At 22 degrees in Earle9s medium, Krebs cells synthesize proteins. After a brief ;pulse9 with [(14)C]valine followed by a ;chase9 of [(12)C]valine the radioactivity appears first in microsomes and is transferred after ;chase9 to the cell sap. Kinetics of labelling of the mitochondrial protein are different from that of either microsomal or cell-sap protein. When Krebs cells in buffer are mixed with ribonuclease in water the nuclease penetrates the cell membrane. The ribonuclease-treated cells are still viable but have lost most of their cytoplasmic ribosomes (electron micrograph). Such cells still synthesize mitochondrial protein at near normal rate but synthesis of microsomal protein is severely inhibited. The results indicate that some mitochondrial proteins are synthesized independently of the microsome-cell-sap system.
