544 



ERIK ZEUTHEN 



shows equally short delays of division i when exposure is immediately 

 after EH. However, for many time points in the cell's cycle the delays of 

 division are much longer than the time for which the cells were in contact 

 with /)-FPhe. 



mM p-FPhe. 



Fk;. 6. " Set-back" against concentration of ^-FPhe in inorganic medium. The 

 exposure is always for 20 min. Crosses: Division i — Cells immersed at 25 min. 

 after EH. Circles: Division 2 — Cells immersed at 55 min. after EH. 



A complicating factor in the experiments described is in the complexity 

 of the growth medium. For cells (" washed cells ") in the inorganic medium 

 the extracellular pool of amino acids is nil and the sensitivity of the cells 

 to amino acid analogues is much increased. For the washed cells we have 



60 90 

 Minutes of exposure to p-FPhe. 



Ekj. 7. " Set-back" (ordinate) as a function of the duration (abscissa) for which 

 exposure is made to o-8 mM p-FPhe in inorganic medium. The curves are for 

 different times after EH of beginning treatment with /)-FPhe. EH is the time when 

 the last temperature shock ends. 



been able to define combinations of concentrations (Fig. 6) of /)-FPhe and 

 of exposure times (Fig. 7) w^hich give maximal delaying efi^ects. 



Figure 6 show^s the relation between response of the washed cells and 

 concentration of ^-FPhe added at 25 (crosses) and 55 (circles) min. after 

 EH. Exposure is for a standard time of 20 min. The analogue is removed 

 by four washings with the inorganic medium. Washing is only complete if 

 I mM DL-phenylalanine is added. So this was done. The response is 

 represented by the "set-back", w^hich is equal to the delay of the division, 

 minus the time for which the cells were in contact with the inhibitor. The 

 crosses represent "set-backs" for division i, the circles relate similarly to 

 division 2. For both divisions maximal effects are obtained with o- 16 mM 



