Sequencing of 16S−23S spacer in a ribosomal RNA operon of <i>Euglena gracilis</i> chloroplast DNA reveals two tRNA genes

Chloroplasts of the unicellular flagellate eukaryote <i>Euglena gracilis</i> contain several copies of a circular 135−140-kilobase pair DNA¹ which codes for chloroplast-specific stable RNAs (16S, 23S (refs 2, 3), 5S rRNAs⁴ and tRNAs⁵) and for an unknown number of chloroplast-specific proteins. The rRNA genes are located within three tandemly repeated DNA regions of approximately 5.6 kilobase pairs each⁶⁻⁸ and the arrangement of the structural genes within each repeat follows the prokaryotic pattern, being 5'-16S-23S-5S-3' (ref. 9). Total chloroplast tRNA hybridizes to fragments of rDNA⁹ and it was suggested that the 16S−23S spacer region contains tRNA coding sequences as is observed in <i>Escherichia coli</i>^10,11 and in spinach chloroplast¹² rDNA. We have therefore analysed <i>E. gracilis</i> strain Z 16S−23S spacer DNA at the nucleotide level, hoping this would allow identification of tRNA genes together with the processing sites of the respective primary transcripts. Maize chloroplast 16S rDNA shows strong sequence homology with <i>E. coli</i> 16S rRNA¹³. Sequence analysis of a total spacer in <i>E. gracilis</i> should demonstrate whether such similarities are also preserved in the chloroplast rDNA spacer region, or if this region has suffered a higher genetic drift rate. The latter is suggested from the 189 bases which have been sequenced from the 2.4-kilobase pair rDNA spacer from maize chloroplasts¹⁴. Flanking sequences, coding for the 3'-terminal region of 16S rRNA and for the 5'-terminal region of 23S rRNA have also been sequenced, to compare the drift rates between the spacer and its adjacent structural genes.