A methyltransferase that methylates one of the proteins involved in chemotactic adaptation to sensory stimuli in Bacillus subtilis was purified to homogeneity. The enzyme utilizes S-adenosylmethionine as donor for a methyl group that is transferred to a glutamate residue in a 69 000-mol.wt. membrane protein and also to a protein of 19 000 mol.wt. The molecular weights of the denatured enzyme by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis and of the native enzyme by gel-filtration chromatography both show the protein to be a 44 000-mol.wt. monomer. Isoelectric focusing of the purified methyltransferase showed the protein to be a single species with isoelectric point pI 5.4. On the basis of a molecular weight of 44 000, the molar absorption coefficient at 262 nm of the enzyme is 10.9 x 10(4) M-1 . cm-1. The Km of the enzyme for S-adenosylmethionine is about 2 microM. The Ki for S-adenosylhomocysteine is about 0.2 microM. Ca2+ is a competitive inhibitor of methylation, with a Ki of 0.065 microM. The enzyme methylates membranes from the wild-type more efficiently than membranes isolated from a mutant strain defective in chemotaxis. The enzyme is unable to methylate Escherichia coli membranes.