COMPOSITION OF THE CARBOHYDRATE GRANULES OF THE CYANOBACTERIUM, CYANOTHECE SP STRAIN ATCC-51142

Cyanothece sp. strain ATCC 51142 is an aerobic, unicellular, diazotrop hic cyanobacterium that temporally separates O-2-sensitive N-2 fixatio n from oxygenic photosynthesis. The energy and reducing power needed f or N-2 fixation appears to be generated by an active respiratory appar atus that utilizes the contents of large interthylakoidal carbohydrate granules. We report here on the carbohydrate and protein composition of the granules of Cyanothece sp. strain ATCC 51142. The carbohydrate component is a glucose homopolymer with branches every nine residues a nd is chemically identical to glycogen. Granule-associated protein fra ctions showed temporal changes in the number of proteins and their abu ndance during the metabolic oscillations observed under diazotrophic c onditions. There also were temporal changes in the protein pattern of the granule-depleted supernatant fractions from diazotrophic cultures. None of the granule-associated proteins crossreacted with antisera di rected against several glycogen-metabolizing enzymes or nitrogenase, a lthough these proteins were tentatively identified in supernatant frac tions. It is suggested that the granule-associated proteins are struct ural proteins required to maintain a complex granule architecture.