A SYSTEMATIC SURVEY OF CONSERVED HISTIDINES IN THE CORE SUBUNITS OF PHOTOSYSTEM-I BY SITE-DIRECTED MUTAGENESIS REVEALS THE LIKELY AXIAL LIGANDS OF P-700
K. Redding et al., A SYSTEMATIC SURVEY OF CONSERVED HISTIDINES IN THE CORE SUBUNITS OF PHOTOSYSTEM-I BY SITE-DIRECTED MUTAGENESIS REVEALS THE LIKELY AXIAL LIGANDS OF P-700, EMBO journal, 17(1), 1998, pp. 50-60
The Photosystem I complex catalyses the transfer of an electron from l
umenal plastocyanin to stromal ferredoxin, using the energy of an abso
rbed photon, The initial photochemical event is the transfer of an ele
ctron from the excited state of P-700, a pair of chlorophylls, to a mo
nomer chlorophyll serving as the primary electron acceptor, We have pe
rformed a systematic survey of conserved histidines in the last sis tr
ansmembrane segments of the related polytopic membrane proteins PsaA a
nd PsaB in the green alga Chlamydomonas reinhardtii. These histidines,
which are present in analogous positions in both proteins, were chang
ed to glutamine or leucine by site-directed mutagenesis. Double mutant
s in which both histidines had been changed to glutamine were screened
for changes in the characteristics of P-700 using electron paramagnet
ic resonance, Fourier transform infrared and visible spectroscopy, Onl
y mutations in the histidines of helix 10 (PsaA-His676 and PsaB-Wis656
) resulted in changes in spectroscopic properties of P-700, leading us
to conclude that these histidines are most likely the axial ligands t
o the P-700 chlorophylls.