588 BIOCHIMICA ET BIOPHYSICA ACTA BBA 96921 MITOCHONDRIAL RNA TURNOVER IN THE PRESENCE OF CORDYCEPIN ESTER A. ZYLBER, S. PERLMAN AND SHELDON PENMAN Massachusetts Institute o/ Technology, Department o/ Biology, 77 Massachusetts Avenue, Cam- bridge, Mass. o2z39 (U.S.A.) (Received February i5th , 1971) SUMMARY Cordycepin inhibits mitochondrial specific RNA synthesis. In contrast to the drug's effect on the nucleolus, no short RNA molecules are produced. After inhibition, prelabeled I2-S and 2I-S molecules decay with a 3-h half-life. The mitochondrial 4-S RNA appears stable. The kinetics of decay of mitochondrial specific protein synthesis resembles that of mitochondrial RNA. INTRODUCTION Mitochondrial specific RNA synthesis in mammalian cells has been demonstrat- ed in many laboratories 1-4. The presence of RNA complementary to DNA of mito- chondria in the cytoplasm of HeLa cells was first reported by ATTARDIAND ATTARDI 1. This was later shown to be RNA specifically located in the mitochondrial fraction and to be composed of several unique species. These include a 4-S component, two descrete RNA species with electrophoretic mobilities corresponding to I2-S and 2I-S (I2-S and I6-S by sedimentation) and higher-molecular-weight heterogeneous RNA 4-e. Most mitochondrial preparations are contaminated with RNA of nuclear origin apparently associated with membrane attached polyribosomes. RNA synthesized in mitochondria is distinguishable from RNA of nuclear origin by several criteria. Mitochondria continue to synthesize RNA when nucleal RNA synthesis is inhibited during metaphase arrest 7 or after virus infection 8. In contrast, low levels of ethidium bromide completely inhibit mitochondrial RNA synthesis but leave nuclear RNA formation unaffected 5,8. This latter property can be used as a diagnostic test for mitochondrial specific RNA. It was previously reported that the high-molecular-weight mitochondrial RNA components decayed after inhibition of new synthesis by ethidium while the mito- chondrial 4-S RNA appeared stable 5,e. This result had to be interpreted with caution since ethidium has many effects on mitochondria and the RNA breakdown observed could have resulted from the presence of the drug. In this report, it is shown that mitochondrial RNA synthesis is inhibited by cordycepin (3'-deoxyadenosine), a drug which has apparently no other direct effect on mitochondria. Following inhi- bition, the I2-S and 2I-S mitochondrial RNA species decay with a 3-h half-life while 4-S RNA is stable. Biochim. Biophys. Acta, 24 ° (I97 I) 588-593