Dear Eleanor -
That is correct. The pseudo-sg is P6, and the structure has been
refined in this sg. The intensity difference between the strong and
weak subsets is quite significant that for most data sets, auto-
indexing routines will miss the weak spots and pick the pseudo-sg
instead. The pseudo-sg is a'=b'=90, c'=56, the true sg is a=b=156,
c=56. Note that a = a' * sqrt(3). So, the sg assignment is certain.
Owen
On May 20, 2010, at 6:33 AM, Eleanor Dodson wrote:
This looks a bit strange..
If you have a hexamer in the asymmetric unit, in P3, then that means
all symmetry copies lie in the same plane. To generate the Patterson
peak, 2/3,1/3,0 the hexamer must be centred at 1/3,1/3, z
(with symmetry equivalents 0,-1/3,z and -1/3,0,z )
I would expect ypu to have a pseudo higher symmetry SG - does
pointless
make any suggestions?
Eleanor
Jürgen Bosch wrote:
Hi Owen,
you should also make the following plot with your data:
y-axis relative intensity of off-origin peak versus x-axis
resolution cutoff used for calculation (30 Å - 4 Å in 2 Å steps).
You can have multiple cases of shifts and I would start with a
perfect hexamer first, take some random monomer and apply a perfect
sixfold, move it along the axis where it should be in your crystal
lattice (things get more complicated if you have a top/down
hexamer, so keep it simple). Now if you shift your hexamer to
2/3,1/3,0 your plot should yield a straight line and be independent
of resolution. Now start rotating the second hexamer relative to
the first clockwise with your sixfold, I would use increments of 3
degrees, which will result in 19 models, then recalculate the off-
origin peak heights and see if they match up with your data. I
should note here, if your real data does not show a strong drop in
peak height of the off-origin peak, then you most likely don't have
a slight rotational translation in your second hexamer.
One other important thing you should look at is the relative
orientation of your sixfold axis, is it truly perfectly aligned
with one of the cell axis ? If not fix this in your model,
otherwise your calculations will be of academic nature. For this
particular case the use of GLRF is more helpful than MOLREP (sorry
Garib, but maybe Garib can come up with a solution to zoom into
certain peaks like you can do in GLRF).
When the tilt is fixed you should be able to figure out the
rotational translation in your second hexamer.
Enjoy your puzzle,
Jürgen
P.S. P3 is certain ? Check with pointless or by human brain visual
inspection (HBVI)
On May 15, 2010, at 11:53 PM, Owen Pornillos wrote:
Dear ccp4bb –
I have questions with regards to crystal disorder that gives rise to
translational pseudosymmetry.
We have a rotationally hexameric protein that crystallized in P3,
with
one hexamer in the asu. The local 6-fold axis of the hexamer is
non-
crystallographic, and is essentially parallel to the
crystallographic
3-fold, which gave rise to translational pseudosymmetry.
Intensities
for the (h,h+/-3n,l) reflections were on average about 8 times
stronger than the weak reflections, and the native patterson gave an
off-origin peak about 70-80% of origin (depending on the crystal) at
fractional coordinates (2/3,1/3,0). We are hypothesizing that the
break in local 6-fold symmetry is caused by small rigid-body
displacements of each subunit (as opposed to conformational
changes in
the protein), and we are trying to estimate the magnitude of the
displacements in the crystal.
To do this, a perfectly symmetric hexamer with the local 6-fold axis
parallel to the crystallographic 3-fold was generated, and then
shifts
were introduced to the atomic coordinates. The direction of the
shift
was chosen randomly for each atom, and a single magnitude applied to
all atoms, which was then changed incrementally. Structure factors
were calculated from these models, and their pattersons were
examined. The magnitude of the off-origin peak could be reproduced
with an atomic shift of say, 1 Å. Because all of these calculations
were made with synthetic structure factors, this is not
necessarily a
reliable estimate. The questions are, how far off are we, and in
what
direction (i.e., are these shifts underestimates or overestimates)?
Is there a way to obtain a reliable estimate?
Thanks in advance,
Owen
-
Jürgen Bosch
Johns Hopkins Bloomberg School of Public Health
Department of Biochemistry & Molecular Biology
Johns Hopkins Malaria Research Institute
615 North Wolfe Street, W8708
Baltimore, MD 21205
Phone: +1-410-614-4742
Lab: +1-410-614-4894
Fax: +1-410-955-3655
http://web.mac.com/bosch_lab/
