Medical Research Council

MRC Laboratory of Molecular Bioloqy
U niversify Postgrad uale Mledical Schmt
Piills Road, Cambridge, CG2 2QH
England

telephone Cambridge ( 0223 ) 4801 1

telex - 81532

7 April. 1977

Dr. F. K. C. Crick, FRS,
The Salk Institute,
Post Office Box 1809,
San Diego,
California 92112, U.S.A.

Dear Francis,

Some things which I forgot to include in yesterday's letter

and some postscripts.

$-

The histone octamer seems to be a pretty compact particle, not

very different from haemoglobin. The partial specific volume of
most proteins is about 0.73 ml/g. Haemoglobin atC.lM salt is
.719 ml/g and in 2h¶ salt is .779 ml/g. Jo Butler finds for the
octamer in 2M saic .767 ml/g. This is very much in favour of the
platysonie model since the bipartite nucleosome one would be very
open.

I enclose X-ray photos of the three main projections. 1 can't

recall whether you have them with you or not. Notice that there are
110 stroug 38 2 reflections, except in the hk0 plane.   I think this
must be the spacing between listones in the layers or turns of the
platysonie. Notice there is no/ spacing in this plane.

A

The relation of the platysome to the nucleofilament and solenoid

is still a mystery. If the platysomes are arranged with their
planes at an angle to the nucleofilament axis there is less trouble
about the mass per unit length (see enclosed photo oi a "model"
made by Michael Levitt) but the putative dyads would no longer be normal
to the axis of the solenoid if this were Kaintained. However
I rather like this type of model since this would explain the tendency
of the nucleofilament to curl and coil. This model
of course is unlike wedge-type ones, but there is "empty space"  for H1.

   A related question is the appearance of beads in electron
micrographs. Hucleof ilaments don't show any unless 331 is removed.. But
Linda Sperling has found (in some experiments she was doing on the
conditions for formation of solenoids) that if uucleofiiaments, containing H1

chloricie, one suddenly sees beatfss;whetheronestarts from nucleofilaments
or from solenoids in the presence of magnesium. My feeling is that
this represents a kind of internal precipitation 'along the length of
the nucleofilament which is reversible when the salt is dialysed out.
On discussing these observatioxs with Roger, he pointed out two things

extracted in .2 mM EDTA are put info as little as 15-30 mM sociiuril

)

si,.

G.

h

.

Dr. F. H. C. Crick -2- 7 April 1977

that might be relevant to this idea of a bead precipitated out of an
otherwise more or less continuous filament. 1) the fact that Georgiev
finds that the tRNA stripping of histones H2A and H2B goes better in
the absence of salt (i.e. in very low salt), contrary to commonsense,
and 2) whereas sheared chromatin loses its micrococcal digestion
pattern, the pattei'n isn't lost if the shearing is done in the presence
of 5 mM magnesium and LOO mP.$ sodium chloride (some unpublished
experiments of Roger and Jean). Roger suggests that facts 1 and 2
go together, implying that added salt keeps H2A and H2B compactly
on the nucleosome.

This makes me wonder whether the two layer or two turn platysome

may not represent a somewhat collapsed structure, at least on the
part of the DNA. After all DNAase I acting on chromatin doesn't
show a pause at 140 base pairs and micrococcal shows a definite pause
at 160. I wonder whether assigning 60 base pairs to the linker may
not be too pat.

Are you going to Spetsai? I am pretty sure that I intend going

this time, and I hope you can manage to get away from the Finland
meeting to attend. I shan't imagine we would have much time at
the Cold Spring Harbor meeting, and I don't think I can visit Aarhus,
hut will of course be at the Copenhagen meeting.

Yours ever,

A. Klug

Encs.

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