Occurrence, Molecular Structure, and Induced Formation of the ‘Stromacentre’ in Plastids

The ‘stromacentre’ is a fibrillar spherulite found in plastids of aldehyde osmium-tetroxide fixed leaves of species in the genus Avena. Fibrils, each up to 0.2 µ x 80-90 Å, are associated in bundles, sometimes in hexagonal close packing, and the bundles in turn are aggregated in the spherulite. Individual bundles, or structures resembling them, occur in the plastid stroma in some other plants. In Avena, the stromacentre develops along with the internal membrane system of the plastids. Its staining reactions suggest absence of nucleic acid, and that it is proteinaceous. It is probably present in all mature Avena plastids. Stromacentre fibrils have been negatively stained. They consist of linearly aggregated particles. In side view these measure about 85-90 Å square, though the outline of the particles varies according to the orientation of the fibril. Particle outlines and staining patterns within particles are illustrated in photographically reinforced images. Micrographs interpreted as illustrating disaggregation into free particles are presented. These free particles are indistinguishable from numerous others in the preparations, and these in turn are thought to be Fraction I protein molecules. A process somewhat similar to stromacentre formation occurs in etioplasts and chloroplasts in Phaseolus leaves that have been dehydrated by plasmolysis, by wilting, or by high-speed centrifugation. These aggregates are not quite the same as the Avena stromacentre, but negative staining shows that they too are composed of units that are about the same size as Fraction I protein molecules. The hypothesis that the stromacentre fibrils consist of linearly aggregated Fraction I protein molecules is discussed.