Presence of photosynthetic reaction centers in cyanobacterial plasma membranes

Abstract

A 2D-separation method described by Norling et al (FEBS Lett. 436 (1998) 189-192) which combines aqueous polymer two-phase partitioning and sucrose density centrifugation offers for the first time a method for isolation of completely pure plasma and thylakoid membranes from the cyanobacterium Synechocystis 6803. Immunochemical analysis of the isolated plasma membranes revealed the presence of small, but significant amounts of the reaction center subunits D1, D2 and the 33 kDa protein, and the presence of a higher amount of the a and b subunits of cytochrome b559, whereas the chlorophyll-a binding proteins CP43 and CP47 are absent (the later demonstrating the purity of the fraction). Furthermore the carboxy terminal processing protease CtpA was present in large quantity. Subunits PsaA, PsaB, PsaD and PsaC of the PSI reaction center were also found in the plasma membranes, but not subunits PsaE, PsaF and PsaL. UV trans-illumination of non-denaturating "green" gels with plasma membranes resulted in two fluorescent bands. The two gel bands were cut and frozen in liquid nitrogen and fluorescence emission spectra at 77K was measured. Band 1 showed fluorescence at 725 nm and band 2 at 680-685 nm. Taken together all these reults strongly suggest the presence of assembled photosynthetic reaction centers of both PSI and PSII in the cyanobacterial plasma membrane. The significance of these unexpected results with respect to the function and biogenesis of the cyanobacterial photosynthetic apparatus will be discussed.