Isolated alpha- and beta -subunits of Thermophilic Bacillus PS3 F(1)ATPase (TF1) bind about 1 Fe(III) equivalent. Upon reassembling in the symmetric alpha (3)beta (3) hexamer, Fe(III) binding capacity decreases, as this complex binds about three Fe(III) equivalents. In accordance, when the hexamer is dissociated in the alpha (3)beta (3), heterodimer, each heterodimer binds about one Fe(III) equivalent. On the contrary, native TF1 exhibits a single Fe(III) site. CD spectra in far UV indicate that upon Fe(III) binding both the whole complex and the isolated beta -subunit undergo structural modifications accompanied by decrease of alpha -helix content, while cu-subunit doesn't. As in alpha (3)beta (3), and in the whole enzyme the number of bound Fe(III) equivalents is consistent with the number of beta -subunits in the "empty" conformation, it is inferred that the single Fe(III) site in TF1 is probably located in beta (E).