Anatomy of the chloroplast ribosomal DNA of chlamydomonas reinhardii

Abstract

Two ribosomal DNA (rDNA) regions exist in the circular chloroplast genome of C. reinhardii. They are located at widely distant sites on the map in an inverted orientation. The chloroplast rDNA has been cloned and a fine structure map has been established. The small chloroplast ribosomal subunit contains 16S RNA, while the large subunit contains the four distinct RNA species 23, 7, 5 and 3S rRNA. The location of the rRNA genes has been determined by electron microscopy and by hybridization of the purified rRNAs to various rDNA restriction fragments. The gene order in the direction of transcription is 16, 7, 3, 23 and 5S. A spacer (Sp) of 1.68 ± 0.13 kb separates the 16S and the 7S RNA genes, and a very small spacer of 0.16 ± 0.04 kb separates the 7S and 23S RNA genes. The 5, 23, 3 and 7S RNAs are transcribed on the same strand. A surprising result is the interruption of all chloroplast 23S rRNA genes by a 940 base pair (bp) intervening sequence at a site 0.27 ± 0.04 kb distant from the 5′ end of the 23S RNA coding strand. This interruption has been observed both in cloned and uncloned chloroplast rDNA restriction fragments. When single-stranded chloroplast rDNA is mounted for electron microscopy, distinctive features of secondary structure which flank the rRNA gene sequences can be seen.