The chloroplast genome of Beta vulgaris L.: Structural organization and transcriptional activity

The structural organization of sugarbeet chloroplast DNA (ctDNA) was studied by electron microscopic and biochemical methods. As a result, levels of ctDNA packing were demonstrated. It was observed that deoxynucleoproteid (DNP) fibrils released from the chloroplasts after a mild osmotic shock have 13–14 nm nucleosome-like globules on their fibrils. The fibrils form rosettes with loops of a contour length of 15–20 μm. Minicircular plasmid-like DNA molecules with nucleosome-like globules were found in the chloroplasts. By means of monoclonal antibodies against the β-subunit of Escherichia coli RNA polymerase and protein A-colloidal gold complexes, the sites of DNP fibrils to which chloroplast RNA polymerase are attached were localized. The highest number RNA polymerase molecules were found to be associated with the relaxed smooth DNA fibrils which are almost devoid of nucleosome-like granules. The compactly packed DNP fibrils with a high number of nucleosome-like granules were free of RNA polymerase molecules. The former regions of the chloroplast nucleoid are presumably the transcriptionally active sites of the genome, while the latter are the transcriptionally inactive. The RNA polymerase molecules were also found to be attached to the minicircular DNA.