The PsbY protein of Arabidopsis Photosystem II is important for the redox control of cytochrome b559

Photosystem II is a protein complex embedded in the thylakoid membrane of photosynthetic organisms and performs the light driven water oxidation into electrons and molecular oxygen that initiate the photosynthetic process. This important complex is composed of more than two dozen of intrinsic and peripheral subunits, of those half are low molecular mass proteins. PsbY is one of those low molecular mass proteins; this 4.7-4.9 kDa intrinsic protein seems not to bind any cofactors. Based on structural data from cyanobacterial and red algal Photosystem II PsbY is located closely or in direct contact with cytochrome b559. Cytb559 consists of two protein subunits (PsbE and PsbF) ligating a heme-group in-between them. While the exact function of this component in Photosystem II has not yet been clarified, a crucial role for assembly and photo-protection in prokaryotic complexes has been suggested. One unique feature of Cytb559 is its redox-heterogeneity, forming high, medium and low potential, however, neither origin nor mechanism are known. To reveal the function of PsbY within Photosystem II of Arabidopsis we have analysed PsbY knock-out plants and compared them to wild type and to complemented mutant lines. We show that in the absence of PsbY protein Cytb559 is only present in its oxidized, low potential form and plants depleted of PsbY were found to be more susceptible to photoinhibition.