Arrangement of the ribosomal RNA genes in chloroplast DNA of Leguminosae

Abstract

The circular chloroplast DNA from three species of plants in the taxonomic family Leguminosae were examined using electron microscopic techniques and restriction endonuclease digestion. Chloroplast DNAs from chickpea (Cicer arietinum), mung bean (Vigna radiata), and soy bean (Glycine max) were found to range in size from 119–151 kilobase pairs by contour length measurements. Sizes of the chloroplast DNAs have been further confirmed using different restriction endonucleases. Two of the chloroplast DNAs examined, soy bean and mung bean, contain a region approximately 15.9–18% of their monomer length that is repeated in reverse polarity. This repeated region separates a small unique region that ranges in size from 18.75–20.4 kilobase pairs and a large unique region that ranges in size from 73.4–85 kbp. This feature was not found in the chloroplast DNA of chickpea. R-loop hybridizations performed using chloroplast ribosomal RNAs demonstrate that the two ribosomal gene sets of the mung been and soy bean are arranged in inverted orientation within this repeated region. In contrast, the chickpea chloroplast DNA posesses a single ribosomal RNA gene set in the circular molecule. In all three chloroplast DNAs examined, the genes encoding the chloroplast 23S and 16S ribosomal RNA genes are separated by a spacer region which ranges in size from 2.2 to 2.48 kbp.