128 M. N. MEISSEL, E. M. BRUMBERG, T. M. KONDRATJEVA AND I. J. BARSKY 



obtained by De Bruyn and co-workers (1953) Morthland et ah. (19o4), Bradley 

 and Felsenfeld (1959) and others.! 



KUZiN : How soon after the irradiation could the shift of the fluorescence maxi- 

 niiun be shown? 



MEISSEL: Practically immediately following exposure. 



GRAY : I would like to ask about the shift of the ultraviolet autofluorescence 

 maximiun. which occurred following irradiation with 25,000 r. In order to obtain 

 this effect is it necessary to use so high a dose, or could you observe it at lower 

 dose levels? 



Meissel: Unfortvmately we have not yet studied the dose dependance of the 

 effect. But just after irradiation beginning with doses of 25,000 r this effect was 

 observed, while at the doses of the order of several thousand roentgens we did 

 not observe it. 



PASSYNSKY: What is the relative role of the chemical and physical alteration of 

 the nucleic acids or nucleojiroteins in the ultraviolet fluorescence changes? Do 

 similar changes occiu- in piu-e RNxV and nucleoprotein preparations or do they 

 become manifest only at the eel hilar level? 



MEISSEL: First may I answer the second question? At present we have at our 

 disposal only preliniinary observations regarding ifltraviolet fluorescence of the 

 isolated nucleic acids, including RNA. Ribonucleic acid isolated from irradiated 

 micro-organisms shows earlier changes in the pattern of its ultraviolet fluor- 

 escence, induced by short wave viltraviolet light, than does RNA from non- 

 irradiated cells. 



As to the first cjuestion. Here, probal)Iy, both types of change occm-. It is 

 difficvilt to say which are prevalent, but i:)robably owing to configuration changes 

 of DNA molecules during its denaturation. an additional capacity for binding 

 acridine orange appears. Initial depolymerization processes are also evidently 

 liberating some free bases which are capable of binding acridine orange; this is 

 one possibility; another possibility lies in the fact that a changed DNA configura- 

 tion or the transition from two-helix to one-helix form brings about more active 

 formation of dimer and trimer complexes of the acridine orange. 



PASSYNSKY: But these structural changes require some time, while in your 

 experiments they were observ^ed inunediately. 



MEISSEL: "Immediately" is but relative. Since it is difficult to make obsei'vations 

 directly under the beam, there was a lapse of several minutes and sometimes 

 10, 20 or even more minutes between irradiation and the recording of the effect. 



TUMERMAN : You have pointed out that most probably the physical cause for the 

 changes in the fluorescence spectrum of the acridine orange consisted in differ- 

 ences in its concentration. A phenomenon is known of a fluorescence spectrum 

 change in the long wave range occurring with the increase of the concentration, 

 i.e. with the increased interaction between molecules of a substrate and the stain. 

 Could the possibility be ruled out that configuration changes allow the develop- 

 ment of trij^let long term luminescence? In the preliminary exiDeriment we 



t De Bruyn, P. P. H., Farr, R. C, Banks, H.. and Morthland. F. W. (1953). 

 Exp. Cell Res. 4, 174. Morthland. F. W., De Bruyn, P. P. H., and Smith. X. H. (1954). 

 Exp. Cell Res. 7, 201. Bradley, D. F., and Felsenfeld. G. (1959). Nalure, LoixL 184, 

 1920. 



