The ␥ and ⑀ subunits of F 0 F 1 -ATP synthase from photosynthetic organisms display unique properties not found in other organisms. Although the ␥ subunit of both chloroplast and cyanobacterial F 0 F 1 contains an extra amino acid segment whose deletion results in a high ATP hydrolysis activity (Sunamura, E., Konno, H., Imashimizu-Kobayashi, M., Sugano, Y., and Hisabori, T. (2010) Plant Cell Physiol. 51, 855-865), its ⑀ subunit strongly inhibits ATP hydrolysis activity. To understand the physiological significance of these phenomena, we studied mutant strains with (i) a C-terminally truncated ⑀ (⑀ ⌬C ), (ii) ␥ lacking the inserted sequence (␥ ⌬198 -222 ), and (iii) a double mutation of (i) and (ii) in Synechocystis sp. PCC 6803. Although thylakoid membranes from the ⑀ ⌬C strain showed higher ATP hydrolysis and lower ATP synthesis activities than those of the wild type, no significant difference was observed in growth rate and in intracellular ATP level both under light conditions and during light-dark cycles. However, both the ⑀ ⌬C and ␥ ⌬198 -222 and the double mutant strains showed a lower intracellular ATP level and lower cell viability under prolonged dark incubation compared with the wild type. These data suggest that internal inhibition of ATP hydrolysis activity is very important for cyanobacteria that are exposed to prolonged dark adaptation and, in general, for the survival of photosynthetic organisms in an ever-changing environment.

Background: A conformational change of the ␥ subunit of ATP synthase may be critical for enzyme regulation. Results: A conformational change of ␥ controls both ADP inhibition and ⑀ inhibition. Conclusion: The ␥ subunit indirectly regulates the activity by way of ADP inhibition and ⑀ inhibition. Significance: Regulation system of ATP synthase based on the unique molecular structure of the ␥ subunit was revealed.

The F o F 1 synthase produces ATP from ADP and inorganic phosphate. The γ subunit of F o F 1 ATP synthase in photosynthetic organisms, which is the rotor subunit of this enzyme, contains a characteristic β-hairpin structure. This structure is formed from an insertion sequence that has been conserved only in phototrophs. Using recombinant subcomplexes, we previously demonstrated that this region plays an essential role in the regulation of ATP hydrolysis activity, thereby functioning in controlling intracellular ATP levels in response to changes in the light environment. However, the role of this region in ATP synthesis has long remained an open question because its analysis requires the preparation of the whole F o F 1 complex and a transmembrane proton-motive force. In this study, we successfully prepared proteoliposomes containing the entire F o F 1 ATP synthase from a cyanobacterium, Synechocystis sp. PCC 6803, and measured ATP synthesis/hydrolysis and proton-translocating activities. The relatively simple genetic manipulation of Synechocystis enabled the biochemical investigation of the role of the β-hairpin structure of F o F 1 ATP synthase and its activities. We further performed physiological analyses of Synechocystis mutant strains lacking the β-hairpin structure, which provided novel insights into the regulatory mechanisms of F o F 1 ATP synthase in cyanobacteria via the phototroph-specific region of the γ subunit. Our results indicated that this structure critically contributes to ATP synthesis and suppresses ATP hydrolysis.

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