296 CELL HEREDITY 



Figures 10.12 a and 10.12/; a mixture of HhA and Hhl i.s .shown before 

 and after reassociation. There has been no change in charge distribu- 

 tion, since either pair of a-chains combined with either pair of /3-chains 

 will produce only the same two types of molecules as were initially 

 present. When HbS is added to this mixture, however, as in Figures 

 10.12c and Figures \0.\2d, a new component appears, and there is an 

 increase in the HhA peak. The equation describing the recombination 

 is written: 



+ 



[a-2^^2^r' 



There are many interesting applications of these experiments, among 

 them, a new method for determining which chain of an unknown hemo- 

 globin contains the abnormality responsible for a net charge difference 

 from HhA. With respect to complementation, the significant point is the 

 ability of a protein to undergo recombination of its constituent poly- 

 peptide chains under mild, virtually physiological, conditions. 



Might complementation also result from recombination of polypeptide 

 chains? In the tryptophane synthetase system of E. coli, any mutant 

 A will complement with any mutant B, in tests carried out by means of 

 abortive transduction. This result is not surprising, since in vitro 

 studies of the A and B components demonstrated that they could be 

 separated and recombined without loss of activity. In this system, com- 

 plementation may well result from a mechanism of recombination analo- 

 gous to that of hemoglobin. A closer comparison with the Neurospora 

 complementation systems would be afforded by studies of complementa- 

 tion within the A and B regions of E. coli, but such studies have not 

 yet been reported. 



In the ad^ region of Neurospora, ten complementation units have been 

 described. If each gives rise to a different polypeptide, then in the 

 heterokaryon, normal units could be substituted for mutant ones, recon- 

 stituting active enzyme. There are many difficulties with this interpre- 

 tation, among them the fact that the linearity of the complementation 

 map remains unexplained, as does the greater efficiency of complementa- 

 tion with increased map distance between mutants. Another proposal 

 considers complementation as the aggregation of identical units, with 

 two inactive enzymes aggregating to form one active unit. On this 

 view, the ad^ region is all one cistron making one polypeptide chain. 



