Discussion 333 



from glycerol phosphate to grow. They may have other phosphatases 

 which are not sufficient in quantity or not active under these conditions 

 and may serve other purposes. 



Gutfreund: May we assume for the moment that what Sir Cyril and 

 you are talking about is what is known as alkaline phosphatase? 



Magasanik : Yes. 



Hinshelwood: No, we used glycerol phosphate for one substrate and 

 phenolpthalein phosphate for the other, and both gave the same cycle. 



Gutfreund: Did you try pH-dependence of the enzyme? 



Hinshelwood: Yes, it was used very near to the optimum. I think it 

 Mas slightly alkaline. 



King: Sir Cyril mentioned that the catalase increases where the 

 process slows down. There is a micrococcus which is resistant to very 

 high radiation dosage, and this micrococcus has many times more 

 catalase content than the ordinary micrococcus. Dr. Laser is studying 

 this at the Molteno Institute. 



Chance : I have made a similar observation on the amount and activity 

 of cytochrome a^ of log and stationary phase Torula suspensions. Early 

 log phase cells grown in a "chemostat" showed a respiratory enzyme 

 content rather low in relation to respiratory activity. The turnover 

 number of cytochrome oxidase was the highest that we ever observed. 

 There appears to be least oxidant capacity when it is most needed. 

 This is, however, compensated by the metabolic control which allows 

 an unusually high turnover number of the available enzyme. 



Siekevitz: Dr. Dean, have you done experiments to determine the 

 turnover rate of RNA during that growth cycle? 



Dean : No. 



Siekevitz : Is it broken down once it is formed in the cell? 



Dean: Towards the end of the growth cycle, when conditions are 

 becoming unfavourable, there is a tendency for the DNA to be kept 

 constant ; if conditions are adverse, a transformation from RNA to DNA 

 may occur, in an attempt to keep the DNA constant. DNA seems to be 

 synthesized from RNA in conditions of phosphorus starvation. 



Krebs : Dr. King, is it thought that the high catalase content of this 

 micrococcus you mentioned is directly connected with lack of radio- 

 sensitivity? One could test it experimentally by adding an inhibitor 

 of catalase, such as hydroxylamine, and examining the sensitivity to 

 ionizing radiations of the treated organism. Has that been done? 



King : No, this organism was first isolated in Oregon from a classified 

 project. 



