Localization of a DNA-binding determinant in the bacteriophage P22 Erf protein

Four amber fragments of the recombination-promoting P22 Erf protein were characterized. The intact Erf monomer contains 204 amino acids. The amber mutations produce fragments of 190, 149, 130 and 95 amino acid residues, all of which are inactive in vivo. The 190 residue fragment is more susceptible to proteolysis in cell extracts than is intact Erf. It breaks down to a stable remnant that is slightly larger than the 149 residue fragment. The 149 and 130 residue fragments are stable; electron microscopy of the purified fragments reveals that they have similar morphologies, retaining the ring-like oligomeric structure, but lacking the tooth-like protruding portions of intact Erf. Intact Erf and the 149 residue fragment have similar affinities for single-stranded DNA; the affinity of the 130 residue fragment is 40-fold lower in low salt at pH 6.0. The 95 residue fragment is unstable in vivo. These observations, combined with previous observations, are interpreted as suggesting that the boundary of the amino-terminal domain of the protein lies between residues 96 and 130, that certain residues between 131 and 149 form part of an interdomain DNA-binding segment of the protein, that the boundary of the carboxy-terminal domain lies to the C-terminal side of residue 149, and that the carboxy-terminal domain is not necessary for assembly of the ring oligomer, although it is essential for Erf activity in vivo.