Purification of the Bacteriophage SP01 Transcription Factor 1

The repressor-like protein, transcription factor 1 (TF1), has been isolated from SP01-infected Bacillus subtilis. A purification method is described which yields homogeneous protein as judged by gel electrophoresis in sodium dodecyl sulfate or urea and Sephadex G-75 gel chromatography. The amino acid analysis of TF1 allows one to estimate a minimum molecular weight of approximately 11,500. Sedimentation equilibrium molecular weight measurements in 0.25 m KCl, 0.01 m Tris cacodylate, pH 6, at 20°, suggest that TF1 is a dimer under these conditions. The amino acid composition of TF1 is characterized by a high content of basic and acidic (or, potentially acidic) amino acids, and by the absence of cysteine, histidine, and tryptophan. The isoelectric point of TF1 is 9.8. The conformation of TF1 in solution is strongly temperature-dependent. The circular dichroism spectrum suggests an α helix content of 20 to 30% at room temperature, but reversible denaturation occurs between 30 and 60°. TF1 is synthesized after phage infection, yet more than 105 molecules of TF1 are present in an infected cell 20 min after infection at 37°.