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

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Abstract
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.