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We've solved structures in this space group with no problem, given that you have a good model. There isn't anything different than in any other space group. One confusion might be that you have it indexed in the trigonal/hexagonal cell (a=b, alpha=beta=90, gamma=120) rather than a rhombohedral cell (a=b=c, alpha=beta=gamma), so the unit cell itself is three times larger. In the rhombehedral unit cell you have 6 asu's in the entire unit cell; in the trigonal/hexagonal unit cell you have 18 asu's in the entire unit cell. Christian is wrong that the signal/noise ratio changes. The unit cell is bigger but you also have three times as many reflections, so the signal/noise ratio doesn't change. All the usual caveats about molecular replacement: If you have a single domain, then you don't need to modify your search model. Your sequence in correct, so that's not the problem. Be sure to use the same B-factors, and not an average one, since that will downweight more flexible regions of the protein. Do remove parts of the N- or C-terminus if they are flexible. If you have two domains, then definately search with only one or the other. Try all MR programs: phaser, molrep, epmr, etc. Finally, is it possible that the space group is really R3, and there is twinning to make it look like R32? R3 is a better choice for a lower symmetry search than the C-centered monoclinic cell. Good luck! Bernie Santarsiero On Fri, January 5, 2007 4:33 am, Christian Schleberger wrote: > *** For details on how to be removed from this list visit the *** > *** CCP4 home page http://www.ccp4.ac.uk *** > > > Hey Jenny, > >> >> The protein is only 90 residues ( 10kDa), so looks to me the cell size >> is a little big. > If your spacegroup is really R32 this means around 3 molecules per > asymmetric unit. This is not unusual. >> >> I heard that it's quite tricky for R32 and I wondered what's that. > R32 has z=18 giving you 3*18=54 Molecules per unit cell. As far as I > understand molecular replacement this gives you a bad signal to noise > ratio for the search. Additionally your molecule is very small. If there > is just a small conformational change between the wt crystal form and > your R32 crystal form this will corrupt your search model giving you a > even worse signal. >> If MR doesn't succeed, does this mean, the new mutant protein is >> very different from the wild type? > Not necessarily. See above. > Why not try SeMet or a good old heavy atom derivate? > > > best regards > > Christian >
