Enzymatic DNA methylation of carbon 5 of cytosines is an epigenetic modification that plays a role in regulating gene expression, differentiation, and tumorigenesis. DNA (cytosine-C5)-methyltransferase-1 is the enzyme responsible for maintaining established methylation patterns during replication in mammalian cells. It is composed of a large (similar to1100 amino acids (a.a.)) amino-terminal region containing many putative regulatory domains and a smaller (similar to500 a.a.) carboxy-terminal region containing conserved, catalytic domains. In this study, murine DNA (cytosine C5)-methyltransferase-1, fused to an amino-terminal hexahistidine tag, was expressed by infecting Spodoptera frugiperda cells for 46 h with a recombinant baculovirus carrying the DNA (cytosine-C5)-methyltransferase-1 cDNA. A total of 3 X 101 infected S. frugiperda cells yielded similar to1 mg of full-length, hexahistidine-tagged DNA (cytosine-C5)-methyltransferase-1, which was purified similar to450-fold from RNase-treated S. frugiperda cell extracts by nickel affinity chromatography. The characterization of hexahistidine-tagged DNA (cytosine-C5)-methyltransferase-1 through DNA methylation and inhibitor-binding assays indicated that the purified enzyme had at least a 30-fold higher catalytic efficiency with hemimethylated double-stranded oligodeoxyribonucleotide substrates than unmethylated substrates and was most active with small oligodeoxyribonucleotide substrates with a capacity for forming stem-loop structures. The expression and purification procedures reported here differ significantly from the original reports of baculovirus-mediated hexahistidine-tagged DNA (cytosine-C5)-methyltransferase-1 expression and purification by nickel affinity chromatography and provide a consistent yield of active enzyme. (C) 2002 Elsevier Science (USA).