Amplification and purification of the bacteriophage Mu encoded B transposition protein

The A and B proteins encoded by the temperate bacteriophage Mu are involved in the high efficiency DNA transposition reaction which is the distinguishing feature of this phage. The genes encoding these early proteins were cloned in an expression vector under the control of the bacteriophage lambda leftward promoter. Under optimal conditions gpB was overproduced to account for 15% of the total cellular protein. The protein was purified to near homogeneity as determined by silver staining. Sequence analysis of the N terminus confirmed the identity of the purified protein as gpB. Proteolytic processing of the B protein does not occur at the amino terminus; the terminal methionine residue is quantitatively deformylated. The protein, which was found to be basic and a general DNA binding protein, was insoluble at low ionic strength in the absence, but not in the presence, of DNA. The B protein also displayed a tendency to aggregate at high ionic strength where it was soluble in the absence of DNA. In addition, the protein was characterized as to its amino acid composition and extinction coefficient at 280 nm. The purified protein is active in a soluble in vitro transposition-replication system.