Mobember 2, 1976

Dr, Aaron Klug
MRC Laboratory of Molecular Biology
University Postgraduate Medical School
Cambridge CB2 2QH
England

Dear Aaron,

You ask in your retter of October 21st LAOW Z arrived at my

interpretation of yourrsuggestion for the packing of the crystals.
The argument went as follows,

   I assumed that there had to be one dyad axis. For simplicity
I chose this to be the shorter; that is, along a. Thus one nucleo-
some haditilits dyad exactly parallet to a. To bring the DNA helices
of the other two into phase with the first one and to gi e an

one needs to put their dyads roughly parallet to the first one.
(Naturally if they are exact1 parallel one gets a shorter repeat,)

might be the same as the relationship in a solenoid with about 6
nucleosomes per turn. This implies that the dyads would have an
gngle of about 60° between them. One can get this by tipping one up
30° and the other down 30Q.

arrangement which look& koughly like 6 turns in the 340 3i repeat
I then assumed that the __1__y. re ationship of these two dyads -- to each other

   I don'& accept your screwing-down arguement because I can see
no strong reason why the ncercleosoreres should be screwed down in the
crystal packing. What is necessary to give the sort of pattern you
have is that the DNA in the necleosomes should everywhere be not far
from a regular helix with about 6 turns - or so I surmise if I
have remembered the diffration pattern more or less correctly.
However, there is no reason to suppose that even the DNA inoone
nucleosome accurately follows a helix, I could have the ends sBlayed
up or splayed down, as explained in my earlier note, Clearly the
DWA "helix" in the crystal is unlikely to be completely regular or
you would have no spots on the axis.

-

   My feeling is that there are so many degrees of freedom that
however good a guess you make only an isomorphous replacement can
decide the issue.

About the density. Lubert Stryer, who is now Professor of

Anatomy at the Medical School at Stanford, says he can measure the
density of small crystals.  Basically, you pop tGm in a density
gradient and try to catch a vdew of them immediate1
scope, before they have had time to alter. +write to him
for details.  I think it most rash to try to solve the structatmle
without the densAty. Ne says, ifiddentally, he can measure
to 1 or 2% which is more than adequate.

sending a copy to Bak.
(copy enclosed). It: indeed is hollww. It shows a 300 A fibre but
very irregular and kinky.
shall have to wait and see what further pictures look like.

using a micro-

7

A short memorandum on the final fold As enclosed. I am also
         I have a picture from Bak of a gross-section
             Of course this may ba an artifact. We

   It occurs to me that you and Bak
proper discussion. Either you should
perhaps EMBO would provide the money.
exchange techniques, €or example? Or
future, one of your trips to Aarhus.
correspondeace is becoming absurd!

should really meet for a
invite him to Cambridge -
My shouldn't he come to
you should make, in the near
I feel this tri-party