TEMPORAL REGULATION IN CELLULAR PROCESSES 237 



in the cell— as distinct from the phase-shifting measurement. We 

 have used as a measure of the effectiveness of different compounds, 

 their effects upon growth rate, cell division, and the incorporation 

 of radioactive sulfate, pliosphate, and caribou dioxide. We have not 

 examined in these ways all the compoimds wliich we have tested for 

 pliase-shifting potential. 



FUDR is structuralh similar to thvmidine monophosphate, and 

 one of its known effects is upon DNA svnthesis, by virtue of the fact 

 that it blocks the conversion of deoxyuridine monophosphate to 

 thymidine monophosphate (Bosch et al., 1958). With Gomjaulax 

 we have not proved directly that FUDR has this effect, but we have 

 shown that it has effects which would be expected as a consequence 

 of this blockage. At concentrations of 10 ~^, FUDR abruptly stops 

 cell division and growth. Moreover, it has a marked inhibitory 

 effect upon tlie rate of inorganic phosphate incorporation at a con- 

 centration of 8.5 X 10 ~'"' M. However, at the same concentration 

 an effect upon the rate of light-dependent C^^O^ incorporation is 

 evident onlv after a lag period of about 12 hours, as might be 

 expected. 



We can also stop growth and division by maintaining the light 

 intensity at a low value. Actually, in all the experiments that we 

 are describing, there was little or no growth. However, only when 

 we add a specific inhibitor can we be more certain that the metabo- 

 lism or turnover in a particular biochemical system is being arrested. 



We undertook to investigate protein synthesis and its possible 

 relationship to the problem of rhythmicity. In view of the fact that 

 the activity of the enzvme lucif erase varies with time of day ( Has- 

 tings and Sweeney, 1957a), we hoped that by blocking or acceler- 

 ating protein synthesis we might alter the phase of the clock. We 

 have used numerous compounds: amino acids, analogs of amino 

 acids, inhibitors of protein synthesis, and intermediates in pathways 

 involved in protein synthesis. Many compounds are without effect. 

 Chloramphenicol does have an effect, but it does not appear to be 

 specifically upon protein svnthesis. At a concentration of 6 X 10 ~^ 

 M chloramphenicol, sulfate is incorporated at approximately 50 per 

 cent of the normal rate, and cell division and growth stop sharply. 

 CO2 fixation drops to about 15 per cent of the control rate. No phase 

 shift occurs (Fig. 9-6). This was repeated on two occasions. 



Certain other compounds, some of which are shown here, have 

 been found to cause very pronounced and stable phase shifts. The 



