Preliminary neutron scattering studies of the Type I restriction-modification enzyme M.Ahdl

Abstract

Type I restriction-modification (R-M) systems encode multisubunit/multidomain enzymes. Two genes (M and S) are required to form the 160 kDa methyltransferase that methylates a specific base within the recognition sequence and protects DNA from cleavage by the endonuclease. Small angle neutron scattering (SANS) revealed an unusually large structural change in the EcoR124I methyltransferase following DNA binding; this involves a major repositioning of the subunits of the enzyme, resulting in a 60 Å reduction in the dimensions of the enzyme on forming a complex with DNA. The related methyltransferase M.Ahdl, with stoichiometry M 2S 2 has been prepared in two protonated/deuterated states (S and M subunits protonated, S deuterated and M protonated) for which SANS data have been collected in a number of H:D solvent contrasts. The contrast match point of the selectively deuterated enzyme confirms the successful reconstitution of the enzyme with deuterated S subunits. Ab initio shape determination using this contrast matched data is in progress to determine the subunit organization of M.Ahdl and the large change in structure that occurs on DNA binding.