sentence: Following 4 h induction at 25 – 28 ° C , bacteria were harvested and stored at − 70 ° C.To isolate His - tagged retroviral IN proteins , thawed bacterial paste was sonicated in buffer B ( 1 M NaCl , 7.5 mM CHAPS , 50 mM Tris – HCl , pH 7.4 ) containing 0.5 mM phenylmethlysulfonyl fluoride and 15 mM imidazole .