Mode of action of the Spiroplasma CpG methylase M.SssI

The cytosine DNA methylase from the wall-less prokaryote, Spiroplasma strain MQ1 (M.SssI) methylates completely and exclusively CpG-containing sequences, thus showing sequence specificity which is similar to that of mammalian DNA methylases. M.SssI is shown here to methylate duplex DNA processively as judged by kinetic analysis of methylated intermediates. The cytosine DNA methylases, M.HpaII and M.HhaI, from other prokaryotic organisms, appear to methylate in a non-processive manner or with a very low degree of processivity. The Spiroplasma enzyme interacts with duplex DNA irrespective to the presence of CpG sequences in the substrate DNA. The enzyme proceeds along a CpG-containing DNA substrate molecule methylating one strand of DNA at a time.