48 THE BIOSYNTHESIS OF PROTEINS 



of the 40 S compound and enzyme synthesis (Wade and Morgan, 1957; 

 Bowen et al., 1959). Protein synthesis is so fast in growing bacteria 

 (Roberts, 1957) that detailed kinetic studies on the incorporation of amino 

 acids into bacterial fractions comparable to the studies which were so 

 successful with rat liver, at first failed to show any difference in rate of 

 protein synthesis between ribosomal and soluble proteins, and many 

 workers thought that ribosomes in bacteria are of no great interest and 

 even that their RNA might be a waste product. Finally, McQuillen et al. 

 (1959) clearly estabhshed that when radioactive methionine is added to 

 exponentially growing E. coli, the radioactivity of the 80 S ribosomes is 

 buih up to saturation within 5 sec, perhaps less, and that it disappears 

 equally rapidly when the tracer amino acid is diluted out by addition of 

 non-labelled methionine. The lost radioactivity appears in soluble proteins. 

 The substance which is rapidly labelled in the microsome has the chemical 

 properties of a polypeptide and it behaves like a precursor of soluble 

 protein. Beside this transient protein which is continuously chased by 

 nascent molecules, another ribosome protein forms more slowly; it is a 

 permanent constituent of the ribosome structure the synthesis of which 

 reflects ribosome formation in the growing bacterial population. 



Ribosomes can now be regarded as an essential piece of the protein 

 forming system in bacteria and in the cytoplasm of mould, plant and 

 animal cells. It is quite possible that no protein synthesis takes place except 

 on ribosomes, for particles which behave like ribosomes have been isolated 

 from nuclei (Frenster et al., 1960), and there are indications of their being 

 responsible for protein synthesis in these organelles as well. The existence 

 of ribosomes within mitochondria, however, is not quite clearly established, 

 although mitochondria do make proteins (see p. 53). 



C. PROTEIN SYNTHESIS IN ISOLATED CELL 

 FRAGMENTS AND ORGANELLES 



1. Protein Synthesis in Enucleate Cytoplasm 



It is clear from the experiments which have been reported above that 

 the cytoplasmic ground substance is the main site where proteins arise in 

 most animal cells. On the other hand, it is well established that the primary 

 structure of proteins is controlled by the nuclear genes. One then wonders 

 how immediate is the nuclear control upon protein synthesis. 



In all the in vivo experiments that have been considered so far, protein 

 synthesis took place in the intact cell ; and even though the cytoplasm was 

 the first site of protein appearance, the nucleus was always present, and one 

 may wonder to what extent it was involved in the process. This problem 



