REGULATION 137 



far-reaching genetical studies. The uniformity of fundamental biochemical 

 mechanisms in all kinds of living cells allows one to assume that the mechan- 

 isms involved in the control of ^S-galactosidase synthesis are probably of wide 

 significance. It would, however, be misleading to suppose that these solve all 

 the problems of regulation and that no other mechanisms can be operating. 

 Other types of processes may be superimposed upon those described 

 above. For instance, evidence that enzyme transformation occurs in certain 

 cases has been presented recently. 



4. Induced Transformation of an Enzyme 



In anaerobically grown yeast, oxygen induces the formation of the 

 enzymes and electron carriers of the respiratory chain (Ephrussi and 

 Slonimski, 1950; Slonimski, 1953, 1954, 1955) and of several peroxidases 

 (Chantrenne, 1954, 1955; Chantrenne and Courtois, 1954). Complete 

 synthesis of the cytochrome-c protein during this adaptation has been 

 demonstrated (Yeas and Drabkin, 1955), but more complex processes also 

 occur. Changes in the absorption spectrum of the yeast cells suggest the 

 replacement of certain haemoproteins by others (Slonimski, 1953; Yeas, 

 1956). This might be due to protein turnover, for at variance with bacteria, 

 yeast readily makes new enzymes in the absence of any external source of 

 nitrogen, at the expense of the intracellular pool of free amino acids and of 

 proteins which can be degraded to amino acids (Halvorson, 1958). How- 

 ever, the formation of lactic dehydrogenase during respiratory adaptation 

 seems to involve a direct transformation of enzymes. 



In anaerobically grown yeast, there is a lactic acid dehydrogenase which 

 catalyses the dehydrogenation of D-lactic acid only; on the contrary, after 

 respiratory adaptation, an enzyme can be isolated from yeast which oxidizes 

 the L-isomer of lactic acid specifically. Meanwhile, the anaerobic enzyme 

 almost completely disappears (Slonimski and Tysarowski, 1958; Labeyrie 

 et al., 1959; Nygaard, 1959, 1960). This replacement of an enzyme by 

 another related enzyme is not due to the destruction of one protein while 

 the other is being made ; there is evidence that the anaerobic D-lactic acid 

 dehydrogenase is transformed into the L-enzyme normally present in 

 aerobic cells (Kattermann and Slonimski, 1960). Amino acid analogues 

 preclude the formation of active catalase, cytochrome oxidase, cytochrome 

 peroxidase, presumably by being incorporated into the proteins ; but these 

 analogues do not interfere with the replacement of the anaerobic lactic 

 dehydrogenase by the aerobic type of enzyme. Stereospecificity is not the 

 only diff'erence between the two enzymes : the aerobic type reduces cyto- 

 chrome-c, the anaerobic does not. Thus the transformation involves the 

 acquisition of cytochrome reductase activity beside the change in stereo- 

 specificity. An intermediary stage in the transformation was detected: the 

 anaerobic enzyme which is specific for D-lactic acid first acquires the 



