Cooperation of plasma and thylakoid membranes for the biosynthesis of chlorophyll in cyanobacteria: the role of the thylakoid centers

Discontinuous sucrose density gradient centrifugation was used to isolate four distinct membrane fractions from cell-free extracts of Synechocystis PCC 6803, viz. two chlorophyll-free plasma membrane fractions, a dark-green thylakoid membrane fraction and a light-green thylakoid membrane fraction with buoyant densities of 1.066, 1.092, 1.208 and 1.162g·cm−3, respectively. The four types of membranes could not be further resolved by recentrifugation on continuous sucrose density gradients; each of them had its genuine complement of cytochrome c oxidase, NADPH-protochlorophyllide reductase, P700, protochlorophyllide and chlorophyllide, which could not be explained by superposition (mixture) of different membrane fractions. Due to the high concentration (activity) of cytochrome c oxidase, NADPHprotochlorophyllide reductase, P700, protochlorophyllide and chlorophyllide in the light-green thylakoid membrane fraction it is tempting to suggest that these membranes are derived from thylakoid centers that appear to represent contact sites between cytoplasmic (plasma) and intracytoplasmic (thylakoid) membranes where the chlorophyllide synthesized from protochlorophyllide in the plasma membrane is esterified with phytol to chlorophyll and the chlorophyll is transferred to the thylakoid membranes. Our findings will be discussed with respect to the long-standing question of continuity between plasma and thylakoid membranes in cyanobacteria.