304 H. FRAENKEL-CONRAT AND B. SINGER 



acids should also contribute to our incipient understanding of the possible 

 origins of life. 



Unfortunately, I will have little definite progress to report, in our under- 

 standing of nucleic acid structure and function. One of the important questions 

 is that of size, or molecular weight. In that regard, Gierer & Schramm [2] and 

 Gierer [7] have taken the firm stand that, in their preparations, the entire and 

 intact nucleic acid complement of a virus particle of about 2 million molecular 

 weight is the only functional entity while smaller molecules occurring in all 

 preparations represent inactive degradation products. 



In contrast, preparations made in our laboratory by splitting the virus with 

 detergents, rather than phenol, appear upon ultracentrifugation to be composed 

 of molecules only about one-tenth as big. Recently we have somewhat clarified 

 this situation, in finding that the sedimentabihty of TMV-ribonucleic acid 

 (RNA) is to an astonishing extent a fimction of the ionic strength. Thus, many 

 of our preparations, when sedimented in 0-02 M phosphate, i.e. under the 

 conditions used by Schramm's group, appear as big as their preparations, with 

 «S20, w values of 20-25. I^i contrast, their type of preparations, when sedimented 

 in 0001 M or lower phosphate concentrations, appear as small as ours. The 

 important question remains to be solved, whether both types of preparations are 

 actually of relatively low molecular weight, and aggregate to the more rapidly 

 sedimenting material under the influence of the salt, or whether both are of the 

 higher molecular weight, but acquire a completely abnormal sedimentabihty in 

 o-ooi M or lower salt media, owing to charge effects. My colleagues and I favour the 

 former concept, but further experimental work is required to settie this question. 



Another line of research which is actively being pursued in our laboratory 

 should also shed Ught on this question. For, we have studied the infectivity and 

 the sedimentabihty of nucleic acid prepared from virus which had been inacti- 

 vated by sonic oscillations [5]. Such virus preparations are largely composed of 

 rods of one-third or less the original length, and usually contain about as many 

 unbroken rods as are required to account for the remaining infectivity. This 

 can be varied, not too predictably, over the range of o-oi% or less to 20^^o of 

 the original infectivity. The release of nucleic acid from such material proceeds 

 more readily than from intact virus. Surprisingly, the sedimentabihty of the two 

 nucleic acid preparations is not significanüy different, and of similar dependence 

 upon the ionic mediiun. And in infectivity the nucleic acid from 'sonicated' 

 virus is usually about one order of magnitude higher than might be expected 

 from the activity of the virus preparation serving as source material. The tenta- 

 tive conclusion from this work, which is still in active progress, is that active 

 nucleic acid subunits, weighing only a fraction of the total nucleic acid of each 

 particle, also do not extend over the entire length of the particle and thus have 

 a fair chance of remaining unbroken when the rods are fractured by sonication. 



Studies concerning the instability of TMV-RNA in salts have yielded inter- 

 esting results, but as yet no definite explanatory hypothesis. It has previously 

 been reported that the nucleic acid loses its infectivity upon incubation in o-i M 

 salts. It has now been found that this loss of infectivity is paralleled by a loss of 

 sedimentabihty (in the presence of salt). All salts tested cause this gradual 



