Initiation of protein synthesis in isolated mitochondria and chloroplasts.

N5-Formyltetrahydrofolate, a competitive inhibitor of the formylation of the initiator Met-tRNAfMet in an in vitro assay, is a powerful inhibitor of amino acid incorporation in isolated Saccharomyces cerevisiae mitochondria and in Euglena gracilis chloroplasts. Thus, a large part of the incorporation is dependent upon new initiation acts. On the contrary, the rate of incorporation can be largely increased by addition of the specific formyl group donor, N10-formyltetrahydrofolate. Experiments are also reported strongly suggesting that the formylation of Met-tRNAfMet is an absolute requirement in order to initiate protein synthesis in chloroplasts, as has been shown in mitochondria.