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

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.