The CpG-specific methylase Sssl has topoisomerase activity in the presence of Mg2

Abstract

A prokaryotic CpG-specific methylase from Splroplasma, Sssl methylase, is now widely used to study the effect of CpG methylatlon In mammalian cells, and can processlvely modify cytosines In CpG dinucleotldes In the absence of Mg2-. In the presence of Mg2+, we found (i) that the methylation reaction is distributive rather than processlve as a result of the decreased affinity of Sssl methylase for DNA, and (ii) that a type l-llke topoisomerase activity is present In Sssl methylase preparations. This topoisomerase activity was still present in Sssl methylase further purified by either SDS-polyacrylamide or Isoelectric focusing gel electrophoresls. We show that methylase and topoisomerase activi..