68 THE BIOSYNTHESIS OF PROTEINS 



they are not necessarily connected. Moreover, the positive correlations 

 which were occasionally observed between incorporation of precursors 

 into protein and net RNA synthesis for instance have lost much of their 

 interest since the plurality and the metabolic heterogeneity of cellular 

 ribosenucleic acids have been recognized. If a cell contains quite an assort- 

 ment of various ribosenucleic acids, data on total net synthesis of RNA, or 

 incorporation into total cellular RNA give but average values; important 

 changes involving certain RNA fractions might pass unnoticed because 

 they can be masked by other processes occurring at the same time in other 

 RNA fractions. It is clear that a direct comparison of the metabolism of 

 total RNA and total protein synthesis cannot be very informative, and one 

 should look for data on the possible connexions between RNA metabolism 

 and protein synthesis at the level of different ribonucleoprotein fractions. 

 A good start in this direction can be found in a metabolic investigation of 

 the ribonucleoprotein particles of guinea pig pancreas by Siekewitz and 

 Palade (1959). Five ribonucleoprotein fractions were isolated by fractional 

 centrifugation and compared as for the rate of in vivo incorporation of 

 adenine into RNA and of amino acids into proteins. The rates and the 

 kinetics of incorporation differ very much between the five fractions. It is 

 striking that fractions especially active in protein synthesis showed 

 practically no incorporation of adenine into their RNA, whereas another 

 fraction incorporated adenine very actively but was very poor in protein 

 synthesis. This indicates that the synthesis of protein and that of nucleic 

 acids are not associated at the level of nucleoproteins. 



A completely different approach to the problem discussed in the present 

 chapter consists in interfering specifically in various ways with the meta- 

 bolism of RNA and in watching the effects on protein formation. The first 

 attempt along this line was probably made by Jeener and Jeener (1952) 

 with Thermohacterium acidophilus. This is an exacting bacterium which 

 requires, among a score of various substances, uracil and thymidine for 

 growth. Deprivation of thymidine stops DNA synthesis and bacterial 

 division, but it has no immediate effect on protein synthesis. On the con- 

 trary, uracil starvation almost immediately stops protein formation. These 

 early observations have been confirmed by Okazaki and Okazaki (1958). 

 Similarly, in pyrimidine-less mutants of E. coli the synthesis of enzymes 

 depends on the exogeneous supply of pyrimidines, except under special 

 conditions when these are provided by the breakdown of certain RNA 

 fractions (Pardee, 1954, 1955; Earner and Cohen, 1958). The ability of 

 resting yeast to make the enzyme a-glucosidase depends on the level of the 

 free nucleotide pool, and the best way of depleting this pool is to force 

 yeast to produce proteins (Spiegelman et ai, 1955). 



Deprivation of nucleic acid precursors, in all these cases, was thus found 

 to be damageable to protein production. Conversely, the provision of 



