The Role of Light-Dark Regulation of the Chloroplast ATP Synthase.

The chloroplast ATP synthase catalyzes the light-driven synthesis of ATP and is activated in the light and inactivated in the dark by redox-modulation through the thioredoxin system. It has been proposed that this down-regulation is important for preventing wasteful hydrolysis of ATP in the dark. To test this proposal, we compared the effects of extended dark exposure in Arabidopsis lines expressing the wild-type and mutant forms of ATP synthase that are redox regulated or constitutively active.

Defects in the Expression of Chloroplast Proteins Leads to HO Accumulation and Activation of Cyclic Electron Flow around Photosystem I.

We describe a new member of the class of mutants in Arabidopsis exhibiting high rates of cyclic electron flow around photosystem I (CEF), a light-driven process that produces ATP but not NADPH. High cyclic electron flow 2 () shows strongly increased CEF activity through the NADPH dehydrogenase complex (NDH), accompanied by increases in thylakoid proton motive force (), activation of the photoprotective q response, and the accumulation of HO. Surprisingly, was mapped to a non-sense mutation in the TADA1 (tRNA adenosine deaminase arginine) locus, coding for a plastid targeted tRNA editing enzyme required for efficient codon recognition.

Limitations to photosynthesis by proton motive force-induced photosystem II photodamage.

The thylakoid proton motive force () generated during photosynthesis is the essential driving force for ATP production; it is also a central regulator of light capture and electron transfer. We investigated the effects of elevated on photosynthesis in a library of mutants with altered rates of thylakoid lumen proton efflux, leading to a range of steady-state extents. We observed the expected dependent alterations in photosynthetic regulation, but also strong effects on the rate of photosystem II (PSII) photodamage.