DNA methylation patterns are generated during development by a sequence of
methylation and demethylation events. We have recently demonstrated that ma
mmals bear a bona fide demethylase enzyme that removes methyl groups from m
ethylated cytosines. A general genome wide demethylation occurs early in de
velopment and in differentiating cell lines. This manuscript tests the hypo
thesis that the demethylase enzyme is a processive enzyme. Using bisulfite
mapping, this report demonstrates that demethylase is a processive enzyme a
nd that the rate limiting step in demethylation is the initiation of demeth
ylation. Initiation of demethylation is determined by the properties of the
sequence. Once initiated, demethylation progresses processively. We sugges
t that these data provide a molecular explanation for global hypomethylatio
n.