EPIGENETIC REGULATION OF THE MAIZE SPM TRANSPOSABLE ELEMENT - NOVEL ACTIVATION OF A METHYLATED PROMOTER BY TNPA

Spm is epigenetically inactivated by C-methylation near its transcript ion start site. We have investigated the interaction between TnpA, an autoregulatory protein that can reactivate a silent Spm, and the promo ter of the element. The promoter undergoes rapid de novo methylation a nd inactivation in stably transformed plants, but only if it includes a GC-rich sequence downstream of the promoter. TnpA activates the inac tive, methylated promoter and leads to reduced methylation. By contras t, TnpA represses the active, unmethylated Spm promoter. Only the inte rnal DNA-binding and dimerization domains of the protein are required for repression, while activation requires an additional C-terminal seq uence. TnpA is therefore a unique regulatory protein with a convention al transcriptional repressor activity and a novel ability to activate a methylated, inactive promoter.