Chloroplast RNA editing required for functional acetyl-CoA carboxylase in plants

RNA editing is an important post-transcriptional. process in chloroplasts a nd is thought to be functionally significant. Here we show a requirement of RNA editing for a functional enzyme. In peas, acetyl-CoA carboxylase (ACCa se), a key enzyme of fatty acid synthesis, is composed of biotin carboxglas e with the biotin carboxyl carrier protein and carboxyltransferase (CT). CT is composed of the nuclear-encoded alpha polypeptide and the chloroplast-e ncoded beta polypeptide in peas. One nucleotide of the beta polypeptide mRN A, which is edited in pea chloroplasts, converts the serine codon to the le ucine codon. We show that this RNA editing is required for functional CT by comparing the unedited and edited recombinant enzymes. In plants not havin g a leucine codon at the same position, editing was shown to take place so as to create the leucine codon, indicating that editing is needed for in vi vo CT activity and therefore for ACCase. To our knowledge, ACCase is an ess ential enzyme, suggesting that the chloroplast RNA editing is necessary for these plants.