Two distinct states of the thylakoid bf complex.

Under normal physiological conditions the state of the cyt bf complex is characterized by rapid reoxidation kinetics of cyt b-563 following flash-illumination. It is known that these kinetics are dramatically slowed down under oxidizing conditions. Here we show that this slow-down of cyt b-563 oxidation is the consequence of a relatively slow (half-time of several minutes) transformation of the cyt bf complex into a distinctly different state (termed state-s). Reversal to the normal state requires strong reductive treatment or light-induced electron transport. The results are in line with a recent model of functional cyt bf dimers [Cramer et al., Annu. Rev. Plant Physiol. Plant Mol. Biol. 47 (1996), 477-5081, if it is assumed that state-s reflects the monomeric state of the bf complex.