106 L. G. AUGENSTINE 



nature of the constraints or the manner in which they arise. The obvious 

 question arises — is the unequal distribution due to unequal availabihty of the 

 amino acids or is it due to constraints imposed in the processes of synthesis, i.e. 

 by 'intersymbol influence' ?* Is the make-up of the pool of amino acids available 

 to the protein-synthesizing centers indicative of the nature of the processes 

 involved in amino acid synthesis or have these processes become adapted to the 

 peculiar demands of the proteins being synthesized? This is essentially the 

 same as looking at a collection of printer's type and asking the question, did 

 the printer select his supply of type because this particular distribution of 

 letters was all that was available to him or did he purposely purchase his 

 particular assortment because he had found that it satisfied his needs? 



The possibility that the unequal availability of amino acids in the cellular 

 pool may produce the unequal distribution does not seem likely. The experi- 

 ments of Roberts, Cowie et al. (2, 3) at the Carnegie Institution indicate that it 

 requires a five to thirty-fold excess of exogenous amino acids, such as valine, 

 leucine and isoleucine, before the incorporation of these amino acids into 

 protein is seriously affected in E. coli. In fact, once a substance has been 

 incorporated into the amino acid pool of yeast, 1000 times the normal con- 

 centration of exogenous amino acid does not affect its incorporation into 

 protein (Cowie). Although these are excellent experiments they do suffer 

 from problems of cell membrane permeability, intracellular diffusion, etc.; 

 however, they, along with numerous experiments involving amino acid deficient 

 mutants, suggest that as long as the minimum required amount of each amino 

 acid is present the frequency distribution of the amino acids in the pool has a 

 relatively small influence on the distribution of amino acids incorporated into 

 protein. 



Two methods have been utilized in searching for intersymbol influence 



in proteins. In the first (reported previously (4)), the behavior of the normalized 



n- 

 amino acid frequencies -rjj— were studied in individual proteins. The average 



normalized frequency of the individual amino acids for the twenty-six proteins 

 was tabulated. Comparing the normalized frequency for the individual amino 

 acids in particular proteins with the corresponding average value from the 

 26 proteins indicated large deviations in many cases. The gross deviations 

 were examined for correlations between pairs of amino acids, both for positive 

 and negative effects. Examination of the 26 proteins indicated that although 

 there are some correlations between the frequencies of individual amino acids 

 combined in single proteins, none was strong enough to be measurable with 

 any degree of confidence for a sample as small as 26 proteins. 



Similar examinations of the normalized letter frequencies in paragraphs 

 were investigated for significant deviations of pairs or groups of letters. Although 

 strong intersymbol influences are known to exist between letters (e.g. between 



* 'Intersymbol influence' is a term commonly used to designate sequential dependencies, 

 i.e. influences upon the identity of a particular element by neighbouring elements, which are 

 not the only types of constraints which might be imposed by a synthesizing center. It is easy 

 to imagine the possibility of unequal 'acceptability' for diff"erent symbols at individual sites on 

 a template in which the factors affecting the specifications of each location are independent of 

 the neighbors. 



