Hi, I suggest you check the following publication: Morales, R. et al. (2000), "Crystallographic studies of the interaction between the ferredoxin-NADP+ reductase and ferredoxin from the cyanobacterium Anabaena: looking for the elusive ferredoxin molecule", Acta Cryst. D56, 1408-1412.
Here is the abstract: Ferredoxin-NADP+ reductase (FNR) and its physiological electron donor ferredoxin (Fd) from the cyanobacterium Anabaena PCC7119 have been co-crystallized. The unit-cell parameters are a = b = 63.72, c = 158.02 Å and the space group is P212121. The crystal structure has been solved with 2.4 Å resolution synchrotron data by molecular replacement, anomalous dispersion and Rmin search methods. For the computations, the crystal was treated as a merohedral twin. The asymmetric unit contains two FNR molecules and one ferredoxin molecule. The packing of the FNR molecules displays a nearly tetragonal symmetry (space group P43212), whereas the ferredoxin arrangement is orthorhombic. This study provides the first crystallographic model of a dissociable complex between FNR and Fd. Slightly different from what you mention (different cell dimensions, in this publication a = b, but the space group is P212121...) Fred. > Message du 21/06/10 22:52 > De : "yang li" > A : CCP4BB@JISCMAIL.AC.UK > Copie à : > Objet : [ccp4bb] Merohedral twining for P212121. > > Dear all, > Is there any possible of twining for a normal P212121 spacegroup and > what is the twin law, say no equal cell dimensions? Some people said there is > no twin law for such symmetry but I am not very sure. Thanks a lot. > BestYang
