Circular DNA in isolated chromoplasts.

Plastid DNA has been isolated from many dif­ ferent sources and characterized in terms of bouyant density, base composition, renaturation kinetics, and sedimentation velocity (for references, c f . 1' 2). For many years, however, it was not possible to isolate plastid DNA in a form sufficiently intact to determine its molecular weight by length measure­ ments in electron micrographs. Only recently the isolation of circular, partly supercoiled DNA mole­ cules and their demonstration in the electron micro­ scope (EM) have been accomplished from chloro­ plasts of algae and higher plants (Euglena gracilis 3, pea 4, bean 4, lettuce 4, spinach 5, and corn 5) . In all cases a contour length of about 40 //m was found for circular DNA from chloroplasts. We now report the electron microscopic demon­ stration of supercoiled DNA rings, obtained from isolated chromoplasts of the flowers of Daffodil. The yellow corona chromoplasts of this monocotyledonous plant have already been investigated by electron microscopy in s i tu 6-8 and after isolation by different procedures 9, and have been character­ ized in terms of the chemical composition of their pigment-carrying internal m em branes10. Isolation and biochemical characterization of DNA from Nar­ cissus chromoplasts, and from chloroplasts, mito­ chondria, and nuclei of the same species as well, is reported by Herrmann u .