MECHANISMS OF GENE ACTION 



287 



come apart after cleavage of the peptide bond hut remain associated, and 

 the enzyme remains fully active. By mild acid treatment, the parts can 

 be separated, and both of the fragments are enzymatically inactive. If 

 these fragments are then mixed, they recombine in equimolar amounts 

 to reconstitute the active enzyme. It is possible that some genetic con- 

 trol devices of the cell employ analogous mechanisms, in which the addi- 

 tion or removal of a short peptide sequence determines enzyme activity. 

 Only isolation and analysis of the enzyme in question could test this 

 proposal. 



Information of the greatest importance about protein structure is com- 

 ing from the incredibly complex and beautiful analysis of the tertiary 

 structure of hemoglobin by means of X-ray crystallography. In these 

 studies, atomic structure is observed by means of X-ray diffraction pat- 

 terns which provide the raw data for calculating relative electron densi- 



r^ 



Treat with 

 subtilizin 



u 



RNA-ase A 

 100% active 



which breaks 

 one particular 

 peptide bond 



RNA-ase S 

 100% active 



Mix 1:1 



^ 



RNA-ase S 

 100% active 



FIGURE 10.7. Spontaneous reaggregation of two inactive fragments of ribonucleose 

 restoring full enzymatic activity (after Richards, 1958, Proc. Nafl. Acad. Sc. Wash., 

 44:162). 



