CELL DIVISION AND PROTEIN SYNTHESIS 545 



/)-FPhe and with concentrations above this. We note once more (cf. Fig. 5) 

 that when^-FPhe is added (at 55 min.) later than at a critical time before 

 division i this division is not delayed. This is not because a permeability 

 barrier to p-FFhe is established at this time. If it were, division 2 would 

 not be delayed, which it is. 



Figure 7 shows the relation between exposure time (abscissa) and "set- 

 back" (delay of division, minus the varied time for which exposure to the 

 drug is made) of synchronized, washed Tetrahymena cells dumped into 

 the inhibitor (o-8 mAi/)-FPhe) at defined times after EH. Separate curves 

 are shown for cells which are immersed at 5, 15, 25, and 35 min. after EH, 

 thus at intervals of 10 min. The maximal set-back takes time to develop 

 but it is nicely defined in each case. It becomes roughly 10 min. longer 

 for every 10 min. by which we postpone the addition of the analogue. 



From Fig. 6 we learned that almost the same effect is obtained whether 

 exposure for 20 min. is to o-i6 or to 4-0 mM /)-FPhe. We shall assume 

 that a given intracellular level of ^-FPhe is attained the earlier the higher 

 is the external concentration of the analogue. Then, with o-8 mM outside 

 concentration the intracellular concentration of the analogue should reach 

 a maximally inhibitory concentration in a fraction of the 20 min. for which 

 (in Fig. 6) exposure is made. If this is so, then the curves of Fig. 7 do not 

 measure penetration rates of ^-FPhe. A different interpretation is based 

 on the idea that^-FPhe penetrates the cells fast enough to produce a quick 

 block for protein synthesis. The washed cells are starving cells, so protein 

 synthesis must be from an amino acid pool which is supplied continually 

 by catabolism working on cellular proteins. The method we use can trace 

 the synthesis only of proteins which are related to division and which for 

 this reason we shall call "division proteins". If also the "division pro- 

 teins" show a turnover then they shall decay as soon as we block protein 

 synthesis with /)-FPhe. Indeed, the curves of Fig. 7 may largely represent 

 decay of "division proteins" piled before the addition of p-FFhe. 



Figure 8 is based on the data presented in Fig. 7. It relates "set-back" 

 and time of the beginningof the exposure to the analogue (o-8 mM^-FPhe). 

 Separate curves are given for the six exposure times of 5, 10, 20, 35, 50, 

 and 75 min. All curves tend to be linear so that intersection points with 

 the time axis at — 6, — 11, — 27, — 29, — 29, and — 30 min. can be defined. 

 The slope is close to 45 . Exposure for 35 min. and more gives maximal 

 set-backs (Fig. 7). In the light of our "block-and-decay " hypothesis, 

 Fig. 8 indicates that a developing store of "division protein" decays fully 

 at any time when /)-FPhe is added to stay for 35 min. or more. Irrespective 

 of when added, when the analogue is again removed, the cells are empty of 

 "division protein". As a consequence the treated cells have a standard 

 time to go before they will divide. This time is no min. and equals the time 

 which the controls take to go from EH to division i (80 min.) plus those 



VOL. II. 2N 



