Evidence for an Unprecedented Histidine Hydroxyl Modification on D2-His336 in Photosystem II of <i>Thermosynechoccocus vulcanus</i> and <i>Thermosynechoccocus elongatus</i>

The electron density map of the 3D crystal of Photosystem II from Thermosynechococcus vulcanus with a 1.9 Å resolution (PDB: 3ARC) exhibits, in the two monomers in the asymmetric unit cell, an, until now, unidentified and uninterpreted strong difference in electron density centered at a distance of around 1.5 Å from the nitrogen Nδ of the imidazole ring of D2-His336. By MALDI-TOF/MS upon tryptic digestion, it is shown that ∼20–30% of the fragments containing the D2-His336 residue of Photosystem II from both Thermosynechococcus vulcanus and Thermosynechococcus elongatus bear an extra mass of +16 Da. Such an extra mass likely corresponds to an unprecedented post-translational or chemical hydroxyl modification of histidine