EXTENSIVE RNA EDITING AND POSSIBLE DOUBLE-STRANDED STRUCTURES DETERMINING EDITING SITES IN THE ATPB TRANSCRIPTS OF HORNWORT CHLOROPLASTS

Three nonsense codons and an unusual initiation codon were located wit hin the putative coding region of the atpB gene of chloroplast DNA of the hornwort Anthoceros formosae. Nucleotide sequencing of cDNA prepar ed from transcripts revealed extensive RNA editing. The unusual initia tion codon ACG was changed to AUG and three nonsense codons were conve rted into sense codons, In total 15 C residues of the genomic DNA were replaced by U residues in the mRNA sequences, while 14 U residues wer e replaced by C residues, This is the highest number of editing events for a chloroplast mRNA reported so far, Partial editing was also show n in a cDNA clone where 23 sites were edited but six sites remained un edited, representing the existence of premature mRNA, The expected two -dimensional structure of the mRNA shows the existence of a sequence c omplementary to every editing site, which can produce continuous base pairing longer than 5 bp, suggesting that mispairing in the double str and is the site determinant for RNA editing in Anthoceros chloroplasts . Comparison of the cDNA sequence with other chloroplast genes suggest s that the mechanism arose in the first land plants and has been reduc ed during evolution.