Purification and characterization of the Ner repressor of bacteriophage Mu

Abstract

The Ner protein of bacteriophage Mu acts as a λ cro-like negative regulator of the phage's early (transposase) operon. Using the band retardation assay to monitor ner-operator-specific DNA-binding activity, the 8 kDa Ner protein was purified to homogeneity. DNase I footprinting revealed that the purified protein bound and protected a specific DNA operator that contains two 12 bp sites with the consensus sequence 5′-ANPyTAPuCTAAGT-3′, separated by a 6 bp spacer region. Moreover, regions corresponding to a turn of the DNA helix flanking these 12 bp repeats are also protected by Ner. Unlike the functionally similar λ cro protein, gel filtration experiments show the native molecular mass of Mu Ner to be approx. 8 kDa. These results, plus the pattern of DNase I protection, suggest that the protein may bind as a monomer to each of its specific DNA substrates.