ON THE PRESENCE AND ROLE OF A MOLECULE OF CHLOROPHYLL-A IN THE CYTOCHROME B(6)F COMPLEX

Highly purified preparations of cytochrome b(6)f complex from the unic ellar freshwater alga Chlamydomonas reinhardtii contain about 1 molecu le of chlorophyll a/cytochrome f. Several lines of evidence indicate t hat the chlorophyll is an authentic component of the complex rather th an a contaminant. In particular, (i) the stoichiometry is constant; (i i) the chlorophyll is associated with the complex at a specific bindin g site, as evidenced by resonance Raman spectroscopy; (iii) it does no t originate from free chlorophyll released from thylakoid membranes up on solubilization; and (iv) its rate of exchange with free, radioactiv e chlorophyll a is extremely slow (weeks). Some of the putative functi onal roles for a chlorophyll in the b(6)f complex are experimentally r uled out, and its possible evolutionary origin is briefly discussed.