COOPERATION OF PLASMA AND THYLAKOID MEMBRANES FOR THE BIOSYNTHESIS OFCHLOROPHYLL IN CYANOBACTERIA - THE ROLE OF THE THYLAKOID CENTERS

Discontinuous sucrose density gradient centrifugation was used to isol ate four distinct membrane fractions from cell-free extracts of Synech ocystis 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. 162 g.cm-3, respectively. The four types of membranes could not be fur ther resolved by recentrifugation on continuous sucrose density gradie nts; each of them had its genuine complement of cytochrome c oxidase, NADPH-protochlorophyllide reductase, P700, protochlorophyllide and chl orophyllide, which could not be explained by superposition (mixture) o f different membrane fractions. Due to the high concentration (activit y) of cytochrome c oxidase, NADPH-protochlorophyllide reductase, P700, protochlorophyllide and chlorophyllide in the light-green thylakoid m embrane fraction it is tempting to suggest that these membranes are de rived from thylakoid centers that appear to represent contact sites be tween cytoplasmic (plasma) and intracytoplasmic (thylakoid) membranes where the chlorophyllide synthesized from protochlorophyllide in the p lasma membrane is esterified with phytol to chlorophyll and the chloro phyll is transferred to the thylakoid membranes. Our findings will be discussed with respect to the long-standing question of continuity bet ween plasma and thylakoid membranes in cyanobacteria.