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The structure of the bacterial signal transduction protein PII has been refined to an R factor of 13.2% using 3σ data between 10 and 1.9 Å. The crystals exhibited twinning by merohedry and X-ray intensities were corrected using the method of Fisher & Sweet [Fisher & Sweet (1980). Acta Cryst. A36, 755–760] prior to refinement. Our earlier 2.7 Å structure [Cheah, Carr, Suffolk, Vasudevan, Dixon & Ollis (1994). Structure, 2, 981–990] served as a starting model. PII is a trimeric molecule, each subunit has a mass of 12.4 kDa and contains 112 amino-acid residues. The refined model includes all 1065 protein atoms per subunit plus 312 water molecules. The high-resolution refinement confirms the correctness of our 2.7 Å model, although it leads to a redefinition of the extent of various secondary-structural elements. The monomeric structure of PII exhibits an interlocking double βαβ fold. This is a stable fold found in a number of proteins with diverse functions. The association of the protein into a trimer leads to a new structure which we describe in detail. The effects of crystal packing forces are discussed and potential interaction sites with other proteins and effector molecules are identified.

Supporting information

PDB reference: 2pii

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