Letter from Rosalind Franklin to H. Fraenkel-Conrat
During her 1954 visit to the United States, Franklin met or renewed acquaintance with many virus researchers, who were able
to send samples of their virus material and brainstorm with her on her x-ray diffraction findings. Heinz Fraenkel-Conrat at
the UC Berkeley Virus Laboratory was especially helpful, sending a variety of heavy-atom substituted TMV preparations. As
Franklin noted in this letter, it was Fraenkel-Conrat's mercury-substituted TMV that enabled her to determine the location
of the RNA in the virus.
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1956-02-21 (February 21, 1956)
University of California, Berkeley
Original Repository: Churchill Archives Centre. The Papers of Rosalind Franklin
Reproduced from the Franklin Collection at the Churchill Archives Centre with the permission of the copyright holder.
I enclose two short manuscripts by Don Caspar and myself, which we are sending to "Nature". You may also have seen
a brief summary which I recently sent to Professor Stanley, describing the same results.
As you will see, it is your Hg-TMV which has made it possible to determine the location of the RNA in TMV. I think there
can be little doubt that the RNA backbone chain(s?) lie on a radius close to 40 A. It is not yet possible to decide whether
there is a single RNA molecule following the line of the main protein helix or 15 molecules running along coaxial helices
of angle about 45 degrees. If anything, the X-ray evidence favours the former, though this is obviously an awkward model
from other points of view.
In my last letter to you I said that there were 46 protein units in the 69 A period. I am now inclined to think that the
number is 49 rather than 46, which makes the molecular weight situation a little easier, though still awkward if the particle
weight is as high as 50 million. In any case there is no evidence left for the earlier suggestions of 31 or 37.
The substance which would help more than any other in the next stage of the work - the determination of the chain direction
and molecular structure of the RNA - is your mercury-substituted repolymerised protein. Unfortunately I was not successful
with what you sent me. I obtained only one specimen which did not depolymerise when orientated and that one was good enough
only for qualitative measurements. If you ever have a few more milligrams of this substance to spare I should be extremely
grateful for them. If you do send me some, would you please specify exactly what buffer solution should be used with it,
as I think it might be best for me to re-suspend it and try to orientate it immediately after centrifuging. Alternatively,
would it be better for you to send it in the form of a solution, for me to centrifuge? (Originally I asked yon to send everything
in the form of pellets because I had no centrifuge available, but I have recently acquired a centrifuge).
Having determined the radius on which the Hg lies, and the magnitude of its contribution to the equator in Hg-TMV, I can now
use the comparison of TMV and Hg-TMV to give the phase of the intensity maxima on the layer-lines of TMV. If I were able
to do the same thing for the repolymerised protein by comparing it with Hg-protein I should be in a position, in theory at
least, to determine the phase and magnitude of the RNA contribution on the layer-lines from a comparison of the TMV and protein
data, and hence to calculate a Fourier for the RNA. All this would be a long job, but, I think, well worth while.