The State of DNA Modification within and Flanking Maize Transposable Elements
Modification of the Mu1-like transposable elements, primarily or exclusively 5-methylcytosine in the sequences 5′-CG-3′ and 5′-CNG-3′, is strongly correlated with the loss of Mutator activity. Chromosomal copies of Mu1-like elements and the maize retrotransposon Bs1 are fully digested by 5-methylcytosine-sensitive restriction enzymes in DNA from plants in which element transposition has been detected. In some cases, Mutator activity may be lost through a process not associated with Mu element modification. Under other circumstances, particularly inbreeding for the Mutator trait, Mu element modification and loss of Mutator activity are concurrent. When Mu element modification occurs, HinfI and MluI sites in and near the Mu1 terminal inverted repeats are often modified first. Modification at these HinfI sites is not seen in some lines that had lost Mutator activity one or more generations earlier, and are fully modified at sites within the Mu element. The Mu1-internal NotI and EcoRII sites are always and exclusively modified in any Mutator-derived line that has lost activity through a modification-associated process. Extensive modification within a Mu element does not lead to detected modification in flanking sequences.
In crosses between Mutator-loss plants with modified Mu elements and active Mutator plants with unmodified Mu elements, the progeny initially contain a mixture of modified and unmodified elements. During development of the plant, the ratio of modified to unmodified elements progressively increases. This gradual dominance of modified elements is approximately equivalent regardless of whether the Mutator-loss plant was the male or female parent.
KeywordsTransposable Element Mutator Activity Cytosine Methylation Transpositional Activity MluI Site
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