Crystallization and preliminary crystallographic analysis of the circadian clock protein KaiB from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1

KaiB is a component of the circadian clock oscillator in cyanobacteria, which are the simplest organisms that exhibit circadian rhythms. KaiB consists of 108 amino-acid residues and has a molecular weight of 12 025 Da. KaiB and Cys-substituted KaiB mutants from the thermophilic cyanobacterium Thermosynechococcus elongatus BP-1 were expressed as GST-fusion proteins in Escherichia coli, purified and crystallized. The crystals of wild-type KaiB belong to the monoclinic space group P2(1), with unit-cell parameters a=89.6, b=71.2, c=106.8 Angstrom, beta=100.1degrees. While the native crystals diffract to 3.7 Angstrom, osmium derivatives, which show an approximately 4 Angstrom shrinkage in the b axis, diffract to 2.6 Angstrom. The crystals of the singly Cys-substituted mutant T64C with Hg, which show different morphology, diffract to 2.5 Angstrom and belong to the monoclinic space group P2, with unit-cell parameters a=63.7, b=33.4, c=93.7 Angstrom, beta=100.1degrees. Anomalous difference Patterson maps of the Os- and Hg-derivative crystals had significant peaks in their Harker sections, suggesting that both derivatives are suitable for structure determination.