CHARACTERIZATION OF DBPM, A PLANT PROTEIN THAT BINDS TO DNA CONTAINING 5-METHYLCYTOSINE

A protein (DBPm) has been isolated from nuclear extracts of soybean seeds, cauliflower florets, corn seed, wheat germ, and pea hypocotyl, seeds, apices, roots, and leaves that specifically binds to double-strand DNA containing 5-methylcytosine residues. In electrophoretic mobility shift assays, non-methylated duplex DNAs competed only slightly, while methylated DNAs were strong competitors. Specific binding still occurred after partial proteolysis of DBPm, but not after heating at 45-degrees-C. By ultraviolet light-crosslinking and sodium dodecyl sulfate polyacrylamide gel electrophoresis and gel filtration, the size of pea seed DBPm was estimated to be in the range 70-90 kDa. From equilibrium binding studies the equilibrium constant for binding of pea seed DBPm to a 34 bp duplex deoxyoligonucleotide containing 12 5-methylcytosine residues was 1.2 . 10(9) M-1. The binding properties of DBPm make it a good candidate for a plant protein capable of mediating the effects of DNA methylation on the activity of some plant genes.