THE REACTION-MECHANISM OF PHOTOSYSTEM-I REDUCTION BY PLASTOCYANIN ANDCYTOCHROME C(6) FOLLOWS 2 DIFFERENT KINETIC-MODELS IN THE CYANOBACTERIUM PSEUDANABAENA SP. PCC-6903

Plastocyanin (Pc) and cytochrome c(6) (Cyt) have been purified to homo geneity from the cyanobacterium Pseudanabaena sp. PCC 6903, which occu pies a unique divergent branch in the evolutionary tree of oxygen-evol ving photosynthetic organisms. The two metalloproteins have similar mo lecular masses (9-10 kDa), as well as almost identical isoelectric poi nts (ca. 8) and midpoint redox potentials (ca. 350 mV, at pH 7). Their reaction mechanism of electron transfer to Photosystem I (PS I) has b een analyzed by laser-flash absorption spectroscopy. The kinetic trace s with Pc correspond to monophasic kinetics, whereas those with Cyt ar e better fitted to biphasic curves. The observed pseudo first-order ra te constant (k(obs)) with Pc and that for the slower phase with Cyt ex hibit saturation profiles at increasing donor protein concentrations, thereby suggesting that the two metalloproteins are able to form trans ient complexes with PS I. The ionic strength dependence of the rate co nstants for complex formation makes evident the electrostatic nature o f intermediate complexes. The experimental findings indicate that the PS I reduction kinetics in Pseudanabaena follow a type II mechanism wi th Pc and a type III mechanism with Cyt, according to the different ki netic models proposed previously [(Hervas M, Navarro JA, Diaz A, Botti n H and De la Rosa MA (1995) Biochemistry 34: 11321-11326)]. From an e volutionary point of view, this reinforces our previous observation th at PS I was first adapted to operate efficiently with positively charg ed Cyt rather than with Pc.