LIT VIVO ASSAY OF P700 REDOX CHANGES IN THE CYANOBACTERIUM FREMYELLA DIPLOSIPHON AND THE ROLE OF CYTOCHROME-C-OXIDASE IN REGULATION OF PHOTOSYNTHETIC ELECTRON-TRANSFER

Photosynthetic and respiratory electron transfers in cyanobacteria not only serve the bioenergetic needs of these prokaryotes during day and night time. The common use of the plastoquinone pool and the cytochro me (cyt) b(6)f complex also establishes possibilities for sharing phot osystem (PS) 2 plus dehydrogenases at the donor side and cyt c oxidase plus PS1 at the acceptor side. Given metabolic conditions and radiant energy supply, the available choices may give rise to unusual combina tions of connected electron transfer activities, for example PS2 and c yt c oxidase. In vivo measurements of energy storage in PS1 cyclic pho tophosphorylation via photoacoustic spectroscopy, and of the P700 redo x state via absorbance changes at 820 nn detected with the pulse ampli tude modulation technique (PAM), as well as of PS2 fluorescence yield, all in the absence or presence of the cyt c oxidase inhibitor KCN wer e combined to demonstrate that the two pathways at the acceptor side c ommunicate in vivo. This type of regulation serves proper poising of e lectron flow through and around PS1. The impaired cyt c oxidase activi ty (in this study achieved by addition of KCN) prevents a bury oxidize d state of P700 to be reached, which hampers electron passage from PS2 . The relative overreduction of PS1 in the KCN intoxicated samples red uces the electron now directed to biosynthesis. The results illustrate the versatility of the P700 redox state measurements at 820 nm as a m eans to study in vivo electron fluxes in cyanobacteria.