Extensive RNA editing of U to C in addition to C to U substitution in the rbcL transcripts of hornwort chloroplasts and the origin of RNA editing in green plants

We cloned and sequenced a portion of chloroplast DNA from the hornwort Anthoceros formosae. A nucleotide sequence of 7556 bp contained structures similar to those of ndhK, ndhC, trnV, trnM, atpE, atpB, rbcL, trnR and accD. The arrangement of these was the same as that of other chloroplast DNA. However, two nonsense codons were located within the putative coding region of rbcL, although they were used as putative termination codons of the genes. RNA was extensively edited in the transcripts of rbcL when cDNA sequences were analyzed. The unusual nonsense codons of TGA and TAA became CGA and CAA respectively. These are examples of U to C type RNA editing, which was never been found before in chloroplast mRNA. In general, 13 Cs of genomic DNA were found as Ts in the cDNA sequence and seven Ts were found as Cs. This is the first finding of RNA editing on the transcripts of rbcL and also in bryophytes. This event had been thought to arise in land plants after the split of bryophytes. The origin of RNA editing is discussed in relation to the landing of green plants.