Epigenetic Regulation of the Maize Spm Transposon
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Expression and transposition of the Suppressor-mutator (Spm) transposon of maize are controlled by interacting epigenetic and autoregulatory mechanisms. Methylation of critical element sequences prevents both transcription and transposition, heritably inactivating the element. The promoter, comprising the terminal 0.2 kb of the element, and a 0.35-kb, highly GC-rich, downstream sequence are the methylation target sequences. The element encodes two proteins necessary for transposition, TnpA and TnpD. There are multiple TnpA binding sites, both in the 5′ terminal promoter region and at the element's 3′ end. In addition to its role in transposition, TnpA is both a positive and a negative regulator of transcription. TnpA represses the element's promoter when it is not methylated. When the element is inactive and its promoter methylated, TnpA activates the methylated promoter and facilitates both its transient and heritable demethylation.