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The identity and rearrangements of substrate water molecules in photosystem II (PSII) water oxidation are of great mechanistic interest and addressed herein by comprehensive analysis of NH 4 + /NH 3 binding. Time-resolved detection of O 2 formation and recombination fluorescence as well as Fourier transform infrared (FTIR) difference spectroscopy on plant PSII membrane particles reveals the following. (1) Partial inhibition in NH 4 Cl buffer occurs with a pH-independent binding constant of ∼25 mM, which does not result from decelerated O 2 formation, but from complete blockage of a major PSII fraction (∼60%) after reaching the Mn(IV) 4 (S 3 ) state. (2) The non-inhibited PSII fraction advances through the reaction cycle, but modified nuclear rearrangements are suggested by FTIR difference spectroscopy. (3) Partial inhibition can be explained by anticooperative (mutually exclusive) NH 3 binding to one inhibitory and one non-inhibitory site; these two sites may correspond to two water molecules terminally bound to the "dangling" Mn ion. (4) Unexpectedly strong modifications of the FTIR difference spectra suggest that in the non-inhibited PSII, ammonia binding obliterates the need for some of the nuclear rearrangements occurring in the S 2 -S 3 transition as well as their reversal in the O 2 formation transition, in line with the carousel mechanism [Askerka, M., et al. (2015) Biochemistry 54, 5783]. (5) We observe the same partial inhibition of PSII by NH 4 Cl also for thylakoid membranes prepared from mesophilic and thermophilic cyanobacteria, suggesting that the results described above are valid for plant and cyanobacterial PSII.

Genetic modification of key residues of photosystems is essential to identify functionally crucial processes by spectroscopic and crystallographic investigation; the required protein stability favours use of thermophilic species. The currently unique thermophilic photosynthetic model organism is the cyanobacterial genus Thermosynechococcus. We report the ability of Thermosynechococcus elongatus to assimilate organic carbon, specifically D-fructose. Growth in the presence of a photosynthesis inhibitor opens the door towards crucial amino acid substitutions in photosystems by the rescue of otherwise lethal mutations. Yet depression of batch-culture growth after 7 days implies that additional developments are needed., (© 2016 Federation of European Biochemical Societies.)