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This review focuses on the epigenetic control of the maize Suppressor-mutator (Spm) transposon and the evolutionary origin of epigenetic mechanisms. Methylation of the Spm promoter prevents transcription and transposition, and the methylation of the adjacent GC-rich sequence renders the inactive state heritable. Spm encodes an epigenetic activator, TnpA, one of the two Spm-encoded transposition proteins. TnpA can reactivate an inactive, methylated Spm both transiently and heritably, and it is also a transcriptional repressor of the unmethylated Spm promoter. Features common to epigenetic mechanisms in general suggest that they originated as a means of decreasing the recombinogenicity of duplicated sequences.
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