An optical and ESR investigation on the acceptor side of the reaction center of the green photosynthetic bacterium <i>Prosthecochloris aestuarii</i>

Abstract

There are several indications that the acceptor chain of green photosynthetic bacteria is basically different from that of purple bacteria. Like photosystem I, green bacteria are capable of direct photoreduction of pyridine nucleotides [ 1,2], a reaction that is dependent on a ferredoxin. Titrations of the ability to photooxidize cytochrome es53 indicate that the oxidationreduction midpoint potential (E,) of the first stable acceptor is about -540 mV [3]. A membrane-bound iron-sulfur protein was reported in Chlorobium [4], with an Em of about -550 mV, low enough to be a component in the electron-transport chain on the reducing side of the light reaction. ESR measurements in [S] at low temperature suggested the photoreduction of an iron-sulfur protein in 'chromatophores' of Chlorobium, but the experiments in [6] were at variance with this conclusion. These ESR experiments, however, were seriously hampered by the extremely high absorbance of the preparations, which have a high antenna BChl to reaction center ratio. We have isolated the photochemically active photosystem pigment (PP) and reaction center pigment-protein (RCPP) complexes from Prosthecochloris aestuarii, with antenna BChl a to reaction center ratios of -75 and 35, respectively [7]. In contrast to the membrane vesicle complex I [8,9], the PP complex could be concentrated without aggregation of the sample. Thus, we were able to obtain concentrated samples of the PP complex with good optical properties, that were suitable for ESR work.