Synthesis of cytoplasmic ribosomes

In higher eukaryotes messenger RNAs for chloroplast and mitochondrial proteins have been localized exclusively to free and not membrane-bound polysomes [4,68]. In yeast, the situation is not as clear a portion of the polysomes for some of the proteins is found to be bound to mitochondria [69,70], This may only reflect the speed at which the import process occurs in the different organisms. Most of the cytoplasmically-synthesized precursors have amino terminal extensions varying in size and nature. The size variation goes from no extension (cytochrome c and ATP-ADP translocator) to approximately 12000 Da (proteolipid of the proton 

ATPase from Neurospora crassa mitochondria). Recently, the sequences of a few of the NHj terminal extensions of mitochondrial polypeptides have become available, and thus far there are no striking sequence homologies [71-73]. One common feature among most precursors is that the NHj-terminal extension is highly basic in nature. This may play a role in recognizing the mitochondrial surface [74,75]. 

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