U. Muhlenhoff et al., CHARACTERIZATION OF A REDOX-ACTIVE CROSS-LINKED COMPLEX BETWEEN CYANOBACTERIAL PHOTOSYSTEM-I AND ITS PHYSIOLOGICAL ACCEPTOR FLAVODOXIN, EMBO journal, 15(3), 1996, pp. 488-497
A covalent complex between photosystem I and flavodoxin from the cyano
bacterium Synechococcus sp, PCC 7002 was generated by chemical cross-l
inking, Laser flash-absorption spectroscopy indicates that the bound f
lavodoxin of this complex is stabilized in the semiquinone state and i
s photoreduced to the quinol form upon light excitation. The kinetics
of this photoreduction process, which takes place in similar to 50% of
the reaction centres, displays three exponential components with half
-lives of 9 mu s, 70 mu s and 1 ms. The fully reduced flavodoxin subse
quently recombines with P700(+) with a t(1/2) of 330 ms. A correspondi
ng flavodoxin semiquinone radical signal is readily observed in the da
rk by room temperature electron paramagnetic resonance, which reversib
ly disappears upon illumination. In contrast, the light-induced reduct
ion of oxidized flavodoxin can be observed only by first-flash experim
ents following excessive dark adaptation. In addition, the docking sit
e of flavodoxin on photosystem I was determined by electron microscopy
in combination with image analysis, Flavodoxin binds to the cytoplasm
ic side of photosystem I at a distance of 7 nm from the centre of the
trimer and in close contact to a ridge formed by the subunits PsaC, Ps
aD and PsaE.