240 



CONTROL MECHANISMS IN CELLULAR PROCESSES 



In Fig. 9-8 are illustrated experiments using several kinds of 

 compounds. Two of the most interesting are CMU and DCMU, 

 mono- and dichlorophenyl dimethyl urea. These are substituted 

 urea compounds developed as herbicides by Du Pont (Bucha and 

 Todd, 1951). Their most important property is that they specifi- 

 cally inhibit photosynthesis (Bishop, 1958). With Euglena, for 

 example, heterotrophic growth in the dark occurs perfectly normally 

 even in high concentrations of CMU or DCMU. But in the light, 

 even in the presence of an organic carbon and energy source, growth 

 is markedly inhibited. 



kine+in 3x10"* M 

 ure+hane S'lO"" M 



chlorpr. SMO"'' M 

 gibberellin 10 M 

 DCMU 5«I0-'' M 



CMU S'lO-* M 



flight 

 -♦control 



18 



05« 



•!8 



06 



06 



06 



06 



Fig. 9-8. Kinetin, urethane, chlorpromazine, gibberellin, di- and mono- 

 chlorophenyl dimethyl urea have relatively little effect upon the phase of the 

 rhythm. (See text and legend to Fig. 9-5 for details.) 



Both of these compounds strongly inhibit light-dependent C^^Oi 

 uptake in Gonyaidax; at a concentration of 10"^ M DCMU, the rate 

 is less than a few per cent of normal. We have not observed any 

 specific phase-shifting effect by these compounds with either pulsed 

 or continuous treatment. Moreover, and this is probably an even 

 more significant result, cells inhibited with DCMU give the normal 

 phase shift when pulsed with light. 



In a photosynthetic form it is significant to exclude photosynthe- 

 sis from an active role in clock function, particularly in view of two 

 facts: (1) there is a diurnal rhythm of photosynthesis and (2) action 

 spectra studies have indicated that chlorophyll and also carotenoids 

 are the photosensitive pigments involved in phase shifting (Has- 

 tings and Sweeney, I960). Thus, photosynthesis may be blocked 



