Physiological Mechanisms of Resistance 169 



residual enzyme activity (tested at 15°) after incubation of the 

 extract at the temperatures and for the times indicated. A 



2.0 



60 



MINUTES 



120 



Fig. 1. Effect of temperature on the stability of the pantothenate- 

 synthesizing enzyme in extracts of mutant 99-1 < and of the wild type. 

 Acetone-powder extracts of the two strains were incubated at the 

 temperatures noted. At the times indicated samples were removed, 

 cooled to 15° and tested for enzymic activity. In these tests, each tube 

 received extract from 40 mg. of acetone powder of the mutant or 

 extract from 3 mg. of acetone powder of the wild type. In addition, 

 the testing tubes contained in mM concentrations : (3-alanine 20, potas- 

 sium pantoate 20, KgATP 10, KCl 100, MgS04 10> tris-(hydroxy- 

 methyl)-aminomethane (Tris) buffer pH 8 • 5 100 ; total volume 1 • ml. 

 After incubation for one hour samples were assayed for pantothenate 

 as described in the text. 100 per cent residual activity equals 86 m[jL 

 moles of pantothenate per ml. per hour for 99-lt, 178 mjji. moles of 

 pantothenate per ml. per hour for the wild type. O-Q- = mutant 

 99-lt ; A- A- = wild type. (Reproduced by permission of University 

 of Chicago Press.) 



large difference in thermal stability is seen. Furthermore, the 

 fact that the difference resides in the enzymes themselves, 

 rather than in their environments in the respective extracts, 



