precursor for ribosomal RNA, this experiment 

 can be interpreted to mean that there is very 

 little, if any, turnover of RNA into new ribo- 

 somes. It is, however, all based on this particu- 

 lar argument which we have yet to prove. 



I can summarize what I have said by point- 

 ing out that we did all this work, really, to 

 prove that our original interpretation was cor- 

 rect. From what I have just said, as well as 

 Turian's work with Allomyces (2, 7), it ap- 

 pears as though the ribosomes actually move 

 and become aggregated in some way within the 

 cytoplasm. I haven't any idea about the physical 

 mechanism for accomplishing this and we are 

 just starting to look at this stage in the develop- 

 ment of the spores in the electron microscope. 



Our results at this point suggest that new 

 ribosomal RNA - and, apparently, transfer 

 RNA - synthesis is not necessary for spore 

 formation. Can we say anything about other 

 kinds of RNA? Obviously the question of mes- 

 senger RNA arises. Does the formation of the 

 spores require production of messenger RNA 

 and is there any way that we can show evidence 

 for it? Our information on this is quite incom- 

 plete, but I would like to describe what we do 

 know very briefly. 



Actinomycin D is a very effective inhibitor 

 of morphogenesis. At 25 //g/ml, the concentra- 

 tion we have normally used, actinomycin effi- 

 ciently reduces the incorporation of uracil into 

 RNA (Fig. 16). In this experiment the actino- 

 mycin was added at the same time as the uracil 

 and even without pre- incubation it caused a 



/'^.CONTROL 

 / \ 

 / \ 



HOURS 



Fig. 17. 

 Inhibition of leucine incorporation by protein Inhibitors. 



60-80% inhibition of incorporation. It is only 

 effective on leucine incorporation if the plants 

 are pre- incubated with the antibiotic. There is 

 no figure for this, but at least 5 min of pre- 

 incubation are required for significant inhibition 

 of leucine incorporation. 



Figure 17 shows the effect of two protein 

 inhibitors on leucine incorporation, puromycin 

 and p-fluorophenylalanine (PFP). Somewhat to 

 our surprise, PFP, the lower curve, was much 

 more effective than puromycin, although this 

 could be, in part, a concentration effect. PFP, 

 however, is also much more effective in causing 

 morphological arrest (Fig. 18). 



These graphs require some explanation. The 

 experimental cultures are on the left while the 

 control cultures on the right show the morpho- 

 logical progression of the parent culture. The 

 top is for actinomycin treatment and the bottom 

 is for PFP- and puromycin-treated plants. 

 Samples were removed as a function of time 

 from a synchronous culture, placed in inhibitor, 



100 



z 

 o 



100 



a: 

 o 



Fig. 18. 



Inhibition of development vs time of treatment. Top left, 

 actinomycin; top right, control. Bottom left, p-fluro- 



phenylalanine ( ) and puromycin ( ); bottom 



right, control. 



173 



