Overproduction and purification of the M.HhaII methyltransferase from Haemophilus haemolyticus.

The HhaII methyltransferase gene from Haemophilus haemolyticus was subcloned in an expression vector under control of the hybrid trp-lac promoter. Induction with isopropyl-beta-D-thiogalactopyranoside results in overproduction of the methyltransferase to about 3% of total cellular protein. The methyltransferase was purified to near electrophoretic homogeneity by phosphocellulose, DEAE, and gel chromatography. Its monomer Mr by sodium dodecyl sulfate-polyacrylamide gel electrophoresis is 25 kDa, in good agreement with that predicted from the nucleotide sequence. Crystals of the methyltransferase were obtained in the presence of a two-fold molar excess of the duplex oligodeoxynucleotide substrate 5'd-GGACTCC.CCTGAGG.