Steve Ealick's Research Group

Bacillus subtilis PurS

PDB files:

1T4A BsPurS (spacegroup C2)

1TWJ BsPurS (spacegroup P21)


PurL exists in two forms: large PurL (lgPurL) is a single chain, multidomain enzyme of about 1300 amino acids, whereas small PurL (smPurL) contains about 800 amino acids but requires two additional gene products, PurS and PurQ, for activity in the fourth step of the purine biosyntheric pathway. PurQ is a glutaminase, but the function of PurS is still unknown. We determined the structure of Bacillus subtilis PurS in two different crystal forms P21 and C2 at 2.5 Å and 2.0 Å resolution. Initial phases were determined by molecular replacement, using a polyalanine dimer model based on the Methanococcus thermoautotrophicum (Mth) structure (PDB code 1GTD) of a PurS homolog determined in a structural genomics project.

The PurS monomer consists of 84 amino acids of which the first 80 are visible in the experimental electron density maps for both the P21 and C2 crystal forms. The C2 crystal form has two molecules per asymmetric unit, but the P21 crystal form has four molecules per asymmetric unit.


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BsPurS forms a tight dimer with a central six-stranded β-sheet flanked by four α-helices. In both the P21 and the C2 crystal forms, the quaternary structure of BsPurS is a tetramer; Based on our recent structure of lgPurL, the biochemical studies on the corresponding B. subtilis smPurL, and the structural studies reported herein, the tetrameric form of the MthPurS cannot be the physiologicially relevant form.


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Anand R, Hoskins AA, Bennett EM, Sintchak MD, Stubbe J and Ealick SE. A Model for the Bacillus subtilis Formylglycinamide Ribonucleotide Amidotransferase Multiprotein Complex. Biochemistry 43:10343 - 10352 (2004).

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