Our laboratory cloned the Spm element some years ago and we have explored the molecular basis of Spm inactivation and reactivation. We’ve done this genetically in maize and we’ve used reconstructed transgenic systems with introduced cloned element-encoded genes and Spm promoter-reporter gene fusions. The essential findings are these:
1. When the element is inactive, its promoter sequence is methylated.
2. The heritability of the inactive or silenced state is correlated with the methylation level of a downstream, very GC-rich seqence.
3. The promoter is rapidly methylated in transgenic plants, but only if it contains the GC-rich downstream sequence.
4. The element encodes two proteins that are necessary for transposition and one of these, TnpA, is both a transposition protein and a regulator.
5. TnpA binds to multiple sites present in both direct and inverted orientations at both element ends.
6. TnpA can reactivate a silenced, methylated element both transiently and heritably, identifying it a sequence-specific epigenetic regulatory protein.