Energy-dependent exciton quenching, or q(E), protects the higher plant photosynthetic apparatus from photodamage. Initiation of q(E) involves protonation of violaxanthin deepoxidase and PsbS, a component of the photosystem II antenna complex, as a result of lumen acidification driven by photosynthetic electron transfer. It has become clear that the response of q(E) to linear electron flow, termed "q(E) sensitivity," must be modulated in response to fluctuating environmental conditions. Previously, three mechanisms have been proposed to account for q(E) modulation:(i) the sensitivity of q(E) to the lumen pH is altered;(ii) elevated cyclic electron flow around photosystem I increases proton translocation into the lumen; and (iii) lowering the conductivity of the thylakoid ATP synthase to protons (g(H+)) allows formation of a larger steady-state proton motive force (pmf). Kinetic analysis of the electrochromic shift of intrinsic thylakoid pigments, a linear indicator of transthylakoid electric field component, suggests that, when CO(2) alone was lowered from 350 ppm to 50 ppm CO(2), modulation of q(E) sensitivity could be explained solely by changes in conductivity. Lowering both CO(2)(to 50 ppm) and O(2)(to 1%) resulted in an additional increase in q(E) sensitivity that could not be explained by changes in conductivity or cyclic electron flow associated with photosystem I. Evidence is presented for a fourth mechanism, in which changes in q(E) sensitivity result from variable partitioning of proton motive force into the electric field and pH gradient components. The implications of this mechanism for the storage of proton motive force and the regulation of the light reactions are discussed.

Light energy is transformed into chemical energy in photosynthesis by coupling a light-induced electron transfer to proton uptake. The resulting proton gradient drives ATP synthesis. In this study, we monitored the light-induced reactions in a 100-kDa photosynthetic protein from 30 ns to 35 s by FTIR difference spectroscopy. The results provide detailed mechanistic insights into the electron and proton transfer reactions of the QA to QB transition: reduction of QA in picoseconds induces protonation of histidines, probably of His126 and His128 in the H subunit at the entrance of the proton uptake channel, and of Asp210 in the L subunit inside the channel at 12 micros and 150 micros. This seems to be a prerequisite for the reduction of QB, mainly at 150 micros. QA- is reoxidized at 1.1 ms, and a proton is transferred from Asp210 to Glu212 in the L subunit, the proton donor to QB-. Notably, our data indicate that QB is not reduced directly by QA- but presumably through an intermediary electron donor.

Energy-dependent exciton quenching, or q(E), protects the higher plant photosynthetic apparatus from photodamage. Initiation of q(E) involves protonation of violaxanthin deepoxidase and PsbS, a component of the photosystem II antenna complex, as a result of lumen acidification driven by photosynthetic electron transfer. It has become clear that the response of q(E) to linear electron flow, termed "q(E) sensitivity," must be modulated in response to fluctuating environmental conditions. Previously, three mechanisms have been proposed to account for q(E) modulation:(i) the sensitivity of q(E) to the lumen pH is altered;(ii) elevated cyclic electron flow around photosystem I increases proton translocation into the lumen; and (iii) lowering the conductivity of the thylakoid ATP synthase to protons (g(H+)) allows formation of a larger steady-state proton motive force (pmf). Kinetic analysis of the electrochromic shift of intrinsic thylakoid pigments, a linear indicator of transthylakoid electric field component, suggests that, when CO(2) alone was lowered from 350 ppm to 50 ppm CO(2), modulation of q(E) sensitivity could be explained solely by changes in conductivity. Lowering both CO(2)(to 50 ppm) and O(2)(to 1%) resulted in an additional increase in q(E) sensitivity that could not be explained by changes in conductivity or cyclic electron flow associated with photosystem I. Evidence is presented for a fourth mechanism, in which changes in q(E) sensitivity result from variable partitioning of proton motive force into the electric field and pH gradient components. The implications of this mechanism for the storage of proton motive force and the regulation of the light reactions are discussed.