air synthetase is found in both mitochondria and plastids; the mitochondrial form has 5 more amino acids than the plastid form. the enzyme is encoded by a single gene in cowpeas despite the fact that it exists in different forms in plastids and mitochondria. this suggests that the different versions may be derived from a single transcript. one study proposes that there is tight transcriptional control of pur5, the gene encoding air synthetase.