LOCALIZATION OF MEMBRANE-PROTEINS IN THE CYANOBACTERIUM SYNECHOCOCCUSSP PCC7942 - RADIAL ASYMMETRY IN THE PHOTOSYNTHETIC COMPLEXES

Localization of membrane proteins in the cyanobacterium Synechococcus sp. PCC7942 was determined by transmission electron microscopy utilizi ng immunocytochemistry with cells prepared by freeze-substitution. Thi s preparation procedure maintained cellular morphology and permitted d etection of cellular antigens with high sensitivity and low background . Synechococcus sp. PCC7942 is a unicellular cyanobacterium with thyla koids organized in concentric layers toward the periphery of the cell. Cytochrome oxidase was localized almost entirely in the cytoplasmic m embrane, whereas a carotenoprotein (P35) was shown to be a cell wall c omponent. The major photosystem II (PSII) proteins (D1, D2, CP43, and CP47) were localized throughout the thylakoids. Proteins of the Cyt b( 6)/f complex were found to have a similar distribution. Thylakoid lumi nal proteins, such as the Mn-stabilizing protein, were located primari ly in the thylakoid, but a small, reproducible fraction was found in t he outer compartment. The photosystem I (PSI) reaction center proteins and the ATP synthase proteins were found associated mostly with the o utermost thylakoid and with the cytoplasmic membrane. These results in dicated that the photosynthetic apparatus is not evenly distributed th roughout the thylakoids. Rather, there is a radial asymmetry such that much of the PSI and the ATPase synthase is located in the outermost t hylakoid. The relationship of this structure to the photosynthetic mec hanism is discussed. It is suggested that the photosystems are separat ed because of kinetic differences between PSII and PSI, as hypothesize d by H.-W. Trissl and C. Wilhelm (Trends Biochem Sci [1993] 18:415-419 ).