During the nucleotide sequencing of chloroplast DNA from the liverwort, M. polymorpha, we found that a coding sequence corresponding to the Esherichia coli ribosomal protein S12 gene (rps 12) is split into three exons. Strikingly, the first exon with the 5'-intron boundary sequence was found on the opposite strand of the chloroplast DNA (120 kilobases long, circular molecules) approx. 60 kilobases away from the rest of the exons. The amino acid sequence deduced from the DNA sequence was highly homologous to the sequences of the S12 ribosomal protein of E. coli (70.2%), and Euglena gracilis chloroplasts (73.6%). Possible mechanisms for the expression of this split gene are discussed.
