Gene-controlled cytosine demethylation in the promoter region of the Ac transposable element in maize.

The transposase product of the Ac transposable element causes demethylation of the BamHI site in the promoter region of Ac. This site remains methylated in Ds9, the deletional derivative of Ac that fails to make an active transposase. In the presence of an active Ac element the BamHI site of Ds9 is demethylated. Transcription of the element is correlated with the demethylation of the promoter region. Evidence is presented that supports the model that the trans-acting protein causes site-specific demethylation by inducing a configuration change in chromatin structure making the site inaccessible to the methylase. A chromosome alteration that involves the insertion of the 4.5-kilobase Ac element results in the methylation of Pvu I target sites about 1 kilobase removed from the point of insertion.