SURFACE LABELING OF THE TYPE-I METHYLTRANSFERASE M.ECOR124I REVEALS LYSINE RESIDUES CRITICAL FOR DNA-BINDING

The type IC methyltransferase M.EcoR124I consists of a specificity sub unit (HsdS) and two methylation subunits (HsdM). Using chemical modifi cation, we have investigated the accessibility of lysine residues in t he free enzyme and in the complex with its DNA recognition sequence. A total of 41 of the 109 lysine residues in the enzyme are susceptible to modification, of which 19 are located in the HsdS subunit and 11 in each of the two HsdM subunits. DNA binding results in extensive prote ction of lysine residues in the HsdS subunit, while those in the HsdM subunit are only protected weakly. The DNA binding activity of the met hylase is abolished when a small fraction of the accessible lysine res idues are modified. Peptide mapping and N-terminal sequencing has been used to locate the rapidly modified lysine residues in HsdS that are critical for DNA binding. Highly modified residues (K297, K261 and K32 7) are found in the C-terminal variable domain that is responsible for DNA recognition, but others (K196, K203 and K210) are found in the co nserved regions that had not previously been implicated in DNA binding . (C) 1996 Academic Press Limited