In thylakoid preparations from spinach and the halophyte Avicennia marina a correlation is observed between functional O2 evolution at low chloride concentrations and the presence of the 23 kDa protein. Addition of spinach 23 kDa protein to polypeptide-depleted halophyte inside-out thylakoid vesicles lowers their chloride requirement for optimal O2 evolution activity from 250 to 5 mM. It is suggested that the specific role for the 23 kDa protein is to increase the affinity of the water oxidation site for chloride.
