Restriction and modification in Bacillus subtilis: two DNA methyltransferases with BsuRI specificity. I. Purification and physical properties.

Two S-adenosyl-L-methionine:DNA (cytosine 5)-methyltransferases, termed M.BsuRIa and M.BsuRIb, were purified 3,000- and 4,000-fold, respectively, from Bacillus subtilis strain OG3R (r+m+) by successive column chromatography. The molecular weights determined by gel filtration were 37,000 for M.BsuRIa and 40,000 for M.BsuRIb. The sedimentation coefficients s20,w were 3.55 for both enzymes as determined by glycerol gradient centrifugation, corresponding to molecular weights of 43,000. Analysis of the two methyltransferases by agarose gel electrophoresis under native conditions, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, showed correspondence of the M.BsuRIa activity with one protein band at a molecular weight of 41,000, whereas M.BsuRIb activity was associated with two protein bands with molecular weights of 42,000 and 39,000, respectively.