Cylic electron flow (CEF) around Photosystem I in photosynthetic eukaryotes is likely to be necessary to augment ATP production, rapidly- and precisely balancing the plastid ATP/NADPH energy budget to meet the demands of downstream metabolism. Many regulatory aspects of this process are unclear. Here we demonstrate that the higher plant plastid NADH/Fd:plastoquinone reductase (NDH) and proposed PGR5/PGRL1 ferredoxin:plastoquinone reductase (FQR) pathways of CEF are strongly, rapidly and reversibly inhibited in vitro by ATP with Ki values of 670 μM and 240 μM respectively, within the range of physiological changes in ATP concentrations. Control experiments ruled out effects on secondary reactions, e.g. FNR- and cytochrome b6f activity, nonphotochemical quenching of chlorophyll fluorescence etc., supporting the view that ATP is an inhibitor of CEF and its associated pmf generation and subsequent ATP production. The effects are specific to ATP, with the ATP analog AMP-PNP showing little inhibitory effect, and ADP inhibiting only at higher concentrations. For the FQR pathway, inhibition was found to be classically competitive with Fd, and the NDH pathway showing partial competition with Fd. We propose a straightforward model for regulation of CEF in plants in which CEF is activated under conditions when stromal ATP low, but is downregulated as ATP levels build up, allowing for effective ATP homeostasis. The differences in Ki values suggest a two-tiered regulatory system, where the highly efficient proton pumping NDH is activated with moderate decreases in ATP, with the less energetically-efficient FQR pathway being activated under more severe ATP depletion.

中文翻译：

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高等植物叶绿体中腺苷酸状态对光合作用循环电子流动路径的调节。

光合真核生物中光系统I周围的循环电子流（CEF）可能对于增加ATP的产生，快速准确地平衡质体ATP / NADPH能量收支以满足下游代谢的需求很有必要。此过程的许多监管方面尚不清楚。在这里，我们证明，在体外，ATP强烈，快速且可逆地抑制了CEF的较高植物质体NADH / Fd：醌醌还原酶（NDH）和拟议的PGR5 / PGRL1铁氧还蛋白：醌醌还原酶（FQR）途径，Ki值为670μM，在ATP浓度的生理变化范围内分别为240μM。对照实验排除了对次级反应的影响，例如FNR和细胞色素b6f活性，叶绿素荧光的非光化学猝灭等。支持以下观点：ATP是CEF及其相关的pmf生成和随后的ATP生成的抑制剂。这种作用对ATP是特有的，ATP类似物AMP-PNP几乎没有抑制作用，而ADP仅在较高浓度下抑制。对于FQR途径，发现抑制作用与Fd具有经典竞争性，而NDH途径与Fd具有部分竞争作用。我们提出了一种直接调节植物中CEF的模型，其中CEF在基质ATP低的条件下被激活，但随着ATP水平的升高而被下调，从而实现了有效的ATP稳态。Ki值的差异暗示了两级监管体系，其中高效的质子泵浦NDH在ATP适度降低的情况下被激活，