The Origin of Specific Protein- 1 I 



of the enzyme. Several mutant strains continued to produce the same 

 number of phosphatases but all the phosphatases were shifted in their 

 electrophoretic mobility to the same extent. These results are con- 

 sistent with the assumption that all five phosphatases are products of 

 the same gene. Mutations in this gene lead to the production of an 

 altered form of a single basic precursor molecule that is then second- 

 arily modified into the five isozymes of alkaline phosphatase. How- 

 ever, the quantitative proportions in which these isozymes are syn- 

 thesized are changed by growth in different media, and in effect, 

 therefore, these different patterns represent different states of cell 

 differentiation. 



Extensive investigations (Markert and Appella, 1961; Appella and 

 Markert, 1961 ) on the physicochemical properties of separated and 

 recrystallized LDH isozymes from beef tissues have demonstrated that 

 each of these isozymes is composed of four dissociable polvpeptide 

 chains. These chains, like those of hemoglobin, may be encoded by 

 two different genes, thus producing two kinds of polypeptides. Assort- 

 ment of these two polypeptides in all possible combinations of four 

 would yield the five LDH isozymes which are found in mouse ( Fig. 8 I 

 and beef tissues. These isozymes would then be under polygenic con- 

 trol although the cytoplasm would doubtless play a role in assembling 

 the finished tetramers in the proportions peculiar to each cell. 



However, it is certain that various isozymes may be converted into 

 alternate isozymes by mechanisms resident in the cytoplasm of the 

 cell. Kaji and collaborators ( 1961 I have shown that one of the two 

 major isozymes of yeast hexokinase may be converted into the other 

 by treatment with trypsin or chymotrypsin in the presence of glucose. 

 Presumably this conversion could also occur in the cell, although a 

 fixed ratio of the two isozymes is normally maintained within the 

 yeast cell. 



The origin of specific proteins through nucleocytoplasmic mecha- 

 nisms is also indicated by the work of Keck ( 1961 ) on two species of 

 the giant single-celled alga, Acetabularia ( Acicularia schenckii and 

 Acetabularia mediterranea) . By ingenious transplantation techniques, 

 Keck combined the nuclei and cytoplasm of the two species in various 

 proportions. Each species was shown to have a distinct acid phos- 

 phatase which could be readily identified by staining techniques after 

 electrophoresis of a cell homogenate in a starch gel. The electro- 

 phoretic mobility of the med phosphatase was considerably greater 

 than that of the acic enzyme. Mixtures of homogenates from the two 

 species showed two distinct bands after electrophones. In the graft 

 combinations shown in Fig. 11, Keck discovered that the med enzyme 



