GENETIC CONTROL 17 



It could be for instance a segment of genetic material which controls the 

 structure of the smallest piece of a protein which can be made independ- 

 ently and which can later be integrated into the finished protein. 



In most cases, all the mutants affecting the synthesis of a specified 

 enzyme are found to be in the same cistron. This means that the genetic 

 information relative to these enzymes cannot be divided, it must be used in 

 one piece. It looks as if most enzymes could not he made piecemeal. 



However, a few very interesting exceptions to this rule have been found 

 recently (Giles et al, 1957; Woodward et al., 1958, 1959; Case and Giles, 

 1958, 1960). Heterocaryons obtained from two mutants both lacking the 

 same enzyme, glutamic dehydrogenase, as the result of closely located 

 mutations showed complementation i.e. partial recovery of the enzyme 

 production (Fincham and Pateman, 1957; Pateman and Fincham, 1958; 

 Catcheside and Overton, 1958). This means that the two mutations are 

 located in two different cistrons. However, the enzyme level in the hetero- 

 caryon reached at most 25 per cent of that in the wild strain. The low level 

 of glutamic dehydrogenase could possibly be explained by assuming that 

 the enzyme is composed of two pieces, namely two polypeptide chains A 

 and B which are made independently, each under the control of one 

 cistron. In the heterocaryon, the nuclei of one type would provide for the 

 synthesis of normal A chains and abnormal B chains, whereas the other 

 type of nuclei would cause the formation of abnormal A and normal B 

 chains. Random association of A and B polypeptides would produce about 

 25 per cent of all good AB protein molecules and 75 per cent of protein 

 molecules in which at least one of the constituent chains is bad. 



Association of polypeptides into a finished protein can indeed occur 

 spontaneously in vitro. Haemoglobin for instance can be made to dissociate 

 into two a and two j8 chains. Under suitable conditions, these polypeptides 

 will reassociate correctly and reconstitute normal haemoglobin molecules 

 (Itano and Singer, 1958, 1959; Itano and Robinson, 1959; Jones et al, 

 1959; Vinograd et al., 1959). Studies by Hunt (1959) on foetal haemo- 

 globin support the idea that the a and j8 chains might be made separately 

 under the control of two functionally independent pieces of genetic 

 material. 



That complementation in certain heterocaryons occurs by association 

 of polypeptides has received a convincing demonstration in Woodward's 

 experiments (1959). By mixing extracts of two auxotroph mutants of 

 Neurospora both lacking adenylosuccinase, some enzyme activity was 

 restored. As the mixing took place at a low temperature under such conditions 

 that no metabolic process could take place, it must be admitted that an 

 active enzyme has been formed by spontaneous association of at least two 

 parts, one coming from each auxotroph, by some process akin to the 

 reassociation of a and ^ chains in the case of haemoglobin. The formation 



