Electron microscopical localisation of the 23S and 16S rRNA genes within an inverted repeat for two chromoplast DNAs

Circular plastid DNA molecules, isolated from flower chromoplasts of the daffodil (a monocotyledon), and the nasturtium (a dicotyledon), have been shown by electron microscopy to contain inverted repeat sequences of 28.5 ± 0.7 kbp and 27.1 ± 1.0 kbp, respectively. The regions separating the repeats have lengths of 16.6 ± 0.8 kb for the shorter region, and 87.8 ± 4.8 kb for the longer in the daffodil, and 18.5 ± 0.5 kb and 82.3 ± 3.1 kb for the corresponding regions in the nasturtium. Further, in both cases, the 23S and 16S ribosomal RNA genes have been located by hybridisation (R-Loop technique) within this inverted repeat. Although the distance between the 16S and 23S genes is comparable, the position of the rRNA gene blocks within the repeats is different in these two chromoplast DNAs, with a shift of 1,500 bp.