lieved to act mainly on the nucleic acids, RNA 

 and DNA in many biological systems. In our 

 experiments we've watched the morphological 

 development of the plants after UV irradiation 

 of the spores. Almost independent of the dose 

 of ultraviolet light, these plants will go through 

 the early stages of development to a point 

 approximately 60% of the way through the 

 normal life cycle and then they'll essentially 

 stop. The development up to that point seems 

 to be quite norm^ except that it proceeds more 

 slowly, the higher the UV dose. We're currently 

 starting studies of nuclear counts, and RNA and 

 DNA content, after exposure of the spores to 

 ultraviolet light. In the OC form, the point 60% 

 of the way through the life cycle, at which 

 growth stops after UV, corresponds to some 

 of the other changes that Cantino has just talked 

 about. This suggests that some critical changes, 

 blocked by earlier UV irradiation, are going on 

 at this point in the life cycle. This point is 

 approximately the point of no return from OC 

 to RS. By that I mean there is a point in the OC 

 development beyond which you can't add bicar- 

 bonate and make it go to the RS form. 



CANTINO: That's right, it is about the 

 point of no return for OC cells. I want to add 

 that this applies only to synchronized plate cul- 

 tures of OC cells. The point of no return for 



OC cells grown in liquid cultures is another 

 matter. 



DEERING: These experiments were on syn- 

 chronized plate cultures. After the UV, the 

 development of these plants stops roughly at 

 the "point of no return." This might indicate 

 that some damage to the RNA or DNA (or both) 

 in the spores has stopped them from supplying 

 information necessary to get them beyond a cer- 

 tain point in development. This might indirectly 

 implicate the involvement of nucleic acids in 

 this change. 



CANTINO: There is one thing I didn't men- 

 tion in respect to the data for RNA shown in 

 Fig. 19. Firstof all, because of the sharp changes 

 in base ratios and in the quantities of RNA insol 

 and RNA Naci-soi , the conclusion seems in- 

 escapable that a large amount of RNA turnover 

 is occurring after the point of no return in an 

 RS cell. (If comparable studies had been done 

 with OC cells, the results might bear directly 

 upon Dr. Deering's comments). Secondly, at the 

 point of no return there occurs a sudden and very 

 fast rise in the free CMP acid pool of the cell, 

 but this change is not counterbalancedby a com- 

 parable rise in the pools of AMP or UMP. The 

 CMP is reutilized, however, as the RS cell 

 proceeds beyond its point of no return. 



References 



1. E. C. Cantino and M. T. Hyatt. Leeuwenhoek 

 ned. Tijdschr. 19, 25 (1953). 



2. E. C. Cantino and J. S. Lovett. Adv. in Mor- 

 phogenesis 3, 33 (1964). 



3. E. C. Cantino and E. A. Horenstein. Myco- 

 logia 48, 433 (1956). 



4. E.C. Cantino. Phytochemistry 1,101 (IQ&l). 



5. E. C. Cantino and M. T. Hyatt. J. Bacterial. 

 66, 712 (1953). 



6. E. C. Cantino. In "11th Symposium of the 

 Society for General Microbiology," 1961, 

 p. 246. 



7. J. S. Lovett and E, C. Cantino. J. Gen. Micro- 

 biol. 24, 90 (1961). 



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