METHYLATION PATTERN OF ACTIVATOR TRANSPOSASE BINDING-SITES IN MAIZE ENDOSPERM

The maize transposable element Activator (Ac) transposes after replica tion from only one of the two daughter chromatids. It has been suggest ed that DNA methylation in conjunction with methylation-sensitive tran sposase binding to DNA may control the association of Ac transposition and replication. We present hem a detailed genomic sequencing analysi s of the cytosine methylation patterns of the transposase binding site s within both Ac ends in the wx-m9::Ac allele, where Ac is inserted in to the tenth exon of the Waxy gene. The Ac elements in wx-m9::Ac kerne ls exhibit intriguing methylation patterns and fall into two distinct groups. Approximately 50% of the elements are fully unmethylated at cy tosine residues through the 256 nucleotides at the 5' end (the promote r end). The other half is partially methylated between Ac residues 27 and 92. In contrast, at the 3' end, all Ac molecules are heavily methy lated between residues 4372 and 4554. The more internally located Ac s equences and the flanking Waxy DNA are unmethylated. Although most met hylated cytosines in Ac are in the symmetrical CpG and CpNpG arrangeme nts, nonsymmetrical cytosine methylation is also common in the hyperme thylated regions of Ac. These results suggest a model in which differe ntial activation of transposon ends by hemimethylation controls the ch romatid selectivity of transposition and the association with replicat ion.