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We characterize a DNA segment of the Euglena gracills chloroplast DNA fragment Eco-N by nucleotlde sequencing and SI nuclease analysis. We show that this region, which is upstream of the previously sequenced tuf A gene, contains the genes for the ribosomal proteins S12 and S7. The gene arrangment is 5'-rps 12-80 bp spacer-rps 7-174 bp 3pacer-tuf A, somewhat similar to the str operon of E. coll. The chloroplast S12 and S7 proteins contain 124 and 155 aminoacids, respectively, and are to 68% and 38% homologous with the corresponding E. coll proteins. The region is transcribed into a distronic mRNA of about 1.1 to 1.2 kb. The rps 12 and rps 7 genes, contrary to the tuf A gene, are not split. INTRODUCTION We have recently sequenced the Euglena qracilis chloroplast gene for the elongation factor EF-Tu (tuf A) (1) which was previously mapped on the DNA fragment Eco-N (2). The chloroplast tuf A gene is expressed as a 1.95 kb mRNA which is the product of three splicing events (1). The coding part of this chloroplast gene is to about 55% homologous with the E_. coli tuf A gene which is known to be part of a large operon containing in the 5' to 3' direction the genes for the small subunit ribosomal proteins S12 and S7 (rps 12, rps 7), the elongation factor EF-G (fus) and the elongation factor EF-Tu (tuf A) (3). The bacterial tuf A (4), rps 12 and parts of rps 7 have been sequenced (5) as well as the S12 (6) and S7 proteins (7). The S12 protein is of particular interest since it is involved in the translation initiation step and controls the fidelity of the reading process (E). Streptomycin interacts with SI 2 abolishing normal protein synthesis (9 and references there). Streptomycin also interacts with chloroplast ribosomes, preventing normal chloroplast growth what can lead to irreversibly bleached cells. Streptomycin resistant Euglena strains have been described (10,11), i.e.. © IRL Press Limited, Oxford, England. 2851 Nucleic Acids Research streptomycin resistance is most likely a convenient chloroplast bound genetic