SYNCHRONIZED GROWTH IN TETRAHYMENA CELLS 



151 



division can be seen to repeat itself from division one to two, and from 

 division two to three ( Figure 12 ) . Thus the synchronous divisions two 

 and three are conditioned by the synthesis of proteins, perhaps before, 

 but at least during and shortly after, the previous division. Figure 12 

 strongly suggests it is the tail of this synthesis that is blocked by the 

 cycling temperature. E.xperiments with short-time exposures to p- 

 fluorophenylalanine clearly indicate that the conditioning of a division 

 begins as early as around the time when the cell has passed the critical 

 phase before the previous division. This is our best evidence— and it 

 seems strong— that we are not simply faced with variations in tlie per- 

 meability of the cell. The analogue p-fluorophenylalanine is completely 

 antagonized by phenylalanine, only incompletely by tyrosine. 



There is, or may be, a block for the growth of the whole cell in 

 every period when the cell conditions itself for a following division— 

 that is, immediately after E.H. and before, during, and after a division 

 {cf. Figure 13, from Hamburger and Zeuthen, 1960). The possibility 

 clearly exists that the limited protein synthesis which conditions divi- 

 sion is separate from other syntheses by which the cell lays down its 

 bulk proteins. In Tetrahymena the microsomes and the "mitochondria" 

 (the latter fraction includes all particles other than the microsomes) 

 are separate protein synthesizers, according to Mager and Lipmann 

 (1958) and Mager (1960). The kinetosomes, too, are capable of pro- 

 tein synthesis (Seamann, 1960). 



Figure 12. Delay of syn- 

 chronous divisions 1,2, and 3 

 by DL p-fluorophenylalanine 

 at 8 X 10-3 M. The cells are 

 synchronized in and remain in 

 medium C {cf. Figure 4). 

 The upper curve gives the di- 

 vision index in the main con- 

 trol. The lower curves, I, II, 

 and III, give the delays of di- 

 visions 1, 2, and 3 when im- 

 mersion in the drug is at the 

 times indicated by the points. 

 Incubation immediately after 

 E.H. delays division 1. Incu- 

 bation during and after divi- 

 sion 1 blocks division 2, and 

 incubation during and after 

 division 2 blocks division 3. In 

 the drug, the cells never per- 

 form more than one division. 



I 



.c: 

 .c: 



f 



^ 



0.5 



0.0 

 40 



20 







12 3 4 



hours after EH 



