Analysis of chlorophyll-protein complexes from the cyanobacterium Cyanothece sp. ATCC 51142 by non-denaturing gel electrophoresis.

The unicellular diazotrophic cyanobacterium, Cyanothece sp. ATCC 51142 temporally separates N(2) fixation from photosynthesis. We are analyzing the mechanism by which photosynthesis is down-regulated so that O(2) evolution is minimized during N(2) fixation. Previous results suggested changes in photosynthesis that are mediated through the redox poise of the plastoquinone pool (a process involving state transitions, in which the redistribution of excitation energy between the two photosystems helps to optimize photosynthetic yield) and the oligomerization state of the photosystems. Our working hypothesis was that the regulation of photosynthesis involved changes in the oligomerization of the photosystems. To analyze this hypothesis, we utilized a low-ionic strength, non-denaturing gel electrophoresis system to study the Chl-protein complexes. We determined that PSI is mostly trimeric, whereas PSII appears mainly as monomers. We demonstrated that most of the Chl-protein complexes in Cyanothece sp. remained constant throughout the diurnal cycle, except for the transient accumulation of a Chl-protein complex (band C) which appeared only during the late light period. Based on the size of this complex, band C represents either an interaction of PSI and PSII or a PSII dimer. These results provide support for the dynamic nature of the photosystems with respect to the diurnal cycle.