The DNA recognition subunit of a DNA methyltransferase is predominantly a molten globule in the absence of DNA

Enzyme-catalysed DNA methylation provides an opportunity for the modulation of protein-DNA recognition in biological systems. Recently we have demonstrated that the smaller of the two subunits of the heterodimeric, cytosine-specific DNA methyltransferase, M.AquI, is largely responsible for sequence-specific DNA recognition. Here we present evidence from a series of NMR, fluorescence and circular dichroism spectroscopy experiments that the DNA binding subunit of M.AquI has the characteristics of a molten globule in the absence of the catalytic machinery. In this metastable state this subunit retains its ability to bind DNA in a sequence-specific manner. We believe this finding offers an insight into the structural flexibility which underpins the mechansim of action of these enzymes, and may provide a possible biological role for molten globules in protein function. © 1994.