30. PHOTOCHEMISTRY OF NUCLEIC ACIDS 81 



gested that this may he useful in action spectra studies to distinguish be- 

 tween nucleic acid and protein effects. 165 



Since nucleic acids in living cells exist predominantly in the form of nucleoproteins, 

 it would obviously be desirable to learn something about the effects of irradiation on 

 isolated nucleoproteins. Some early observations 121 on DNA-histone in salt solutions 

 of different concentration suggested little difference in behavior from that of free 

 DNA. An investigation has also been made of the photochemical behavior of nucleo- 

 protein from sea urchin sperm in salt solution, which gradually loses the ability to 

 form long threads on transfer to water. 166 It is nonetheless difficult to draw any def- 

 inite conclusions from these findings in the absence of some criterion as to the degree 

 of dissociation of the nucleoproteins in the salt solutions in which they were irradi- 

 ated (Section IV, 5; see also Volume I, Chapter 10). 



More definite information on this subject is forthcoming from photochemical stud- 

 ies of T.MV and infectious RNA from TMV, from which it appears that the protein 

 portion of a nucleoprotein complex may profoundly affect the behavior of the nucleic 

 acid fraction. 



It appears to the writer that, apart from viruses, studies on the behavior of nucleo- 

 proteins are likely to be more informative in those instances where the nucleic acid 

 portion exhibits biological activity; the best model for this purpose at the present 

 time being bacterial nucleoproteins containing transforming DNA. 



2. Biological Inactivation 



Although an understanding of the chemical and physicochemical modi- 

 fications resulting from irradiation of nucleic acids is of unquestionable 

 value in attempts to interpret photobiological phenomena, it is nonethe- 

 less obvious that the usefulness of such information may be considerably 

 enhanced if combined with quantitative data on biological inactivation. 

 The possibility of such studies has, however, hitherto been limited by the 

 fact that the known biological activities of free nucleic acids, susceptible 

 to quantitative study, are relatively few. 



a. Infectious RNA from TMV 



The inactivation of infectious RNA from TMV is first-order 167 " 170 to a 

 residual activity of about 1 % which is highly resistant to further irradia- 

 tion. 168 The reaction has been found to proceed with a quantum yield of 

 about 3 X 10~ 4 assuming a molecular weight of 2.5 X 10 6 . Particularly 

 instructive is the fact that following complete inactivation there is no 

 reduction in viscosity or intrinsic viscosity, but only a less than 1 % drop 

 in extinction of the maximum at 260 nux which is about that to be ex- 



165 R. Setlow, Advances in Biol, and Med. Phys. 5, 37 (1958). 



166 A. C. Giese, Anat. Record 99, 116 (1947). 



167 A. Siegel, S. G. Wildman, and W. Ginoza, Nature 178, 1117 (1956). 



168 A. D. McLaren and W. N. Takahashi, Radiation Research 6, 537 (1957). 



169 A. Siegel and A. Norman, Virology 6, 725 (1958). 



110 F. C. Bawden and A. Kleezkowski, Nature 183, 503 (1959). 



