The organization of the chloroplast ribosomal RNA genes in Spirodela oligorhiza has been determined. We have therefore characterized two cloned BamHI fragments containing the genes for the large and small rRNA genes, and two PstI fragments, each containing one of the two large, invertedly repeated regions of chloroplast DNA. Characterization was performed by restriction endonuclease mapping, DNA/RNA hybridization and electronmicroscopic R-loop analysis.
The results show that the rRNA genes are present in the order 16S, 23S, 4.5S and 5S. The 16S (1.5 kb) and 23S (2.9 kb) rRNA genes are separated by a spacer of 2.25 kb. There appears to be a close resemblance between the organization of the chloroplast rRNA genes in Spirodela oligorhiza and other higher plants so far examined.