Struktur und Funktion der genetischen Information in den Plastiden: XI. DNA in normalen und mutierten Plastiden der Sorte „Mrs. Parker‟ von Pelargonium zonale

Abstract

As a consequence of a plastid mutation the white plastids en: alba-1 of the variety “Mrs. Parker” of Pelargonium zonale do not possess ribosomes, and therefore they are not able to perform plastidal protein synthesis. By means of biochemical and electron microscopic methods the question was studied, whether the mutant plastids contain DNA. The DNA of isolated wild type chloroplasts of Pelargonium zonale has a buoyant density in CsCl of 1,697 ± 0.001 g/cm 3 (fig. 1), and a melting temperature of 85°C (fig. 2). The chloroplast DNA of Pelargonium cannot be separated by buoyant density centrifugation in CsCl and melting temperature from the nuclear DNA. The base composition of both chloroplast and nuclear DNA was calculated to be 37–38% GC. However, the chloroplast DNA renatures almost completely, whereas nuclear DNA renatures only to a low extend. In the DNA of pure white mutant leaves of en: alba-1 there is no indication of the presence of any other component differing in the density (fig. 1c). The electron microscopic study of plastid DNA by means of three independent techniques gave the following results for both normal chloroplasts and mutated en: alba-1 plastids: In situ studies show an intraplastidal localization of DNA (fig. 4 b) in nucleoplasmatic areas, occupying 39.3 ± 8.2 % of total plastid capacity (fig. 4a) in the case of mutant plastids. The identity of the fibrillar structures with DNA was proved in positive DNase-tests. Spreaded DNA from osmotically disrupted plastids is associated with membranes in a consistent way. It was possible to find good visible, loop-like moleeules from wild type (fig. 8) and from the mutant plastids (fig. 6 a). Apart from this there are large “displays” of DNA with numerous membrane attachment points (fig. 7 a). Thereby the DNA has contact with the membrane either directly in polyfibrillar form or by building solid stick-like structures (fig. 7 b). A single DNA molecule from en: alba-1 plastids is presented with partially distinct diameters of its strand. This phenomenon was interpreted as a possible result of replication (fig. 6 b). Evidence for the ability for replication of en: alba-1 DNA was obtained by electron microscopic autoradiography (fig. 8). As in wild type chloroplasts, about four labelled centres are scattered over the plastid sections. DNase-treated samples do not show any specific labelling. It is discussed, whether the DNA is present in the plastid as a single molecule or as a multitude of independent molecules.