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 activities are not functionally Interdependent, since conditions exist where only one or the other enzymatic activity Is detectable. The catalytic domains of Sssl methylase and prokaryotic topoisomerases show similarity at the amlno acid level, further supporting the idea that the topoisomerase activity Is a genuine activity of Sssl methylase. Mycoplasmas, including Splroplasma, have the smallest genomes of all living organisms; thus, this condensation of two enzymatic activttes Into the same protein may be a result of genome economy, and may also have functional implications for the mechanism of methylatlon.
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