Synthesis of protein and nucleic acid in enucleate cytoplasm

Abstract

It is now well established that the formation of the individual proteins is controlled by the nuclear genetic material. The question which immediately arises is: are the proteins synthesized in the nucleus? If the liver of a rat which has been injected with a labelled amino acid is homogenized and if the cellular components are separated by the usual fractionation techniques, the microsome fraction contains the most radioactive proteins, and the nuclei are not especially active. Moreover, if the incorporation is carried out in the homogenate in vitro , the presence of the nuclear fraction has no effect on the amino acid uptake by the microsomal proteins (Siekevitz 1952; Keller, Zamecnik & Loftfield 1954; Hultin 1950, 1955). Since the microsomes are pieces of a major cytoplasmic structure (Bernhard, Gautier & Rouiller 1954; Palade & Siekevitz 1956), it would seem that protein synthesis is a cytoplasmic process. Experiments with homogenates are not entirely convincing, however, because the destruction of the cell structure might change the distribution of certain constituents among the fractions; it is known, for instance, that a considerable part of the nuclear proteins are released in the medium used for fractionation (Stern & Mirsky 1954). A direct way to find out whether cytoplasmic proteins are synthesized in the nucleus or in the cytoplasm is to study the capacity for protein synthesis of non-nucleate cytoplasm, or better to compare a nucleus-free fragment of cytoplasm with a fragment of the same cytoplasm containing the nucleus. Two very different unicellular organisms have been used for studies of this type by Brachet (1954 b , 1956; Brachet & Chantrenne 1956); Amoeba proteus and a green alga Acetabularia mediterranea . Both can be cut into nucleate and enucleate parts which will survive for a considerable time.