Sequencing of the 3′-terminal region of a 16S rRNA gene from Zea mays chloroplast reveals homology with E. coli 16S rRNA

Abstract

CHLOROPLASTS are semi-autonomous organelles which possess their own DNA coding for organelle-specific RNAs and proteins. The translation process within a chloroplast is mediated by its own ribosomes of the 70S type, which are different from the 80S ribosomes of the cytoplasm. In view of this key function of 70S ribosomes for the organelle-specific protein synthesis we have undertaken to analyse rDNA from Zea mays chloroplasts at the nucleotide level, as we considered that this should allow a deduction of rRNA primary structures, and analysis of their mode of function and evolutionary relationship. DNA from Zea mays chloroplasts is a circular molecule of about 135 kilobase pairs, on which two rRNA coding regions are positioned in opposite orientation within two 22-kilobase pair inverted repeats¹. Each region includes one copy of a 16S rRNA, a 23S rRNA and a 5S rRNA gene and a spacer between the 16S and 23S rRNA genes. One such rDNA region has been linked to the plasmid vector pMB9 within the Escherichia coli clone pZmc134 (ref. 1), and this clone was therefore used for isolation, mapping and sequencing of the respective DNA fragments (Fig. 1)². We present here the sequence coding for the 3′-terminal part of chloroplast 16S rRNA and for the adjacent part of the spacer region, and report that there is extensive homology between this region of 16S rRNA and the 16S rRNA gene from E. coli.