New insights on Chl function in Photosystem II from site-directed mutants of D1/T179 in Thermosynechococcus elongatus.

The monomeric chlorophyll, Chl, which is located between the PP chlorophyll pair and the pheophytin, Pheo is the longest wavelength chlorophyll in the heart of Photosystem II and is thought to be the primary electron donor. Its central Mg is liganded to a water molecule that is H-bonded to D1/T179. Here, two site-directed mutants, D1/T179H and D1/T179V, were made in the thermophilic cyanobacterium, Thermosynechococcus elongatus, and characterized by a range of biophysical techniques. The MnCaO cluster in the water-splitting site is fully active in both mutants. Changes in thermoluminescence indicate that i) radiative recombination occurs via the repopulation of Chl itself; ii) non-radiative charge recombination reactions appeared to be faster in the T179H-PSII; and iii) the properties of PP were unaffected by this mutation, and consequently iv) the immediate precursor state of the radiative excited state is the ChlPheo radical pair. Chlorophyll bleaching due to high intensity illumination correlated with the amount of O generated. Comparison of the bleaching spectra with the electrochromic shifts attributed to Chl upon Q formation, indicates that in the T179H-PSII and in the WT3-PSII, the Chl itself is the chlorophyll that is first damaged by O, whereas in the T179V-PSII a more red chlorophyll is damaged, the identity of which is discussed. Thus, Chl appears to be one of the primary damage site in recombination-mediated photoinhibition. Finally, changes in the absorption of Chl very likely contribute to the well-known electrochromic shifts observed at ~430 nm during the S-state cycle.