Site specific cytosine methylation in rice nonautonomous transposable element nDart.

The mobile nonautonomous element nDart, which is active in intact rice plants, exhibits locus specific transposition. Due to the high homogeneity of nDart elements, the locus specificity of nDart transposition might be controlled by factors other than genetic differences. In this study, we elucidated the regulation of the locus specificity of nDart transposition. The difference of transpositional activities in 10 nDart elements among rice varieties exhibiting nDart transposition was clearly correlated with the methylation state of nDart elements. Both hyper- and hypo-methylated nDart elements were inactive, while site specific methylation in both subterminal regions was identified in active nDart loci. The specific methylation sites contain the pentamer motif GCC/ACG. The repeated motifs in the subterminal region of nDart elements may contribute to the stable secondary structure of nDart elements with low free energy. Our results suggested that site specific cytosine methylation may loosen the stable secondary structure of the nDart element to allow it to bind TPase, which then perform the excision of nDart elements from genomic loci.