546 ERIK ZEUTHEN 



30 min. which is represented by the distance from EH to the most left 

 intersection point in Fig. 8. This figure also suggests that by the time EH 

 the synchronized cells contain some "division protein", though not 

 enough for a division. How much they contain we cannot say. Our graphs 

 only compare times measured in two different ways. Linearity of a curve 

 which slopes 45° means that time spent on synthesis before the addition 

 of /)-FPhe is fully lost at the time w^hen the analogue is again removed. 



Returning to the experiments recorded in Fig. 5 it is observed that 

 both curves I and H ascend towards the right in a non-linear fashion. In 

 those experiments cells in proteose-peptone were treated for 20 min. with 

 16 mM p-FPhe. This combination of growth medium, exposure time, and 



30 60 



Age, in minutes after 



EH. 



Fig. S. "Set-back" against time since Ell when the cells were immersed into 

 08 mM /)-FPhe in inorganic medium. The separate curves represent various 

 exposure times as indicated. The dashed parts of the curves are extrapolations. 

 [Error in Figure: for 30 min., read 35 min.] 



inhibitor concentration failed to give maximal set-backs as defined for the 

 washed cells (Figs. 6 and 7). For this reason linearity of the rising limbs 

 of the curves in Fig. 5 could not be expected. 



Other amino acid analogues have been tested for comparison with 

 /)-FPhe. Ethionine, yS-thienylalanine and canavanine seem to act in the 

 same way as p-FPhe. So do chloramphenicol and puromycin, well- 

 established inhibitors of protein synthesis. It is the results with the two 

 antibiotics which have induced us to ignore the possibility of a significant 

 synthesis of false proteins in the presence of an amino acid analogue. This 

 possibility is under further study. 



5. Conclusions 



We have no evidence from the study of the synchronous cells that 

 before every division Tetrahymena must produce RNA. With regard to 

 DNA we have evidence that no new synthesis of this substance needs take 



