The PsbH protein belongs to a group of small protein subunits of the photosystem 2 (PS2) complex and genes encoding PsbH homologues have been so far found in all studied oxygenic phototrophs. This single helix membrane protein is important for the proper function of the PS2 acceptor side and for stable assembly of PS2. Its hypothetical function as an analogue of the H subunit of the bacterial reaction centre as well as a putative role of its phosphorylation is evaluated. and J. Komenda, D. Štys, L. Lupínková.