276 CELL HEREDITY 



the particular properties of which determine the observed mutant trait. 

 A few such systems have been described, and they are listed in Table 

 10. 1 together with a brief comparison of some of their properties. 



TRYPTOPHANE SYNTHETASE 



For purposes of illustration, we shall discuss some of the properties 

 of the tryptophane synthetase system, for which there are extensive 

 genetic and biochemical data, proxiding examples of most of the 

 phenomena which have been found in other systems as well. A unique 

 feature of this system, which makes it particularly useful for the present 

 discussion, is that mutants affecting the same enzyme have been studied 

 both in Neurospom crassa and in E. coll. 



The enzyme tryptophane synthetase (previously called tryptophane 

 desmolase, the gene symbol being td) catalyzes the condensation of 

 serine with indole to form tryptophane, and mutants have been identified 

 initially by their tryptophane requirement. Recently it has been shown 

 that the enzyme catalyzes three related reactions: (1) indole + L- 

 serine — » tryptophane; (2) indole + triose phosphate ^ indole-3-glycerol 

 phosphate; and (3) indole-3-glycerol phosphate + L-serine — > trypto- 

 phane + triose phosphate. In Nettrospora, one enzyme catalyzes all 

 three reactions, whereas in E. coli, the enzyme has been separated into 

 two components, A and B. Of these, A is active in reaction 2 and B in 

 reaction 1, but neither A nor B exhibits more than a small fraction of its 

 activity in the absence of the other. It has been proposed that A con- 

 tains the indole-glycerol-combining site, and B the serine-combining 

 site of the enzyme. 



The A and B fractions have been separated by column chroma- 

 tography. When they are subsequently mixed, full enzymatic activity is 

 restored. Altered A or B components which have been isolated from 

 mutant strains of E. coli are tested for activity by mixing them with the 

 complementary component from wild type in order to approximate 

 optimal conditions. Antibodies prepared against the enzyme can effec- 

 tively block its activity. For example, antibodies against component A 

 block all three reactions, and anti-B antibodies block reactions 1 and 3 

 but not 2. The antibody against the Neurospora enzyme blocks all three 

 reactions. 



The complexity of this system provides a number of parameters for 

 comparison of mutant enzymes with wild type, including enzyme activi- 

 ties in the three reactions, based on comparisons of rates, substrate affini- 

 ties, pH optima, activation energies, and immunological cross reactivity. 



