RNA AND CONTROL OF CELLULAR PROCESSES 107 



template (Hoagkiiid, 1959). It is not entirely certain yet that this 

 is the only median ism of protein formation, since it has been ob- 

 served that ribosomes can incorporate amino acids into proteins 

 directly, in the absence of activating enzymes or adaptors (Beljan- 

 ski and Ochoa, 1958; Zalta;1960). 



Other systems of protein syntliesis have been proposed and stud- 

 ied. Mitochondria were found to incorporate amino acids into 

 proteins, especiallv into cytochromes (McLean et at, 1958; Bates 

 et al., 1958 ) . The mechanism of incorporation is not clear yet, but 

 tlie role of RNA could not be excluded. All mitochondrial prepa- 

 rations contain small amounts of RNA which would suffice to explain 

 the amount of protein produced. The fact that ribonuclease cannot 

 inhibit protein svnthesis does not necessarily mean that RNA is not 

 required. The RNA mav be protected and not open to ribonuclease 

 in intact mitochondria. Indeed, Rendi (1959) could isolate, from 

 mitochondria destroyed by desoxycholate, RNA particles which 

 seemed to be responsible for most of mitochondrial protein synthesis 

 and which became sensitive to the action of ribonuclease. Similar 

 results were obtained after the destruction of mitochondria by soni- 

 cation (Kalf and Simpson, 1959). 



Nuclei were frequently considered as protein-producing sites 

 even before the modern advances of cell biology. Caspersson ( 1941 ) 

 still assumed that the important production of proteins occurred in 

 nuclei and in their immediate surroundings. It has been possible to 

 show since that the main protein synthesis occurs inside the cytoplasm. 

 In insect cells this could be demonstrated directly by autoradiogra- 

 phy. The salivarv glands of Drosophila, the nurse cells of Drosoph- 

 ila and the silk glands of Malacosomma americana were treated with 

 H'^ leucine for 5 to 15 seconds (Zalokar, 1960a, and unpublished 

 results). After fixation and elution of unincorporated precursors, 

 autoradiographs showed high incorporation inside the cytoplasm 

 and practicallv none inside the nucleus (Fig. 4-2). The nucleus 

 became labeled onlv later and certainly could not contribute to the 

 cytoplasmic labeling. In silk glands treated with H'" glycine, the 

 main component of silk fibroin, the nucleus remained practically un- 

 labeled still after one hour, while the cytoplasm and the excreted 

 silk were heavily labeled. 



There can be no doubt that proteins are made inside the nucleus 

 also (Lang et al, 1953; Allfrey and Mirsky, 1957; Allfrey et al, 

 1957). Is this due to the same mechanism as the protein synthesis 



