Epigenetic mechanisms in the regulation of the maize Suppressor-mutator transposon.

Transcription and transposition of the maize Suppressor-mutator (Spm) transposon are epigenetically controlled. Methylation of specific element sequences prevents transcription and transposition in a heritable manner. Reactivation and demethylation occur in the presence of an active element, implying the existence of an element-encoded epigenetic activator. The methylation target sequences are the 0.2 kb promoter and an 0.35 kb GC-rich downstream sequence. Two Spm-encoded proteins, TnpA and TnpD, participate in transposition. In addition, TnpA has positive and negative regulatory activities. TnpA represses and activates the unmethylated and methylated Spm promoters, respectively, and it participates in the transient and heritable demethylation of the promoter and GC-rich region. There is evidence that TnpA-mediated repressor and epigenetic activator functions occur by different molecular mechanisms.