Does Chl b bind to the Chl a-binding sites in PSI RC complexes?

Abstract

PsaA/B polypeptides of PSI RC bind about 100 Chl a, but no Chl b. Most Chl a in the complexes was removed without any loss of P700 by ether treatment, yielding the antenna-depleted PS1 RC complexes that have a Chl a/P700 ratio of 12. (1) On addition of Chl b with a phosphatidylglycerol, 9-14 Chl b per P700 bound to the antenna-depleted complexes. (2) The quantum efficiency of the P700 photooxidation on illumination with 360-480 nm light was increased to about 170% in the Chl b-reconstituted complexes, suggesting that the bound Chl b acts as an efficient antenna as Chl a remaining in the complexes. (3) The Chl b-reconstituted complexes emit the fluorescence peaking at 660, 680 and 695 nm, which are suggested to be emitted by Chl b, Chl a-675 and P700, respectively. (4) The excitation spectrum of Fl660 resembled the absorption spectrum of Chl b, whereas that of Fl680 showed the peak at 465 nm higher than that at 440 nm, which is in inverse to the absorption spectrum. The excitation spectrum of Fl695 was similar to that of Fl680, but with a little smaller peak at 465 nm. (5) These results indicate that even though a small part of Chl b binds far from Chl a, most Chl b can transfer its excitation energy to Chl a. Moreover, a part of Chl b is suggested to bind very close to P700 with a quite efficient energy transfer to P700.