THE ROLE OF RNA EDITING IN CONSERVATION OF START CODONS IN CHLOROPLAST GENOMES

Open reading frames (ORFs), encoded by the plastid genomes of tobacco, liverwort, rice and maize were aligned with a view to studying the co nservation of translational start and stop codons created by RNA editi ng of homologous genes. It became evident that most of the homologous ORFs have conserved translation start and stop signals at the gene lev el. However, some of the ORFs show differences with respect to extensi ons of their 3' and 5' terminal regions. For example, the proposed N-t ermini of the ndhD-encoded peptides from different plant species are v ery variable in length and amino-acid composition. Sequence analysis o f ndhD and the corresponding cDNA shows that editing of an ACG triplet in tobacco, spinach and snapdragon leads to the creation of an AUG co don, corresponding to the start codon in other species. Conservation o f translational start codons of plastome-encoded genes can, therefore, be achieved by editing of transcripts, and the definition of plastome -encoded ORFs must take potential editing events into consideration.