Cytosine 5 DNA methyltransferases share ten conserved motifs. Motif IV contains an absolutely conserved proline-cysteine dipeptide. The cysteine residue of this motif is involved in catalysis by forming a covalent bond with the 6-position of cytosine prior to methyl group transfer. AquI DNA methyltransferase recognising the sequence CCCGGG is a heterodimer unlike other C5 DNA methyltransferases. We changed the conserved cysteine (Cys82) of M.AquI to serine and glycine. The presence of mutations was confirmed by automated DNA sequencing. Mutants were tested by in vivo plasmid protection assay and also by transformation into mcrA+BC+ strain of E. coli. Replacement of the conserved cysteine with serine led to an apparent loss of the methyltransferring ability of the enzyme. Interestingly, it was found that substitution of cysteine with glycine is not cytotoxic to E. coli in the case of M.AquI.

Cytosine 5 DNA methyltransferases share ten conserved motifs. Motif IV contains an absolutely conserved proline-cysteine dipeptide. The cysteine residue of this motif is involved in catalysis by forming a covalent bond with the 6-position of cytosine prior to methyl group transfer. AquI DNA methyltransferase recognising the sequence CCCGGG is a heterodimer unlike other C5 DNA methyltransferases. We changed the conserved cysteine (Cys82) of M.AquI to serine and glycine. The presence of mutations was confirmed by automated DNA sequencing. Mutants were tested by in vivo plasmid protection assay and also by transformation into mcrA+BC+ strain of E. coli. Replacement of the conserved cysteine with serine led to an apparent loss of the methyltransferring ability of the enzyme. Interestingly, it was found that substitution of cysteine with glycine is not cytotoxic to E. coli in the case of M.AquI.