76 Martynas Ycas 



Protein Species 



Virus (48) 



. . . thr-ser-gly-pro-ala-thr* TMV (M, YA strains) 



. . . thr(thr,ala)pro-ala-thr* TMV (HR strains) 



It is possible that a mutation may suppress an amino acid determining site 

 altogether. This is indicated by the tentative finding of Akabori (quoted in 

 (41)), that the 'B' chain offish insuHn has the sequence . . . pro-lys*, as compared 

 with the sequence . , . pro-lys-ala* in cattle. 



In some cases (ACTH, TMV), two adjacent replacements differentiate 

 one homologous protein from another. It is not probable that this is due to 

 two independent but adjacent mutations, but rather that a single mutational 

 event has affected two residue-determining sites. Such a view is made plausible 

 by the work of Benzer (49). He has shown that mutations in bacteriophage 

 involve small sections of DNA, of molecular dimensions, but that these sections 

 can be of diflferent lengths. Presumably the length of the mutated section deter- 

 mines the number of residues changed in the protein. It is perhaps not too 

 sanguine to hope that eventually it may become possible to measure crossover 

 values in terms of distance in residues along a protein chain, and thus obtain 

 an estimate of the number of bases in DNA determining a single residue 

 selecting site. The present difficulties of such an approach are of course obvious 

 (50). 



It would be of interest to determine if there are any restrictions on the 

 replacement process. Restrictions might be expected on the following grounds. 

 More than one nucleotide must determine an amino acid site. If the process 

 of mutation were predominantly to change some, but not all nucleotides 

 determining a site, then obviously not all sites would be interconvertible in 

 one step. A study of any such restrictions would be of great value, since their 

 nature would depend on the coding principle and could be used to infer the 

 latter. 



Table II. Replacements Inferred from Table I and their 

 Frequency of Occurrence 



Occurrence 



3 

 2 

 2 

 2 

 2 

 2 



