the oxygenic photoreduction (i.e., by water) of ferredoxin was blocked by plastoquinone inhibitors, dibromothymoquinone or dinitrophenol ether of iodonitrothymol, the photoreduction of ferredoxin was restored by each of four chemically diverse uncouplers, similar only in their ability to facilitate proton movement across membranes. Similar results were obtained for the oxygenic reduction of NADP'. Our results suggest that the light-induced electron flow from water cannot be maintained unless the simultaneously liberated protons are removed from inside the membrane via plastoquinone. The new evidence is embodied in a concept of an oxygenic photosystem for photosynthetic electron and proton transport, which we propose as an alternative to the Z scheme, to account for photoreduction offerredoxin-NADP+ by water and the coupled oxygenic (formerly noncyclic) ATP formation without involving PSI. The role of the anoxygenic photosystem (formerly called PSI) is ATP formation by cyclic photophosphorylation.Photooxidation of water, 2H20-) 4 e-+ 4H + 02, is a key reaction in plant photosynthesis. Aside from its all-important by-product, oxygen, the liberated electrons eventually reduce ferredoxin (1, 2) and then NADP+ (3), thereby accounting for photosynthetic reducing power; the liberated protons contribute to the protonmotive force (4, 5) which accounts for the ("noncyclic") ATP formation that is coupled to NADP+ reduction (6).Photooxidation of water takes place inside the thylakoid membranes (7). These contain two photocenters identified with photosystems I and II (PSI and PSII) and a chain of carriers responsible for the transport ofelectrons and protons within and across the membrane. According to the now-prevalent concepts embodied in the so-called Z scheme (8, 9), PSII photooxidizes water but cannot reduce ferredoxin because it can energize the released electrons only to an intermediate potential; the electrons are transported from PSII t...
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