with HKL 24 or XDS 25 . Crystals belong to space group P2 1 2 1 2 1 with unit cell dimensions a ¼ 56.7 Å , b ¼ 67.7 Å and c ¼ 135.6 Å , and contain three molecules per asymmetric unit.
Structure determinationSelenium sites were located with SnB 26 . MAD phases were calculated with SHARP 27 and improved by density modification. A CID model was built with O 28 and refined with CNS 29 . The refined model has excellent stereochemical quality and a free R-factor of 25.2%. A difference Fourier for the peptide-soaked crystal was calculated with phases from the CID model and identified strong density for the CTD peptide bound to one CID molecule (chain B). The two other CID molecules pack against each other, burying their peptide-binding sites. After peptide building, the CID-CTD model was refined to a free R-factor of 25.7%. Peptide binding does not result in significant conformational changes in the CID domain. Soaking experiments with phosphoserine did not reveal any additional electron density. D 54, 799-804 (1998). 27. Terwilliger, T. C. Automated structure solution, density modification and model building. Acta. Crystallogr. 58, 1937Crystallogr. 58, -1940Crystallogr. 58, (2002 Competing interests statement The authors declare that they have no competing financial interests.Correspondence and requests for materials should be addressed to P.C.(cramer@lmb.uni-muenchen.de). Atomic coordinates and structure factors for the Pcf11 CID domain and the CTD-CID complex have been deposited in the Protein Data Bank under accession numbers 1SZ9 and 1SZA, respectively.
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