Structure, composition, and distribution of plastid nucleoids in Narcissus pseudonarcissus

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Abstract
The size, frequency and distribution of the nucleoids of chloroplasts (cl-nucleoids) and chromoplasts (cr-nucleoids) of the daffodil have been investigated in situ using the DNA-specific fluorochrome 4′6-diamidino-2-phenylindole. Chromoplasts contain fewer nucleoids (approx. 4) than chloroplasts (> 10), and larger chromoplasts (cultivated form, approx. 4) contain more than smaller ones (wild type, approx. 2). During chromoplast development the nucleoid number decreases in parallel with the chlorophyll content. Each nucleoid contains 2–3 plastome copies on average. In chloroplasts the nucleoids are evenly distributed, whereas they are peripherally located in chromoplasts. The fine structure of isolated cl-and cr-nucleoids, purified either by Sepharose 4B-CL columns or by metrizamide gradients, was investigated electron microscopically. The cl-nucleoids consist of a central protein-rich core with ‘naked’ DNA-loops protruding from it. In cr-nucleoids, on the other hand, the total DNA is tightly packed within the proteinaceous core. The protein-containing core region of the nucleoids is made up of knotty and fibrillar sub-structures with diameters of 18 and 37 nm, respectively. After proteinase treatment, or incressing ion concentration, most of the proteins are removed and the DNA is exposed even in the case of cr-nucleoids, the stability of which proved to be greater than that of cl-nucleoids. The chemical composition of isolated plastid nucleoids has been determined qualitatively and quantitatively. Chromoplast-nucleoids contain, relative to the same DNA quantity, about six times as much protein as cl-nucleoids. Accordingly the buoyant density of cr-nucleoids in metrizamide gradients is higher than that of cl-nucleoids. In addition to DNA and protein, RNA could be found in the nucleoid fraction. No pigments were present. The cr-and cl-nucleoids have many identical proteins. There are, however, also characteristic differences in their protein pattern which are possibly related to the different expression of the genomes of chloroplasts and chromoplasts. Nucleoids of both plastid types contain some proteins which also occur in isolated envelope membranes (probably partly in the outer membrane) and thus possibly take part in binding the DNA to membranes.