THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 315 



ultracentrifugal studies which showed that these objects likely were aggrega- 

 tion products of the A-protein. Why the degradation seems to give a frag- 

 ment one-third the size of TMV is still unknown, but these results would 

 suggest that chemical studies may reveal heterogeneity of structure along 

 the rodlike particles. The one-third fragment was shown to form dimers 

 which were resistant to alkaline degradation, as though the resistant end of 

 each of the one-third particles was on the outside in the dimer (Harrington and 

 Schachman, 1956). Recently Lauffer et al. (1958) found that the aggregation 

 of the A-protein at neutral pH was affected markedly by temperature, with 

 the unaggregated form predominant at 5°C and the aggregates being formed 

 at 25°C. It is important to note that the studies of the aggregation of small 

 units to form either doughnut-like objects or rod-like particles are confined 

 to the sub-units of molecular weight about 10^. The chemical sub-unit is 

 much smaller than this, with a molecular weight of about 18 X 10^ (see 

 Ramachandran, 1958), but very little physical chemical work has been done 

 with this material. The ribonucleic acid isolated from TMV has been the 

 object of numerous investigations (e.g. Cohen and Stanley, 1942; Hopkins 

 and Sinsheimer, 1955; Gierer, 1958). The physical properties depend upon 

 the ionic environment during the measurements and upon the method 

 employed for the degradation of the virus. The more recent studies are 

 consistent with the view that there is one RNA molecule with a molecular 

 weight of about 2 X 10*^ in each virus particle. These molecules may them- 

 selves be composed of sub-units held together by weak covalent bonds or by 

 secondary forces. Information on this subject is currently being sought, and 

 it is likely that soon the older data which give molecular weights of about 10^ 

 will be combined with the results of more recent investigations thereby 

 famishing the basis for a model which accounts satisfactorily for all of the 

 data. The results of the X-ray inactivation of TMV (Buzzell et al., 1956) have 

 yielded a target volume about 1/12 the volume of the anhydrous particle. 

 These workers suggested that the sensitive target is the nucleic acid. 



It has been possible to locahze the RNA withui TMV by the application 

 of X-ray analysis. From a comparison of native TMV with polymerized 

 A-protein on a radial distribution plot the position of the phosphorus of the 

 RNA is fomid to be 40A from the axis (Caspar, 1956b). The same kind of plot 

 also shows that there is a central tube of 40A diameter, and that the extreme 

 diameter of the particle is 190 A. The reconciliation of this diameter with the 

 150 A figure for packed arrays is accomplished by assuming that in the packed 

 configuration the rods fit together, groove to ridge, as would a number of 

 aligned, packed machine screws of the same pitch. There is certainly X-ray- 

 dense material within the region of the RNA. The grooves are now beheved to 

 be so deep that, if it were not for the RNAase resistance of the virus, it would 

 be tempting to conclude that the virus is assembled by wrapping the RNA 



