What is the sequence identity of your best search model? Finding that many copies in P1 with 3A data is a challenge but certainly not impossible if there is a reasonably close (>20-25% identity) search model available. I would suggest spending some time on preparing a very good search model with tools like Sculptor as well as manually trimming loops and using ensembles of conserved folds from several homologous structures.
It is also worth remembering that there are a number of different programs available for molecular replacement and it is worth investing some time in learning how to use Molrep and Amore as well as Phaser as they all have different strengths and weaknesses. Is your data otherwise devoid of any other problems like pseudo-translational symmetry? These can be readily identified with tools like phenix.xtriage. PST can complicate matters quite considerably in molecular replacement. SAD phases, even if obtained at low-resolution, can still be very useful if combined with molecular replacement, so it is well worth pursuing all lines of attack simultaneously. Hope this helps. Eugene On 19 January 2014 19:30, Chris Fage <[email protected]> wrote: > Thank you all for your responses. I already have a few ideas about how to > approach the problem. > > One of my concerns with so monomers per asymmetric unit at lower > resolution was the failure of MR software. Neither PHENIX nor Phaser MR > have made progress. I am fairly new to anomalous methods, having solved > only two structures by SeMet-based SAD. I've certainly picked up on a > number of tricks from the recent messages on heavy atoms, but I thought my > case might be a little unusual. I am confident the space group is P1, as it > was the only viable option when I indexed four clean albeit low-res > datasets. > > The monomers are ~38 kDa, and the crystals diffracted to 3.4-3.0 at a > synchrotron. > > The conditions for both native and SeMet crystals are: > 8-12% PEG 2000 MME, 0.2 M ammonium sulfate, 0.1 M sodium acetate pH 5.5. > > Macromolecular seeding of native crystals into SeMet drops yields the > needle-like crystals. > > Any further input is greatly appreciated! > > Regards, > Chris > > > On Sat, Jan 18, 2014 at 11:14 AM, Chris Fage <[email protected]> wrote: > >> Hello Everyone, >> >> I am currently trying to phase a structure with an asymmetric unit >> predicted to contain 20-24 monomers (space group P1). The native crystals, >> while beautiful in appearance (see attached), only diffract to ~3.4-3.0 >> angstroms at best, and SeMet-derived crystals grow with poor morphology >> (small needles). Also, based a fluorescence scan, I know that mercury does >> not bind appreciably. Other than screening for a new space group, what >> options might I have for phasing this many monomers at lower resolution? Is >> there any real chance of solving the structure in this space group? >> >> Thank you in advance for any suggestions! >> >> Regards, >> Chris >> > > -- Dr Eugene Valkov Room 3N049 Division of Structural Studies MRC Laboratory of Molecular Biology Francis Crick Avenue Cambridge Biomedical Campus Cambridge CB2 0QH, U.K. Email: [email protected] Tel: +44 (0) 1223 267358
