264 UNITY AND DIVERSITY IN BIOCHEMISTRY 



This mechanism impHes the activation, in the presence of ATP, of the 

 free carboxyl group of the amino acids which go to form the new protein 

 molecule. This idea is supported by a large number of experimental obser- 

 vations. The activation of the carboxyl group is brought about by specific 

 enzyme for a particular amino acid, which first attaches it to AMP. The re- 

 sulting amino acid-AMP-anhydride is then joined to a short soluble piece 

 of an RNA chain, the whole taking place on a complementary sequence of 

 an RNA template. 



1. The ijitracellular pool of amino acids 



The existence of an intracellular pool of amino acids not combined as 

 protein, either free or in the form of amides, etc., is now generally admitted. 

 In vertebrate tissues, the amounts of these non-protein anmino acids, acting 

 as a source for protein synthesis, are small. On the other hand they are 

 much greater in the tissues of such animals as the marine crustaceans, and 

 in plant cells, in yeast cells, or in the cells of certain Gram-positive bacteria. 



The factors regulating the total amount and composition (which in 

 animals appears to be specific for each tissue of a particular species or, for 

 a given tissue, specific for each species) of the intracellular pool of amino 

 acids, are of special interest. The composition of the pool of non-protein 

 amino acids differs from the overall composition of the proteins in the cell 

 which contain it, and this has been demonstrated in widely different cases, 

 for example in animal cells and in infusoria. 



A particularly detailed study of the pool of non-protein amino acids in 

 yeast was carried out in Spiegelmann's laboratory and has given some 

 highly interesting results. It is possible to modify the composition and the 

 amount of the intracellular pool by various changes in the culture medium. 

 If the medium contains no nitrogen and contains glucose, the intracellular 

 pool in the yeast decreases in amount and this decrease affects all the 

 constituents. If the cells are then placed in a medium containing both 

 glucose and nitrogen the pool is replenished, but in a manner which differs 

 according to the nature of the source of nitrogen. If the nitrogen is provided 

 by a casein hydrolysate the pool replenishes all its constituents. If ammon- 

 ium chloride is the source the restoration is much slower and certain amino 

 acids such as methionine, threonine, proline, lysine and histidine, only 

 return to their former concentration after some considerable time. On the 

 other hand, irradiation with ultraviolet light increases the amount of the 

 pool in the yeast cell, and a considerable number of observations seem to 

 indicate that in yeast cells there is an internal mechanism for replenishment 

 of the pool which depends upon the degradation of a labile protein 

 compound. 



In Staphylococcus aureus, which has been studied from this aspect by 

 Gale, the synthesis of a large number of amino acids is not possible and 



