PSBY-1 and -2 are manganese-binding polypeptides with L-arginine metabolizing enzyme activity. They are a component of the core of photosystem II. They have also a minor catalase-like activity since they cause evolution of oxygen from hydrogen peroxide in a reaction stimulated by manganese. Leaves. Not present in stems and roots. Developmentally regulated. Photocontrolled. Steady-state concentrations increase for the first 3 hours of illumination and then decline. The central hydrophobic segment does not form a membrane-spanning region but could serve as a targeting signal for processing of the precursor in the thylakoid membrane. Belongs to the PsbY family.