Specificity of the histone lysine methyltransferases from rat brain chromatin

The histone lysine methyltransferases catalyze the transfer of methyl groups from S-adenosylmethionine to specific epsilon-N-lysine residues in the N-terminal regions of histones H3 and H4. These enzymes are located exclusively within the nucleus and are firmly bound to chromatin. The chromosomal bound enzymes do not methylate free or nonspecifically associated histones, while histones H3 and H4 within newly synthesized chromatin are methylated. These enzymes can be solubilized by limited digestion (10-16%) of chromosomal DNA from rapidly proliferating rat brain chromatin with micrococcal nuclease. Histone H3 lysine methyltransferase remained associated with a short DNA fragment throughout purification. Dissociation of the enzyme from the DNA fragment with DNAase digestion resulted in complete loss of enzyme activity; however, when this enzyme remained associated with DNA it was quite stable. Activity of the dissociated enzyme could not be restored upon the addition of sheared calf thymus or Escherichia coli DNA. Histone H3 lysine methyltransferase was found to methylate lysine residues in chromosomal bound or soluble histone H3, while H3 associated with mature nucleosomes was not methylated. The histone H4 lysine methyltransferase which was detectable in the crude nuclease digest was extremely labile, losing all activity upon further purification. We isolated a methyltransferase by DEAE-cellulose chromatography, which would transfer methyl groups to arginine residues in soluble histone H4. However, this enzyme would not methylate nucleosomal or chromosomal bound histone H4, nor were methylated arginine nucleosomal or chromosomal bound histone H4, nor were methylated arginine residues detectable upon incubating intact nuclei or chromatin with S-adenosylmethionine.